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METAL-COLOURING
AND BRONZING

BY THE SAME AUTHOR

IRON AND STEEL MANUFACTURE. Globe
8vo. 3s. 6d.

A TEXT-BOOK OF ELEMENTARY METAL-
LURGY. Globe 8vo. 3s.

QUESTIONS ON METALLURGY. GlobeSvo. is.

PRACTICAL METALLURGY AND ASSAYING.
Globe 8vo. 6s.

PRINCIPLES OF METALLURGY. Globe 8vo. 6s.

MIXED METALS, OR METALLIC ALLOYS.
Globe 8vo. 6s.

METALLOGRAPHY. Globe 8vo. 6s.

STEEL AND IRON FOR ADVANCED STU-
DENTS. Globe 8vo. 10s. 6d.

MACMILLAN AND CO., Ltd., LONDON.

METAL-COLOURING

AND BRONZING

ARTHUR H. HIORNS

■ »
HEAD 07 METALLURGICAL DEPARTMENT, BIRMINGHAM

MUNICIPAL TECHNICAL SCHOOL

SECOND EDITION

MACMILLAN AND CO., LIMITED
ST. MARTIN'S STREET, LONDON

1907

First Edition 1892
Reprinted 1902, 1907

15017 S

•Htl
'2

PREFACE TO SECOND EDITION

Since the publication of the first edition additional
experience has been gained in working the various solu-
tions, and although the recipes then given are correct so
far as they go, it has been thought advisable to add
further information to guide the operator, and such addi-
tions are given in the Appendix.

PREFACE TO FIRST EDITION

The attention of the author was directed to the subject of
Bronzing some years back, and more particularly about
eighteen months ago, by the apparent absence of any
reliable information on the subject in this country. It is
a subject in which those engaged in metal trades,
especially in Birmingham, should be deeply interested, as
that city was formerly considered to be one of the chief
centres for the production of beautiful colours on metal
work. A member of the Birmingham Technical School
Committee, Mr. W. J. Davis, suggested to me that we
should endeavour to improve the ordinary methods of
working by establishing a class for the cultivation of this
special branch of industry, and for studying in a systematic
manner the principles on which the actions are based, so
as to attempt to bring back the almost lost art to its
former home, and, if possible, to greatly expand it in
beauty and usefulness.

The present work is a record of the investigations
carried out by means of many hundreds of experiments in
every way that seemed likely to yield reliable information.
The first thing was to find out, by inquiries in different

viii METAL-COLOURING AND BRONZING

directions, and by reference to works which were likely to
contain any information on the subject, what others had
done, and how much of their work was published. A
great number of recipes were thus collected, many of them
bearing the stamp of age by the quaintness of the wording
and the old-fashioned way in which different chemical
bodies were named. Some were in quite a modern garb,
as though they had been recently discovered, but in many
cases their effectiveness was doubtful.

The experiments have been carried on almost unin-
terruptedly for eighteen months by the author and his
colleague, Mr. T. J. Baker, who has conducted the greater
part of the experiments with the assistance of Mr.
A. Hiorns, jun.

Our aim has been to take nothing for granted, but to
test every method in various ways by direct trials, and to
faithfully record such results as were likely to be useful to
our students and to any others who desire the information.

It is possible that, in cases where we have failed to
obtain a good effect from a given solution, the fault may
have been in the manipulation rather than in the reagent,
and as this work is intended to stimulate further inquiry,
as well as to give the results of our experience to the public
generally, some other workers may succeed where we have
failed, and this is the reason for recording some methods,
recommended by others, but which proved to be useless
under the conditions in which our experiments were
performed.

In several instances we have found a certain solution
barren of good results when first tried, but after perhaps
several months of additional experience the secret of
success has been discovered, and an apparently worthless

PREFACE «

method found to be a very useful one. Sometimes it was
the right temperature, or the proper strength of the
solution that was needed, but more often the non-success
has resulted from an excess of acid or alkali, as the pre-
ponderance of one or other of these chemical conditions
may altogether alter the result. In some cases it is
essential that the bronzing solution shall be perfectly
neutral in character, as the deposit is dissolved if any acid
or alkali is present in excess.

The term Bronzing, as now understood, may be
applied to the formation of any kind of colour on metals
for artistic effects, but as the word seems hardly appli-
cable to those cases where one metal is deposited upon
another by electrical or other means — as, for example,
the deposition of copper or brass upon iron or zinc — the
term Metal-colouring has been adopted as embracing
every kind and shade of colour which can be produced on
metals by any means whatever.

The essential portion of the work is treated under
three principal divisions, viz. — Chemical Metal-colouring,
Electro-chemical Metal-colouring, and Mechanical Metal-
colouring, the first of which is by far the largest and most
important, while the third, embracing methods which aim
at superimposing something on a metal, without necessarily
uniting it intimately with the metal, forms but an in-
significant part of the whole, and is only included in
order to make the work as complete as possible.

It has been considered advisable to include in the
introductory portion a brief account of the properties of
the ordinary metals and their chemical relations with
regard to such elements as oxygen, sulphur, chlorine, etc.,
as bronzing is essentially a chemical subject and requires

x METAL-COLOURING AttD BRONZING

that the metal-colourer should have at least a knowledge
of the elementary principles on which the science is based.

The first portion deals with the chemical effects of the
atmosphere on metals, the relation of metals to colour,
chemical principles and changes, and the nature and
object of metal-colouring.

The second portion deals with cleaning, dipping,
scratch-brushing, polishing, etc.

The third portion treats of chemical metal-colouring.

The fourth portion is devoted to the subject of Electro-
chemical Metal -colouring, embracing the deposition of
metals on each other.

The fifth portion refers to the subject of Mechanical
Metal-colouring.

The following works among others have been consulted :
— Watt's Dictionary of Chemistry ; Workshop Receipts,
(Spon) ; Die Metallfarhung, by Buchner ; Bronzage des
Metaux et du Platre, by Lagombe; Electro-metallurgy,
by Gore ; Electro-deposition, by Watt ; Electro-deposition,
by Langbein and Brannt; Colour, by Church; Design,
by Dresser.

The author gratefully desires to express his indebtedness
to his colleagues, Messrs. Baker and Stansbie, for their
care and attention in reading the proof-sheets.

Birmingham Municipal Technical School,
September 1892.

CONTENTS

PART I

INTRODUCTORY

Nature of the atmosphere

,, metals ....
Character of colour
Metals in relation to colour .
Effect of mixed metals .
Japanese colouring
Fundamental chemical relations .
Properties of non-metals

,, metals

Nature of alloys ....
Nature and object of metal-colouring
Classification of metal-colouring .
Development of ,,

Metal-colouring in relation to art
Effect of time and temperature on metal-colouring
Influence of physical conditions on ,,

,, different metals in colouring

PAGE

4
5
12
14
15
18
26
31
52
57
60
62
63
66
69
71

Xll

METAL-COLOURING AND BRONZING

PART II

PRELIMINARY TREATMENT OF METALS

PAGE

Cleaning

„ copper and its alloys

Bright dipping

Dead ,, ....

Cleaning zinc ....

,, silver ....

,, iron and steel

„ lead, tin, and their alloys

,, aluminium .

Mechanical treatment .

Scratch -brushing ....

Grinding and polishing

PART III

CHEMICAL METAL-COLOURING

Colouring of copper

Various Shades of Brown on Copper

,, „ >> and graphite

. 100

,, ,, ,, and copper tartrate

. 102

„ ,, the English method

. 103

,, ,, the Chinese „ ...

. 104

,, ,, Langbein's and Dienst's methods .

. 106

,, ,, copper sulphate

. 108

CONTENTS

xin

Colouring copper by various mixtures .
Observations on colouring by means of a paste

PAGE

110
112

Light Brown Colours on Copper

Colouring copper in the wet way

by copper chloride ....
„ ,, and other chlorides

by platinum chloride
by mixed sulphides and chlorides
by copper nitrate ....

and acetate
„ ammonia
„ „ sodium hydrate
Remarks on black bronzes

»i

»»

113
124
127
136
138
140
142
143
144
144

Light to Dark Brown Colours on Copper

Colouring copper by copper sulphate and other salts
,, ,, by Japanese methods ....

146
151

Dark Brown to Black Colours on Copper

Colouring copper by ammonium sulphide

,, potassium „ ...

mercury and other salts
sodium or potassium sulphantimoniate
arsenic salts ....

copper nitrate and potassium sulphide
barium sulphide ....

>»

»>

156
157
159
161
163
163
164

Green Colours on Copper

Production of a green patina on copper and brass
Experiments on public monuments

167
169

xiv

METAL-COLOURING AND BRONZING

Different methods of producing an antique patina

Buchner's antique bronzing solutions

Various solutions for antique bronzes

Bronzing of copper alloys

Brass ......

Different colours on brass
Effect of copper chloride on brass
ferric „ „

„ soluble chlorides and copper salts on brass

PAGE

173
180
182
187
187
190
193
194
197

Black Colours on Brass

Eisner's method

Effect of mixed sulphides and chlorides on brass

various mixtures on brass

mercury salts on brass .
solutions containing arsenic on brass

copper nitrate in colouring brass

mercury and antimony salts on brass

ammoniacal solutions on brass

205

207

208

209
211

211

212

214

Brown Colours on Brass

French method for brass

220

Bronze Barbldienne

222

Effect of barium sulphide on brass

223

Smoke-bronze . . . .

223

Effect of copper sulphate on brass

. 224

, , „ nitrate ,,

227

,, lead acetate ,,

230

, , sodium thiosulphate on brass

t *

231

CONTENTS

Bronzing and Colouring of Zinc

PAGE

Zinc ...........

232

Black colour on zinc

233

Grayish-black colour on zinc

237

Effect of platinum chloride on zinc

240

Brown colour on zinc

240

Gray coating on zinc

241

Different colours on zinc . . .

242

Parcel coppering and bronzing zinc ....

. 243

Bronzing and Colouring of Iron and Steel

Iron ..........

. 245

Gray colour on iron and steel

. 247

Blue „ ,, „

. 248

Various colours „ ,, . .

. 249

Black colour ,, ,,

. 249

Bronze colour ,, ,,

. 252

Colouring of Tin

Tin ...
Colouring of tin .
Crystalline surface on tin

254
255
256

Colouring of Gold

Ormolu colour 257

Colouring of gold 259

Colouring gilt work ....;... 263

Colouring of Silver

Colours on silver generally

263

XVI

METAL-COLOURING AND BRONZING

PAGE

Oxidising processes

. 265

Colouring silver by platinum chloride .

. 266

,, ,, barium sulphide .

. 266

,, ,, potassium sulphide

. 267

,, ,, ammonium sulphide

. 268

,, ,, sulphuretted hydrogen .

. 269

„ ,, means of chlorine .

. 270

, 270

PART IV

ELECTRO-CHEMICAL METAL-COLOURING

Nature of electro-deposition

Deposition by simple immersion .

Coppering by a separate current .

Brassing „ „

Deposition of bronze by a separate current .

nickel and its alloys

German silver

silver by a separate current .

gold on other metals
Recovery of gold and silver from old solutions

Water gilding

Electro-chrome

Coppering of zinc by simple immersion

cast-iron goods ....
various metals ....
Brassing articles by simple immersion .
Coating other metals with tin .

»>

>»

zinc

271
272
275
279
285
286
289
290
293
294
295
296
299
301
304
306
308
312

CONTENTS

xvu

PART V

MECHANICAL METAL-COLOURING

Bronzing with metallic powders and leaf

PAGE

. 314

,, wood, porcelain, etc

. 316

Materials used in bronzing .....

. 316

Bronzing operation ......

. 317

,, of plaster of Paris articles

. 321

Remedies for accidents, etc. .....

. 325

Removal of stains, etc. .....

. 328

APPENDIX

. 329

PART I

INTKODUCTOKY

§ 1. The word bronzing is derived from the Italian
Brormno — a brown colour, and was originally applied to
the browning of metallic articles, but has now come to be
applied to the production of all colours alike.

Bronzing is an ancient art, probably dating back to the
time when the common metals were first discovered by
the human race, and is not confined to any particular
nationality. The love of ornament is inherent in human
nature, and the peculiar adaptability of base metals to
take a kind of beauty when exposed to ordinary atmospheric
influences would very soon attract the attention of a lover
of the beautiful, and excite a desire to imitate by artificial
means the effect produced by the slow continued action of
the atmosphere. Every one is charmed with the magnificent
display of colours produced in a soap-bubble when its
envelope is gradually diminishing in thickness, and in a
similar way the film of oxide, which forms on the surface
of a piece of bright steel when gradually and slowly heated,
calls forth the admiration of the beholder.

2 METAL-COLOURING AND BRONZING part

The change produced in metals, chiefly by the action of
oxygen, would be noted long before any attempt was made
to give an explanation of the fact, and ages before the
grand discovery of oxygen was made by Priestley. It
would further be noted that the air was most effective in
producing a colour on metals when in a moist state, and
almost inoperative when very dry. At any rate it is a well-
known fact at the present time that most metals, when in
the massive state, remain perfectly bright and unacted
on in dry oxygen or air ; but in moist air many of them
become slowly oxidised. The coating of oxide first formed
frequently protects the metal from more than a superficial
oxidation, as is notably the case with lead. On the other
hand, some of the ordinarily permanent metals, when in a
very finely divided state, such as lead for instance, obtained
by the ignition of its tartrate, and iron, as produced by the
ignition of Prussian blue, undergo so violent an oxidation
that spontaneous combustion results from their mere
exposure to air or oxygen. This is in accordance with the
well-known fact that various other bodies, which expose a
large surface to the action of air or oxygen, become gradu-
ally heated through slow oxidation and may finally undergo
spontaneous combustion. Greasy cotton waste and woollen
refuse are peculiarly liable to this change. Many bodies
when heated to redness in air unite with oxygen and give
rise to the phenomenon of combustion or burning.

§ 2. Under certain circumstances, oxygen gas acquires

a greatly increased chemical activity, together with a

peculiar smell. This is observed in the atmosphere during

1 a thunderstorm, or in the vicinity of a machine giving off

j electric sparks. This modification of oxygen is termed

i ozone, and like ordinary oxygen is most active as an

INTRODUCTORY S

oxidising agent when in a moist state. The oxygen
evolved by the action of light on plants is largely in the
form of ozone. When fresh slices of apple are exposed to
the air, ozone is produced ; and the same occurs in the
processes of fermentation, putrefaction, and decay. The
general characters of ozone are those of an oxidising agent.
Thus it corrodes organic matter, such as corks and india-
rubber ; it bleaches most vegetable colours ; it oxidises
many metallic sulphides to sulphates; changes lower
oxides into the higher forms ; and not only oxidises,
when in a moist state, iron, copper, brass, and bronze,
but also silver and mercury with the production of
their various oxides. Moist silver is even converted
into the state of peroxide. Even dry ozone is absorbed
by dry mercury.

The air is a mixture of various gases, viz. nitrogen,
oxygen, water vapour, carbonic acid, with minute quantities
of ozone, hydroxyl, ammonia, nitrous and nitric acids and
hydrocarbons. These gases are not present in any constant
ratio, but vary with circumstances. Thus a room with
living beings will cause a difference in composition from
that of the open air. Or the air of large towns differs from
that of the country districts and the sea-coast. Air over
marshes contains little or no ozone, and the same remark
applies to the air of large towns and inhabited houses. It
appears also that the amount of ozone varies with the
seasons : it is greatest in spring and least in winter.
Ozone is more frequently observed on rainy days than in
fine weather; thunderstorms, gales, and hurricanes are
generally accompanied by relatively strong manifestations
of it. It is highly probable that many so-called ozone
manifestations are due to hydroxyl, which is probably in

4 METAL-COLOURING AND BRONZING part

greater quantity in the air than ozone, and possesses similar
oxidising powers.

§ 3. The quantity of aqueous vapour in the air varies
with the temperature, being greater at warm temperatures
than when the air is near the freezing point. The air is
seldom saturated with moisture, although in our moist
climate saturation is occasionally very nearly attained.

The presence of carbonic acid in air is easily proved,
and the quantity is greater in large towns than in the
country, as any circumstance which interferes with the
ready diffusion of the products of respiration and the com-
bustion of fuel will of course tend to increase the relative
amount of carbonic acid in the air of a town ; hence during
fogs the amount may become relatively very great.

Minute quantities of ammonia and nitrous and nitric
acids, as already mentioned, are present in the air. The
amount of ammonia which exists mainly as a carbonate
varies considerably, but is greatest in towns, owing to the
influence of animal life and the constant presence of
decaying organic matter in the air. The amount of
nitrous and nitric acids is still less than that of ammonia.

Sulphuretted hydrogen is always present in the atmos-
phere where coal-gas is burnt for heating purposes or for
illumination.

All the constituents of atmospheric air, except free
nitrogen, have an influence on metals, modifying their
colour, and more or less forming compounds with the
surface exposed to them. The films of colour thus pro-
duced may be due to oxides, carbonates, sulphides, nitrites,
nitrates, and possibly hydrocarbonaceous deposits.

§ 4. Metals may, for industrial purposes, be divided
into two classes : — Those which are fundamental and

INTRODUCTORY

contain only one kind of matter, and those which con-
tain two or more fundamental metals. The fundamental
metals, with one or two exceptions, have peculiar tints by
which they are distinguished from each other. Thus gold
is yellow, copper is red, silver is white, bismuth is reddish-
white, lead is bluish-white, etc. But when metals are
alloyed together an almost infinite variety of tints may be
produced, but always of a subdued tone and quite peculiar
to the metallic character.

Hence metals form a class of bodies different from
every other form of matter and require special treatment
when used for decorative purposes, either in their natural
colours or when operated upon so as to produce on the
surface the effects produced by the atmosphere or other
corroding agents. Nature seems therefore to teach that
variety of colour in metals is a most desirable thing,
and that it is a laudable action to exalt and intensify
the natural chromatic effects, always provided that the
colours aimed at are strictly in keeping with the metallic
character.

We know that form may exist without colour, but it
seldom has any important development without the chro-
matic adjunct, and history teaches that form alone is in-
capable of satisfying the instincts of the human mind, for
no system of national decoration has ever. existed in the
absence of colour. Dr. Dresser says, "Mere form, mere
light and shade, is pleasing to a certain extent, but it falls
far short of what we require, and it is only when we get
perfectly proportioned forms in combination with colours
harmoniously arranged that we are satisfied." Colour may
be applied to almost every inanimate object, and many
articles which are now colourless might be coloured with

6 METAL-COLOURING AND BRONZING part

advantage. Colour lends to bodies a new charm ; it assists
in separating parts of an object and thus gives assistance
to form, elevating the parts which require prominence, and
throwing back the parts which need to be in shadow.

§ 5. It is a common idea that colour is inherent in a
body which retains it under all circumstances and condi-
tions. This is not so, for the colour of any object depends
entirely on its power of retaining or rejecting certain of the
constituent colours of white light. All coloured bodies
possess the power of absorbing, or in other words, stopping
some of the rays which fall upon them, and reflecting the
others. For example, a violet body absorbs all the rays
that fall upon it, except the violet, which it reflects ; a
green body absorbs all but the green, which it reflects, and
so on. A body which absorbs all the rays that fall on it
is black ; one which reflects equally all the visible rays is
white. A body which absorbs all the rays of light partially
but equally is gray. It should be remembered that all
bodies, including coloured ones, reflect white light from
their immediate surfaces ; it is only the light which pene-
trates bodies that is, so to speak, sifted and in part reflected
so as to give rise to colour.

White light is composed of seven distinct colours, viz.
violet, indigo, blue, green, yellow, orange, and red, as may
be readily proved by looking at the light through a tri-
angular shaped piece of white glass termed a prism, which
has the power of decomposing the white light transmitted
through it into its constituent colours. Although white
light is composed of the seven primary colours it is not
essential that all these colours should combine to form
white light. For example, blue and yellow lights when
mingled produce white light.

INTRODUCTORY

Artists generally recognise only three primary colours,
viz. blue, red, and yellow, since they cannot be formed by
the admixture of other colours. All other colours except
these three may be produced by admixture of these three
primary colours. But we must here carefully distinguish
between the effects produced by mixing coloured lights
and mixing coloured pigments. As already stated, blue
and yellow lights produce white light, but a blue and
yellow pigment produces a green pigment, because in the
latter case the incident light suffers two absorptions — one
due to the blue pigment, and the other to the yellow. It
is the light which escapes the absorption of both which
gives its colour to the mixture. If the blue and yellow
pigments used were pure colours — that is, reflected a
colour that a prism could not decompose — then their
mechanical mixture would be black or gray, for between
the two pigments the whole of the light would be
absorbed.

But pigments never are pure colours, and the practical
effect in the above example is that the blue and yellow
both conspire to absorb the red of white light, the yellow
absorbs the blue, and the blue the yellow, so that the only
part of white light reflected is the green, because neither
of the pigments can absorb it. The colours produced by
the admixture of two primary colours are termed secondary
colours ; the admixture of two secondary colours is called
a tertiary colour and so on.

When a light colour is contiguous to a dark colour, the
light colour appears lighter than it is, and the dark colour
darker ; and different colours juxtaposed become modified
as to their hue. Thus when red and green are placed side
by side, the red appears redder than it actually is, and the

8 METAL-COLOURING AND BRONZING t»ART

green greener. When blue and black are in proximity,
the blue manifests but little alteration, while the black
assumes an orange tint. Any two coloured lights which
when mixed produce white light are termed comple-
mentary; the following pairs of spectrum colours are
complementary : —

Red and Greenish-blue,
Orange and Prussian Blue,
Yellow and Indigo Blue,
Greenish-yellow and Violet.

No colour can be persistently viewed by the eye with-
out another being observed. Thus if red be looked at,
the eye soon perceives a greenish-blue, and this green is
cast on whatever is near. These are termed subjective
colours. If the eye has been subject for some time to a
brilliant white light and then passes into a room only
moderately lighted, the room appears dark. Most persons
have experienced the effect of passing from a place illum-
inated by the electric light to one where a sickly-looking
gas-flame is burning. In the case of the complementary
colours, red and green, above mentioned, the eye having
been exposed to red light for a time becomes less sensitive
to that colour, and on looking at a white body, such as a
sheet of paper, its red constituent is virtually absent, and
the complementary colour green appears.

In the same way, if we view a blue, the complementary
colour orange or yellow respectively appears ; and so we
may say that the eye demands white light because that
is its natural illuminant, and if only a portion of white
light is presented, the eye creates for itself the rest. In
the same way coloured shadows are produced. If a strong
red light fall on a white screen, and an opaque body be

INTRODUCTORY 9

interposed between the source of light and it, the shadow, as
seen when moderately illuminated by a white light, will be
green. If the original light be blue, the shadow will be
red. All these properties of colour are termed subjective,
because they do not express external facts but depend on
the condition of the eye.

§ 6. Another cause of colour is what is known as inter-
ference. If a stone be thrown into water, a circular wave
is produced, and if a second stone be thrown into the
same water, a second wave will be produced, which will
influence the first. If the two stones fall into the water
at precisely the same time and place, the two waves will
beat in unison, and the amount of disturbance in the water
will be double what it would have been if only one stone
had been thrown in. If the second stone had been thrown
into the water exactly a wave-length behind the first, the
same thing would happen, for the first crest of the second
wave would coincide in point of time with the second crest
of the first wave and produce perfect unison. But if the
second stone were thrown into the water exactly half a
wave-length behind the first, the crest of one wave would
be opposed by the depression of the other; and if the
height of each wave be the same, the motion of the water
will be destroyed.

The same action occurs in light which is propagated by
wave-motion in a similar way to that in water. Take a
soap-bubble for illustration. When light impinges on the
soap film, part of it is reflected from the exterior surface,
and part enters the film and is reflected from the interior
surface. This latter portion has to traverse the thickness
of the film twice, and is therefore retarded with respect
to the former, so that the two sets of waves interfere with

10 METAL-COLOURING AND BRONZING part

each other and produce a coloured light in the place of
white light. If two waves are equal and there is exactly
half a wave-length difference between them, they will
destroy each other, and therefore extinguish the light.

But white light is comppsed of several colours, each of
which has a different wave-length, so that if the white
light which enters a soap-bubble is resolved into its con-
stituent colours, some may so interfere as to destroy each
other, but some will remain unaffected, the longer waves
requiring a greater thickness of film than the shorter to
produce their extinction ; hence the alteration in the
colours of a soap-bubble as its film gradually alters in
thickness. In this way we may account for the iridescence
of oil upon water, of steel when being tempered, the hues
of some insects' wings, and the colours of thin plates or films
generally.

When light passes from one transparent medium to
another it is bent or refracted, except at certain angles
when it may be totally reflected. If a surface contains a
number of fine lines very close together, the light reflected
from the different portions will be in interference, and
variegated colours will result. In this way it is supposed
that the magnificent chromatic display often observed in
mother-of-pearl is due to reflection from its striated surface.
The colouring of metals and many other objects may often
be traced to the same cause.

§ 7. When contrasted musical notes give a pleasing
sound we call the result harmony. In the same way there
are certain contrasted colours which produce an agreeable im-
pression on the eye, and these may also be said to harmonise.
Those colours which most perfectly harmonise improve one
another to the utmost, and this is effected by a combina-

INTRODUCTORY 11

tion of the three primary colours, red, blue, and yellow,
either pure or in combination with each other. Eed and
green harmonise, because green is a secondary colour con-
sisting of blue and yellow. Blue and orange, yellow and
purple, respectively are in harmony, for in each case the
three primary colours are present. On the other hand,
there are combinations of colour which are perfectly
hideous, producing the completest discord, although it is
true that the rarest harmonies frequently lie close on the
verge of discord.

It has been already stated that colours are modified in
tone by the proximity of other different colours, so that
the nature of the light by which a coloured body is viewed
will influence the effect on the retina of the eye. For
example, by ordinary gaslight blue becomes darker, red
brighter, and yellow lighter. By this artificial light a
pure yellow appears lighter than white itself, when
viewed in contrast with certain other colours. In this
way highly polished brass is often mistaken for silver. At
twilight blue appears much lighter than it is, red much
darker, and yellow slightly darker.

Colours also have the power of influencing the beholder
in various ways ; thus blue looks cold and appears to re-
cede from the eye. Eed is a warm colour and exciting ;
it remains stationary as to distance. Yellow is the colour
most nearly allied to white light ; it appears to advance
to the spectator. By certain combinations colour may be
made to cheer or sadden ; to convey the notion of purity,
richness, or poverty, and in fact affect the mind in much
the same way as does music.

The following principles with regard to colour in design
are given by Dr. Dresser : — " (1) When a colour is placed

12 METAL-COLOURING AND BRONZING part

on a gold ground it should be outlined by a darker shade
of its own colour. (2) When a gold ornament falls on a
coloured ground, it should be outlined with black. (3)
When an ornament falls on a ground which is in direct
harmony with it, it must be outlined with a lighter tint of
its own colour. Thus, when a red ornament falls on a
green ground, the ornament must be outlined with a lighter
red. (4) When the ornament and the ground are in two
tints of the same colour, if the ornament is darker than
the ground, it will require outlining with a still darker
tint of the same colour, but if lighter than the ground, no
outline will be required." There are three colours which
are neutral with regard to colour, viz. gold, black, and
white, and these may be employed to separate colours
where such separation is necessary or desirable.

§ 8. It has already been said that metals form a class
of bodies quite peculiar to themselves, and differ from
every other known substance. They therefore should have
such special treatment in any system of colouring as
will enable the metallic character still to be evident, or at
least to be suggested. Anything which completely hides
the metallic nature and represents it as something which
it is not, may be very ingenious and display a considerable
amount of manipulative skill, but it is a spurious form of
art. In my opinion nothing should be added to any form
of metal for the purposes of metal-colouring except what is
capable of becoming a part of itself. For this reason
paint in any form is not admissible, as it forms a layer of
matter, non-metallic in character, simply superposed on
the metal and not in any sense uniting with it to form a
compound effect.

In the same category are lacquers and varnishes of all

INTRODUCTORY 18

kinds, however beautifully they may be coloured. It is
true that lacquers and varnishes are very useful and even
necessary to preserve many metals from the corrosive
action of the atmosphere and other agencies to which they
are subjected, but they should only be used when absol-
utely necessary, and then only the uncoloured and trans-
parent variety should be employed.

What a disappointment it is to see some really meri-
torious brass or bronze work highly lacquered so as to
shine like a French-polished piece of furniture instead of
having the subdued lustre peculiar to the metallic char-
acter. It is a grand and beautiful thing to imitate
beautiful things, but it is degrading to true taste to make
a metal look like wood or glass and vice versa. Let us
beautify metals so as to present as great a variety of charms
to the beholder as possible, but never make the metal to
lose all semblance of its real nature.

It is a good sign that at the present time the national
taste is setting in in the direction of plain metal work.
Brass is finished as brass, iron as iron, copper as copper,
showing the repugnance felt by a people advancing in
artistic taste to the shams and paints and varnishes which a
past generation endured. One occasionally sees in private
and public rooms metal picture rails painted in various
tints to match the wall decoration or the furniture, with
nothing whatever to distinguish between the material of
which the rail is composed and that of the plaster wall.
This is utterly wrong in principle ; it may be called har-
monious blending of colours, tasteful decoration, and so
on but it is a perversion of true art and should never be
encouraged. Have we not metals of various shades which
lend themselves more perfectly to decoration than any

14 METAL-COLOURING AND BRONZING part

paint superimposed upon them can do? Moreover, we
have yellow gold, and white tin and silver, which, being
quite neutral as to colour, will always lend themselves
perfectly to metallic decoration.

It is truly marvellous to see what effects may be pro-
duced by certain pickling solutions on impure metals,
compared with similar effects on the same metal when in
a pure state, and this is often true when the foreign
matter is present in such minute quantity as to seem to a
non-scientific person infinitesimal and unworthy of con-
sideration. Yet a minute quantity of one metal can
change the whole of another metal in which it is hidden,
causing it to behave in a different way in relation to light,
and consequently to possess a colour different to that
which it would have when pure. That various shades of
colour may be produced in metals by alloying them
together in various proportions is well known. Take, for
example, a mass of red copper and an equal weight of
gray antimony ; the union of the two by fusion produces
a beautiful violet alloy. The same colour may be obtained
by depositing copper on an article, such as a brass ash-tray,
and then immersing the same for a few seconds in a solu-
tion containing antimony. The thin film of antimony
thus deposited gives to the surface the well-known violet
tint characteristic of the above-mentioned alloy, termed
Regulus of Venus. This alloy is too brittle to work, but
the colour may be imparted to base metals in the manner
indicated above.

The colouring power of metals in alloys is very vari-
able, as shown by the following table by Ledebur : —

I INTRODUCTORY

Tin

Zinc

•

Nickel

Lead

Aluminium

Platinum

Manganese

Silver

Iron

Gold

Copper

• • •

Each metal in the above series has a greater decolour-
ising action than the metal following it. Thus, the colour
of the last members is concealed by comparatively small
amounts of the first.

That tin and nickel readily whiten copper in gun metal
and nickel-silver respectively is well known. On the
other hand, gray zinc requires to be in considerable excess
in brass before the yellow tint characteristic of that alloy
is completely destroyed.

§ 9. The Japanese, however, are the real authorities to
whom we must turn for examples of the highest kind of
coloration in metals. They combine the most perfect
artistic taste with marvellous manipulative skill in execut-
ing any design their imagination may suggest. In certain
branches they are inimitable, and as colourists they are
almost perfect. Their commonest works generally contain
a bit of good colouring, and their best efforts are marvels
of harmony. Their colours are usually warm, simple, and
quiet, but always effective. Even the bloom on the surface
of fruit is very faithfully reproduced by them. These
facts are all the more remarkable when we consider how
impure their ordinary commercial metals are compared
with ours, and how infinitely we excel the Japanese in the
knowledge of metals from a scientific standpoint.

It is true that impure metals are sometimes better adapted
for producing varieties of colour on metals by means of

16 METAL-COLOURING AND BRONZING part

bronzing solutions ; but we, by means of our great army of
analysts and scientific men, can ascertain the exact com-
position of the alloys they use, and with our richer stores
of mineral wealth and metallurgical knowledge can pro-
duce any variation we choose in the composition of alloys,
as every known metal, if not existing in the British Isles,
is procurable by us. If complexity of composition is the
secret of success, we have the knowledge and the skill to
make such complexes.

But the same cannot be said of our art knowledge. It
is true there are individual Englishmen who are the equals
of the greatest artists of any other nation ; but there is
not among us the same desire to cultivate that artistic
taste generally which seems to pervade every section of
the community in Japan, high and low, rich and poor,
and with regard to art metal work we are vastly inferior
as a nation. But if we cannot equal them we can strive
towards greater perfection, and for the present be content
to imitate the admirable skill attained by this truly artistic
people.

One sad feature with regard to the closer European con-
tact with Japan is said to be a deterioration of Japanese art
work to a lamentable extent. Contact with Europeans, and
especially with Englishmen, causes a much greater demand
for Japanese wares, so that quantity is being produced, it
is feared, at the expense of quality. We cavil about
prices, and by thus creating a demand for inferior work we
raise the price even of that which is comparatively bad,
and soon we shall have to pay for inferior wares a price
for which superior articles could formerly be obtained.

Happily at present nothing from Japan and India, of
native workmanship, is utterly bad. Inharmonious colour-

INTRODUCTOKY 17

ing does not appear to be produced by these nations. Sir
Frederick Leighton, in his Address to the Art Congress at
Liverpool, said : " Our complaint with respect to the great
majority of Englishmen is that the appreciation of art is
blunt, superficial, desultory, and spasmodic ; that our
countrymen have no adequate perception of the place of
art as an element of national greatness ; that they do not
count its achievements among the grounds of their
national pride ; that they do not appreciate its vital im-
portance in the present day to certain branches of national
prosperity; that while what is excellent receives from
them honour and recognition, what is ignoble and hideous
is not detested by them ; that the aesthetic consciousness
is not with them a living force, impelling them towards
the beautiful and rebelling against the unsightly."

FUNDAMENTAL CHEMICAL KELATIONS

§ 10. Before proceeding with the description of the
production of the various colours produced on bodies by
different bronzes, it will be necessary to give a general idea
of the physical and chemical properties of the metals, and
to explain the nature of chemical change which plays such
an important part in bronzing.

With about seventy exceptions all known substances,
by various chemical processes, may be decomposed, and
are therefore termed chemical compounds ; while the
seventy known bodies which have never been resolved into
simpler parts are termed chemical elements. Of the latter
more than three-fourths possess metallic properties, and are
distinguished by the name of metals t in contradistinction
to the remainder which are termed non-metals.

The chemical elements possess almost every shade of
physical character, and if we include the infinite variety of
metals which can be produced by the process of alloying,
the significance of this statement is still more marked.
Among elementary substances we have oxygen, nitrogen,
hydrogen, chlorine, and fluorine, which are gases under all
ordinary conditions. Bromine and mercury are liquids.

With regard to the solids it may be stated that some are
readily volatile, such as iodine and arsenic, some easily
fusible, such as phosphorus, and some which require the

part I CHEMICAL ELEMENTS 19

very highest temperatures to effect their fusion, such as
platinum. Hydrogen is the lightest and osmium the
heaviest of all known bodies. Many elements occur in
nature in the free state, such as oxygen, nitrogen, sulphur,
gold, silver, copper, etc.; but by far the larger portion
only exist in combination with other bodies, and those
which occur in the free state, with three or four exceptions,
only do so in comparatively small quantities.

The elements are very unequally distributed throughout
nature, some being exceedingly abundant, others only
occurring in the minutest quantities in any one place.
One -half the solid crust of the globe, eight -ninths of
water, and one-fifth of the atmosphere is the element
oxygen. Moreover, many other elements are most fre-
quently found in combination with oxygen, so that this
element may be considered as the cement by which the
elementary parts of the world are held together. Of the
known elements, thirteen alone make up ^^ of the whole
mass of the earth.

The ultimate particles or " atoms " which compose any
element differ in weight from the atoms of any other
element, and the relative weight compared with hydrogen
is termed the "atomic weight." The atoms are usually
represented by symbols. The following table gives the
chemical elements with their symbols, atomic weights,
and specific gravities : —

METAL-COLOURING AND BRONZING

PART

Metals.

Names.

Symbols.

Atomic
Weights.

Specific

Gravity.

Aluminium ....

2-56

Antimony

120

6-72

Arsenic

5-67

Barium

137

375

Bismuth

207-5

9-8

Cadmium

112

8-6

Caesium

133

• • •

Calcium

1-58

Cerium .

141

6-68

Chromium

52 4

6-8

Cobalt .

58-6

8-5

Copper .

63*2

8-8

Didymium

145

6-5

Erbium

112-6

• • •

Glucinum

Gold .

196*2

19-32

Indium

113-4

7-4

Iridium

192*5

22-4

Iron

7'86

Lanthanium .

138-5

6*2

Lead

206*4

11-37

Lithium

•58

Magnesium

1-74

Manganese

Mercury

200

13*59

Molybdenum

8-6

Nickel .

58*6

8*8

Niobium

6 27

Osmium

195

22-48

Palladium

106*2

11-5

Platinum

194-3

21-5

Potassium

39*1

•87

Rhodium

104

12-1

Rubidium

85-2

1-52

Ruthenium

103-5

12*26

Silver .

107-6

10-5

Sodium

•97

CHEMICAL ELEMENTS

Metals — Continued.

Names.

Symbols.

Atomic
Weights.

Specific
Gravity.

Strontium ....

87-2

2 54

Tantalum

182

10*78

Thallium

203*5

11*9

Thorium

232

11-1

Tin

117-4

7'3

Titanium

• • •

Tungsten

184

19-1

Uranium

240

18-7

Vanadium

5-5

Yttrium

617

• • •

Zinc

65*2

7-15

Zirconium

90*4

4*15

Non-Metala

Names.

Symbols.

Atomic
Weights.

Specific
Gravity.

Boron

2'G8

Bromine

2 96

( Graphite

• • •

2*2

Carbon -|

• • •

[ Diamond .

• • •

3 5

Chlorine

35-5

• • •

Fluorine

• • •

Hydrogen

• • •

Iodine .

127

4-95

Nitrogen

• • •

Oxygen .

• ••

Phosphorus ,

1-8-2*1

Selenium

79*5

4-28-4-8

Silicon .

28-1

2*49

Sulphur

1-97-2-07

Tellurium

126*3

6-25

22 METAL-COLOURING AND BRONZING part

§ 11. Molecules. — Every body, whatever its substance
may be, is formed by the assemblage of minute particles of
the same kind, which cannot be further subdivided without
destroying the identity of the substance. Thus a lump of
sugar is an aggregate of minute particles of sugar, and if the
sugar be burnt, these particles will be further subdivided ;
but the sugar will be changed into new substances. The
smallest particles of any substance which can exist by
themselves we call Molecules. So long as the identity of
a substance is preserved its molecules remain undivided ;
but when, by some chemical change, its identity is lost
and new substances are formed, the molecules are broken
up into still smaller particles which are termed atoms.
Indeed, this division of the molecules is the very essence
of a chemical change, and the atom may be defined as
the smallest mass of an element that can exist in any
molecule.

§ 12. Chemical Nomenclature. — There are several
systems in use of naming chemical compounds, but no
one of these is rigidly followed. Previous to 1787 no
general rule was followed, and even now many of the old
empirical names are more extensively used than the more
modern ones. Thus we still speak of oil of vitriol, calomel,
corrosive sublimate, saltpetre, common salt, borax, cream of
tartar, Epsom salts, etc. The simplest of the nomencla-
tures^ — for compounds containing two elements — is that
of writing the name of the metal first and the non-metal
or least metallic element last, giving it the termination
ide. When two non-metals unite the one which is least
like a metal is written last.

In addition to this, Greek prefixes are in common use,
such as mono, di, tri, tetr, etc., to indicate the number of

CHEMICAL NOMENCLATURE

atoms present. Thus, carbon dioxide, C0 2 , carbon
monoxide, CO. Another system consists of making the
metal terminate in ic or ous. That compound which con-
tains the greater proportion of the non-metallic constitu-
ent is distinguished by the suffix ic, and that containing
the smaller is ous. The following list will illustrate these
points : —

Name.

Iron oxide

Ferrous oxide

Iron protoxide

FeO

Iron trioxide

Ferric oxide

Iron sesquioxide

Fe^O,

Iron tetroxide

Triferric \
tetroxide J

Black oxide 1
of iron j

Fe,0 4

Manganese oxide

Manganous oxide

Manganese |
protoxide

MnO

Manganese )
dioxide (

Manganic oxide

Manganese
peroxide J

Mn0 3

Aluminium oxide

Aluminic oxide

Alumina

A1 2 3

Calcium oxide

Calcic oxide

Lime

CaO

Magnesium oxide

Magnesic oxide

Magnesia

MgO

Titanium dioxide

Titanic oxide

Titanic acid

Ti0 2

Carbon monoxide

Carbonic oxide

...

Carbon dioxide

• • •

Carbonic acid

C0 2

Silicon dioxide

Silicic oxide

Silica

SiO a

Phosphorus \
pentoxide J

Phosphoric oxide

Phosphoric acid

P 2 8

Sulphur dioxide

Sulphurous oxide

Sulphurous acid

S0 2

Sulphur trioxide

Sulphuric oxide

Sulphuric acid

S0 8

When three or more elements — one being a metal and
another oxygen — are combined together, the name of the
second is made to end in " ate." In the following list a
few compounds are given to illustrate this, but it should be
observed that the order of placing the symbols is imma-
terial: —

METAL-COLOURING AND BRONZING

PART

Name.

Formulae.

Lead acetate

Iron sulphate
Calcium carbonate
Silver nitrate
Potassium chlorate

Plumbic acetate

Ferrous sulphate
Carbonate of lime
Argentic nitrate
Potassic chlorate

J Pb(CaHs02) or
1 Pb0 2 (C2H 8 0) 2

FeS0 4 or FeO.S0 3

CaCOjor CaO.CO a

AgNO,

KC10 8

§ 13. Equations. — When two or more elements unite
to form a compound, or two compounds unite to form a
more complex compound, the change may be represented
by a chemical equation, thus —

Copper.
68*2

CuO

Cupric oxide.

Oxygen.
16

+ C0 2

Carbonic acid.

CuO

Cupric oxide.
79*2

CuC0 8
Copper carbonate.

Any chemical change may be so represented, the bodies
taking part in the change being placed on the left-hand
side of the sign of equality " = ," and the bodies formed
after the change on the right-hand side.

The quantities involved in any such change can be seen
at a glance, since the symbol of an element represents a
definite weight, given in the table, p. 20, as the atomic
weight. Thus, in the first equation 63*2 parts of copper
unite with 16 parts of oxygen to form 79*2 parts of cupric
oxide.

When a chemical change occurs, the production or ab-
sorption of heat is the result, the former by the union of
elements or compounds, the latter by the forcible separation
of the constituents of a compound. When the increase in

CHEMICAL AGENTS

temperature produced is considerable, it is usual to speak
of the change as combustion ; thus, carbon combines with
oxygen to produce carbonic acid and generates intense heat.

Carbon.

20
Oxygen.

CO a
Carbonic acid.

In common language it is said to burn, and the burning
of fuel is simply the result of chemical combination.

§ 14. When substances combine with oxygen they are
said to be " oxidised," and the substance which imparts
the oxygen is termed an oxidising agent Conversely,
substances which remove oxygen from a body are termed
reducing agents. The following lists give some examples
of both kinds : —

Oxidising Agents.

Seducing Agents,

Oxygen (0).

Carbon (C).

Air (0 and N).

Carbonic oxide (CO).

Iron tetroxide (Feg0 4 ).

Hydrogen (H).

Manganese dioxide MnOj.

Compounds of carbon and

Slags containing the above

hydrogen, such as coal-gas.

or similar oxides.

All fuels.

Carbonic acid (C0 2 ).

Sometimes metals.

Water (H a 0).

Examples of Oxidation.

3PbO +

Pb 3 4

Lead oxide. Oxygen.

Bed lead.

3FeC0 8 +

Feg04 + 3C0,

Iron carbonate. Oxygen.

Iron tetroxide. Carbonic acid.

3FeO + C0 2 =

Fe 8 4 + CO

Iron oxide. Carbonic acid.

Iron tetroxide. Carbonic oxide.

Examples of Reduction.

2PbO + C

C0 2 + Pb 2

Lead oxide. Carbon.

Carbonic acid. Lead.

26 METAL- COLOURING AND BRONZING pakt

Examples of Reduction. — Continued.

FeaOj

+ 6H 2Fe

+ 3H 2

Ferric oxide.

Hydrogen. Iron.

Water.

ZnO

+ CO = Zn

+ C0 2

Zinc oxide.

Carbonic oxide. Zinc.

Carbonic acid.

Fe 2 8

+ CO 2FeO

+ C0 2

Ferric oxide.

Carbonic oxide. Ferrous oxide.

Carbonic acid.

PbS

+ Fe = Pb

+ FeS

Lead sulphide.

Iron. Lead.

Ferrous sulphide.

It will be obvious that in the cases of oxidation and
reduction the change may be partial or complete. Thus,
iron is completely oxidised when converted into Fe 2 3 , and
oxide of iron is completely reduced when all the iron is
brought to the metallic state.

The following is a short description of the chief sub-
stances mentioned in the present part, and which are
subsequently alluded to : —

§ 15. Oxygen (O). — This is the most abundant element,
forming probably one-half the solid crust of the earth, eight-
ninths of all water, and about 21 per cent by volume of the
air. It is necessary for life and all ordinary processes of
combustion. In the air it is a gas, but its compounds are
chiefly solid or liquid. Its oxidising action has been al-
ready mentioned. It is the chief supporter of combustion,
that is, it forms the active medium in which bodies burn.

Oxides, as the compounds of oxygen with other elements
are termed, may be roughly divided into two groups. 1°.
Those which have an acid character, chiefly oxides of the
non-metals, and often termed acids, such as carbonic acid
C0 2 and silica Si0 2 . 2°. Those of a basic character,
chiefly oxides of the metals, which are termed bases.
These two classes are opposite in character, and when
united in equivalent proportions, generally neutralise each

NITROGEN 27

other, forming what are termed " neutral " bodies, which
do not possess the characteristic properties of either kind.
Thus silica Si0 2 will neutralise oxide of iron FeO, forming
a silicate, which is neither acid nor basic. If any compound
contain an excess of acid or base, it is classified either
as an acid or as a basic substance, according to the kind
which predominates. Thus, 3FeO.Si0 2 is a basic silicate,
and FeO.Si0 2 an acid silicate, because in the former there
is more FeO than is required to neutralise the acid Si0 2 ,
and in the latter less than is necessary for this purpose.

§ 16. Hydrogen (H) is chiefly found in nature in com-
bination with oxygen, forming water H 2 0, which contains
one-ninth its weight of hydrogen. It differs from other non-
metals in not generally uniting with metals to form com-
pounds, but metals such as palladium and iron absorb it in
large quantities, when it is said to be occluded. It burns
in air or in pure oxygen, forming water, and evolving
great heat- 2H + =H 3 0.

It is a constituent of wood, peat, coal, and coal-gas, part
of it probably existing in these bodies as water ; and in
combination with carbon it forms what are termed hydro-
carbons, such as marsh -gas CH 4 , and defiant -gas C 2 H 4 .
When the latter are burnt the hydrogen forms water, thus —

CH4

CO a +

2H 2

Marsh-gas.

Oxygen.

Carbonic acid.

Water.

C2H4

2C0 2 +

2H 2

Olefiant-gas.

Oxygen.

Carbonic acid.

Water.

On account of the readiness of hydrogen to unite with
oxygen it is used as a reducing agent and thus removes
the oxygen from a compound containing it.

§ 17. Nitrogen (N) forms about 79 per cent by volume

28 METAL-COLOURING AND BRONZING part

of the air, its chief function being to modify the active
properties of oxygen. It neither burns nor supports com-
bustion, so that the nitrogen which enters a furnace, for
the most part, comes out unchanged, thus robbing it of a
large amount of heat, without contributing any itself.

Air is chiefly a mixture of oxygen and nitrogen along
with small quantities of water and carbonic acid. Omit-
ting the latter, its composition may roughly be taken as —

By Volume.

By Weight.

100 100

A ton of air thus contains about 515 lbs. of oxygen. Air
resembles oxygen in its properties, but is less active on
account of the inactive nitrogen.

§ 18. Silicon (Si). — This non-metal is a grayish-black
substance. It is generally present in iron and a few other
metals, and supposed to exist, like carbon, in the " free " and
in the " combined " state. It is of little importance as an
element, but in combination it forms about one-fourth of the
earth's crust. It burns in oxygen, forming silica, thus —

Si + 20 Si0 2

Silicon. Oxygen. Silica.

Silica (Si0 2 ) plays a prominent part in the reduction
of metals from their ores, being the chief slag-forming
substance. It exists largely as sand, and in combination
with "bases" it forms silicates. The various slags are
chiefly combinations of Si0 2 with alumina A1 2 3 , lime
CaO, and other metallic oxides which fuse at high tern
peratures. Uncombined silica is practically infusible.

§ 19. Carbon (C). — This non-metal is an essential con-

CABBONIC ACID 29

stituent of all living matter, and of all ordinary fuels, such
as coal. It exists in the free state as the diamond, and as
graphite or black-lead. In the latter form it is used in
the manufacture of crucibles, etc., because of its inf visi-
bility, and its non-tendency to form fusible slags with acid
or basic substances. It will burn away in contact with
air, but will not melt or vaporise. It exists in pig-iron
and steel in the free and in the combined state. Part of
the free carbon of pig-iron sometimes rises to the surface
of the molten mass when allowed to stand, and is known
as "kish." Charcoal and coke are almost entirely com-
posed of carbon, with a little earthy matter, which is left
as ash when the carbon is burnt. Either form of carbon
will burn in oxygen, forming oxides. When carbon is
strongly heated in the presence of steam the latter is
decomposed and the carbon oxidised, thus —

+ H 2 = H a + CO

Carbon. Water. Hydrogen. Carbonic oxide.

30 + 2H 2 = CH 4 + 2CO

Carbon. Water. Marsh-gas. Carbonic oxide.

Carbon dioxide or Carbonic .acid (C0 2 ) is a gas about
1£ times the weight of air, and is formed when carbon is
burned in oxygen or in a free supply of air, thus —

C + 20 = C0 9

Carbon. Oxygen. Carbonic acid.

Also when carbonic oxide is burned in air or oxygen,
thus — CO + = CO a

Carbonic oxide. Oxygen. Carbonic acid.

If carbon dioxide is brought in contact with red-hot carbon,
it takes up some of the latter, forming twice its own volume
of carbonic oxide (CO), thus —

COa + C 2C0

Carbonic acid. Carbon. Carbonic oxide.

SO METAL-COLOURING AND BRONZING part

In this case carbonic acid is oxidising. C0 2 is not poison-
ous, but it will not support life or ordinary combustion.

Carbon monoxide or Carbonic oxide (CO) is a colour-
less gas, about the same weight as air, extremely poisonous,
and burns in air or oxygen with a blue flame producing
carbonic acid, and evolving considerable heat. The com-
bustible gas formed in gas-producers is chiefly carbonic
oxide. It is a powerful reducing agent, probably the chief
agent in reducing oxide of iron in the blast-furnace, and
zinc oxide in zinc muffles. At high temperatures CO is
decomposed, especially in the presence of other bodies,
such as iron, which combine with carbon.

2CO = C + CCfo

Carbonic oxide. Carbon. Carbonic acid.

This is probably the case in the blast-furnace, and in the
cementation process for steel.

§ 20. Phosphorus (P). — This non-metal is generally a
waxy-looking crystalline solid, which readily melts and
vaporises. It is highly inflammable in air, forming a
white cloud of phosphorus pentoxide P 2 5 , also called
phosphoric acid. The red variety of phosphorus is much
less inflammable than the above. It combines with oxy-
gen in two proportions, forming oxides of phosphorus.
One of these oxides unites with bases to form compounds
termed phosphates. It probably exists in metals as an
element, but in slags as a phosphate.

§ 21. Sulphur (S) is a non-metal, and solid at ordinary
temperatures. It readily melts and vaporises, and unites
with metals forming sulphides, such as ferrous sulphide
FeS. With oxygen it forms oxides, viz. sulphur dioxide
S0 2 , and sulphur trioxide S0 3 .

§ 22. Chlorine. — This element exists in nature mainly

I PROPERTIES OF GOLD 81

in combination with sodium, calcium, potassium, mag-
nesium, etc. At ordinary temperatures and pressures
chlorine is a greenish -yellow gas, having a pungent and
irritating smell, but by great pressure may be liquefied to
a dark greenish -yellow liquid, and at low temperatures
may be solidified. It is readily soluble in water. It is
an active chemical agent, and combined with most metals,
forming a class of bodies termed chlorides. Indirectly it
acts as a powerful oxidising agent, and is thus used in
bleaching and as a disinfectant. In combination with
hydrogen it forms hydrochloric acid HC1.

PROPERTIES OF THE METALS
Noble Metals — Gold, Platinum, Silver

§ 23. Gold is usually found in the metallic state in
nature (generally associated with silver, and sometimes
with copper, iron, and platinum). It is often found in
ores of lead, zinc, iron, and copper. Gold is a yellow
metal, with a brilliant lustre; it exceeds all others with
regard to malleability and ductility; its specific gravity
is 19*32 ; its melting point about 1050° C. ; and it is only
volatile at very high temperatures. It is almost as soft as
lead, and can be welded by pressure in the cold ; it is one
of the best conductors of heat and electricity.

Gold does not oxidise in air, nor is it acted on by any
single acid except selenic, but is dissolved by substances
like aqua regia which yield chlorine. Chlorine gas unites
directly with gold to form gold chloride AuCl 8 . It is
unacted upon by sulphur or its compounds, so that gold
exposed to sulphurous fumes does not become tarnished
like silver under similar circumstances. Pure gold is too

82 METAL-COLOURING AND BRONZING pabt

soft for general use, so that it is usually alloyed with silver
and copper, which harden it, without seriously impairing
its malleability and ductility. Antimony, arsenic, and
lead are most injurious substances in gold, even when
present in minute quantities.

Platinum is a white metal, with a brilliant lustre;
highly malleable and ductile; as soft as silver, and can
readily be welded; it is very tenacious, being only ex-
ceeded in this respect by iron and copper among the
elementary metals ; it only melts at the highest tempera-
tures, such as those of the oxy-hydrogen flame, and the
electric arc. It does not oxidise at any temperature, and
resists the action of all single acids, its best solvent being
aqua regia. It is one of the heaviest metals, having a
specific gravity of 21*5. Like silver it absorbs oxygen
when melted, giving it out again on cooling, causing the
mass to spit. It absorbs considerable quantities of hydrogen
and other gases when strongly heated with them, especially
the spongy variety called platinum black ; if this substance
be introduced into a mixture of oxygen and hydrogen it
causes them to combine, with the development of great heat.

Platinum occurs in nature, like gold, in the metallic state,
in the form of grains or nuggets, and is often associated
with iron, copper, gold, silver, and several rare metals.

Silver is remarkable for its whiteness and brilliant
lustre, although when precipitated from its solutions it
often forms a gray powder; it is harder than gold, but
softer than copper, the relative hardness being as 4 : 5 : 7*2.
Silver is extremely malleable and ductile, with a tenacity
of about 14 tons per square inch ; its specific gravity is
10*5, which may be slightly increased by the operations of
coining, rolling, hammering, etc. ; it melts below 1000°

I PROPERTIES OF SILVER 33

C. ; is one of the best conductors of heat and electricity ;
is volatile at high temperatures, and at the temperature of
the electric arc it may be boiled and distilled. When
heated in a current of hydrogen it volatilises at 1330° C.
It does not oxidise when heated in air, but molten silver
mechanically absorbs oxygen and emits it on solidifying ;
this is termed " spitting." Silver in a finely divided state
is oxidised when heated with certain metallic oxides, such
as cupric oxide, manganese dioxide, red lead, etc., these
bodies being reduced to lower oxides. Silver is soluble in
nitric and sulphuric acids. Silver unites readily with
sulphur when heated, forming silver sulphide Ag 2 S, which
is a dark gray, crystalline body, with feeble lustre ; some-
what soft and malleable. When heated in air it does not
form oxide or sulphate, like most other metallic sulphides,
and at a red heat is decomposed into metallic silver and
sulphur. Dilute hydrochloric acid has no action on
silver sulphide, but the strong acid attacks it. Lead,
copper, or iron decomposes it when the two bodies are
fused together. When silver sulphide is heated with
common salt,' in the presence of moist air, silver chloride
is formed.

Silver and all its salts dissolve in sodium thiosulphate,
forming a soluble double thiosulphate (Na^SgOg + Ag 2 S 2 3 ),
when the sodium salt is in excess. Silver combines
directly with chlorine to form silver chloride AgCl. The
same substance is formed by adding hydrochloric acid,
or a solution of common salt to a solution containing
silver, when AgCl is precipitated as a white powder;
if, however, a large excess of strong salt solution be
used the AgCl is dissolved, a double salt being formed
thus —

84 METAL-COLOURING AND BRONZING pakt

AgN0 8 + NaCl = AgCl + NaN0 8
AgCl + NaCl=(Ag01, NaCl).

Silver chloride fuses at a low red heat to a yellow liquid
and readily volatilises at a strong red heat. It is insoluble
in acids, but soluble in ammonia, sodium chloride, sodium
thiosulphate, and potassium cyanide. It may be reduced
by hydrogen, carbonate of soda, zinc, iron, and several
other metals, and partially by sulphur. It unites with
oxide of lead in all proportions, and partially so with
sulphide of lead and some other sulphides.

Silver occurs in nature in the metallic state ; as sulphide
in silver glance, which is often associated with the sul-
phides of lead, antimony, and iron; as bromide and
iodide \ as chloride in horn silver ; in many lead, zinc, and
copper ores ; and sometimes in iron pyrites.

Silver is too soft to be worked by itself for most pur-
poses, pure silver being only used in special cases, where
the presence of another metal would exert an injurious
effect. In most cases silver is alloyed with copper, and
occasionally with other metals, as in silver solders.

Copper Group — Copper, Mercury, Lead, and

Bismuth

§ 24. Gopper has a red colour ; is highly malleable,
ductile, tough, and tenacious ; it melts at about 1050° C,
is not sensibly volatile, except at very high temperatures ;
its specific gravity is 8*8, which may be slightly increased
by hammering and rolling. Copper is one of the best con-
ductors of heat and electricity, but this property is con-
siderably interfered with by the presence of small traces of

I PROPERTIES OF COPPER 35

impurities. When a bar of pure copper is freshly broken
it exhibits a fibrous ^ilky fracture of a light salmon colour.
It readily unites with oxygen at a red heat, forming one
or both of the two oxides, known respectively as black and
red oxide, in virtue of their colour. The red or cuprous
oxide is highly basic, and unites with acid substances, such
as silica, forming copper salts. Cuprous oxide is soluble
in molten copper, making it dry in appearance and brittle
in character. This may be remedied by remelting the
copper with a little charcoal, and stirring with a pole of
green wood. Commercial or tough-pitch copper is never
pure, but the impurities are neutralised by the presence
of a little oxygen. If the poling referred to above be
continued too far, the neutralising oxygen is removed
and the other impurities present act on the copper pre-
judicially, making it brittle. The copper is then said to be
over-poled.

Copper unites directly with sulphur when the two
bodies are heated together forming cuprous sulphide,
which is of a dark bluish -gray colour, shows a finely
granular fractured surface when broken, and has a metallic
lustre.

Phosphorus is highly injurious to copper when allowed
to remain in it, but a small quantity may, under certain
circumstances, exert a refining influence, provided the
whole of it is afterwards removed.

The element silicon, when reduced from sand by the
action of carbon, unites with copper, making it much
harder, and causing it somewhat to resemble gun-metal in
colour, but diminishes its toughness and malleability.

Lead, arsenic, and antimony have a very injurious
action on copper, making it hard, brittle, and cold-short

36 METAL-COLOURING AND BRONZING part

The common impurities in copper are iron, arsenic,
antimony, and cuprous oxide; sometimes tin, bismuth,
sulphur, lead, nickel, and cobalt are present. The varieties
of commercial copper are — rosette or Japan copper, the
surface of which presents a peculiar red colour, due to a
coating of oxide, formed by throwing water on the surface
of the metal while in a heated state. Bean-shot and
feathered-shot copper, which are obtained in the form of
globules and flakes respectively, by running the metal
into hot or cold water. Tough-cake is a variety cast into
rectangular slabs, convenient for rolling, etc. Best-selected
is the name applied to the purest variety of commercial
copper, special care being taken to free it from sulphur,
arsenic, antimony, and iron. Kussian - copper gener-
ally contains traces of iron, but is otherwise very pure.
Chili-bars. This variety, as imported into this country, is
prepared in bars weighing about 200 lbs. each ; the copper
being in a raw state, requires to be refined before it is
ready for use.

Mercury. — This is the only one of the useful metals
which is liquid at ordinary temperatures ; it is also called
quicksilver, and has been known from the most remote
times. It has a silver-white colour with a brilliant lustre ;
is devoid of taste or odour when pure ; at a temperature
of 360° C. it boils, and at - 39 '4° C. it solidifies, forming
a soft white malleable mass, exhibiting a granular structure
on the freshly fractured surface. It has a high and fairly
regular coefficient of expansion for heat, which renders it
suitable for thermometers and similar instruments; its
specific heat is -0332, and its density at 4° C. is 13*59.
Liquid mercury does not oxidise in air, except near its
boiling point, which forms a ready means of detecting the

I PROPERTIES OF LEAD 37

presence of base metals, such as lead and antimony, added
as adulterations, or present as impurities. Impure mer-
cury, when exposed to air or oxygen, becomes coated with
a gray film, due to the oxidation of the impurities. At its
boiling point mercury is slowly oxidised to mercuric oxide
HgO. It combines directly with sulphur, forming an
important compound, mercuric sulphide or vermilion
HgS.

Mercury unites with most metals forming " amalgams,"
some of which are liquid, others semi-liquid, and some
solid. The solid amalgams are regarded as chemical com-
pounds, while the liquid amalgams may be solutions of
compounds in excess of mercury, but the affinity is feeble,
as the mercury is partially expelled by pressure, and com-
pletely so, in most cases, by heat. Amalgams are formed
— (1) by rubbing the metal in a finely divided state
with mercury, an increase of temperature facilitating the
amalgamation ; (2) by dipping a metal into the solution
of a mercury salt ; (3) by voltaic action, as when a metal
is placed in contact with mercury and an acid ; (4) by
mixing a metal, such as gold, with an amalgam of a highly
positive metal, such as sodium.

Mercury sometimes occurs in the metallic state, some-
times as an amalgam with silver, and occasionally as
chloride, bromide, and iodide of mercury. The chief
source of the metal is the sulphide HgS, known as
cinnabar.

Lead. — This metal has a bluish-gray colour, and pos-
sesses considerable lustre; it is malleable, ductile, and
tough, but has a feeble tenacity. The lustre of a freshly
cut surface soon becomes dim when exposed to the air,
owing to the formation of a film of suboxide of lead.

38 METAL-COLOURING AND BRONZING part

Pure lead emits a dull sound when struck, but the pres-
ence of impurities renders it more sonorous ; also when
the pure metal is cast in the form of a hollow sphere it
becomes somewhat sonorous. Its specific gravity is 11*37,
and all base metals, when alloyed with it, lower its density.
Its melting point is about 330° C, and it is not well
adapted for castings, since it contracts considerably on
solidifying. It is so soft that it can easily be cut with
a knife, and squirted into the form of tubes or rods ; two
clean surfaces of lead can easily be welded together by
pressure in the cold, and also when in a finely divided
state the metal can be pressed into a compact mass. Its
specific heat between 0° and 100° C. is '0314:, and its
coefficient of expansion is '00003 for each degree between
0° and 100° C.

If lead is boiled with water containing oxygen it is
partially dissolved, and the liquid affords an alkaline
reaction. The metal is oxidised when exposed to moist
air; it is somewhat volatile when heated in air, form-
ing lead oxide PbO, and this oxide acts as an oxidising
agent on many metals, such as copper, zinc, iron, etc.
Lead and sulphur unite when heated together, forming
lead sulphide PbS, which is a bluish-gray, brittle, and
crystalline body.

Commercial lead is often nearly pure, but it generally
contains some silver, copper, antimony, tin, and sulphur ;
and occasionally iron, arsenic, zinc, and manganese.

Bismuth is a hard grayish- white' metal with a reddish
tint and bright metallic lustre. Its specific gravity is 9*8,
which may be reduced by pressure ; it melts at 270° C.
and volatilises at a high temperature, burning with a
blue flame, forming flowers of bismuth Bi 2 8 ; it expands

I PROPERTIES OF TIN 39

in the act of solidifying. When the metal is melted, and
allowed to cool until its surface begins to solidify, the
crust broken, and the metal poured out, line large crystals
are obtained. They oxidise in air, and frequently become
covered with an iridescent film of oxide. Bismuth unites
with sulphur, forming a dark gray metallic-looking sulphide
Bi 2 S 3 .

Bismuth serves for the preparation of many pharmaceu-
tical products and cosmetics. The chief use of the metal
is in the preparation of fusible alloys, the melting points of
which can be altered according to the proportions of their
constituents. It occurs in nature in the metallic state, as
bismuth-glance Bi 2 S 3 , as bismuth-ochre BigOg, and often
in company with silver, lead, tin, copper, and cobalt ores.

Tin Group — Tin, Antimony, and Arsenic

§ 25. Tin is a white metal with a brilliant lustre ; very
malleable, as seen by the thinness to which tin-foil can be
reduced. A bar of tin when bent produces a crackling
sound, known as the " cry of tin," supposed to be due to
the grinding action of its crystals over each other. Its
specific gravity is 7*3, it melts at 230° C, and may be
somewhat strongly heated without volatilising. When
raised to a temperature near its melting point, and
alloved to fall from a considerable height, the metal
breaks up into the form of long grains, known as grain-
tin. When tin is melted, and poured into a mould at a
temperature little removed from the point at which it
solidifies, the surface remains bright, if pure, but the
preence of a little lead, iron, or other base metals im-
pacts a more or less dull and frosted appearance, so that

40 METAL-COLOURING AND BRONZING paht

the brilliancy of the surface is a test of purity. Tin
is easily crystallised superficially by treating its surface
with a mixture of dilute sulphuric and nitric acids ; the
ornamental appearance, known as Moiree MetaUique, is
obtained in this way. Tin is an inferior conductor of
heat and electricity; it takes a high polish, and the
radiation of heat from its surface is small. It forms a
valuable metal for coating culinary vessels. It is little
affected by air at ordinary temperatures, and is therefore
used for coating iron to protect it from rust. It unites
readily with sulphur on the application of heat, fDrming
stannous sulphide SnS. Commercial tin often contains
small portions of lead, iron, copper, arsenic, antimony,
bismuth, tungsten, and sometimes manganese and zinc.
The tin of commerce is quoted as common, refinsd, and
grain-tin. The refined-tin is made from the beat ores,
and is more perfectly refined than common tin. Grain-tin
is obtained from the best pigs, which are heattd and
dropped from a height as referred to above.

The only important ore of tin is tin-stone, which con-
tains the dioxide Sn0 2 .

Antimony. — Ordinary commercial antimony ii very
impure, containing iron, lead, arsenic, and sulphur, and is
called "regulus of antimony." Antimony is a briliant
bluish -white metal, highly crystalline, with fern-like
markings on the surface, and very brittle, so that it may
be easily powdered; its specific gravity is 6*72 ; it nelts
at 632° C, and volatilises at a higher temperature. It
does not oxidise at ordinary temperatures, but wtan
heated in air, antimonious oxide Sb 2 O s is formed ; anc at
a red heat it burns with a bluish-white flame, producng
dense white fumes of Sb 2 8 .

I PROPERTIES OF ARSENIC 41

Antimony and sulphur readily unite when heated
together, forming Sb 2 S 3 ; the same compound is also
formed by heating the oxide with sulphur, thus —

2Sb a O s + 9S = 3S0 2 + 2Sb2Ss.

Antimony unites with other metals to form valuable
alloys, in consequence of its hardening properties, but it
impairs the malleability and ductility of the malleable
metals. The effect of even small quantities of antimony
on the malleable metals, such as copper, gold, iron, etc.,
is most injurious, making them hard and brittle.

Antimony occurs native, and in combination with
other ores, but the chief ore is " stibnite " Sb 2 S 3 .

Arsenic. — This metal has a brilliant, dark steel-gray
colour, and metallic lustre; it is crystalline, exceedingly
brittle, and may be readily reduced to powder. When
heated to 180° C. in a closed vessel it begins to volatilise
without fusing, and crystallises as it condenses in forms
similar to those of antimony. Its vapour is colourless,
and possesses a peculiar garlic-like odour. The metal may
be exposed to dry air without undergoing change. If
heated in air it absorbs oxygen, and burns with a bluish
flame, forming arsenious acid As 2 3 , which is condensed
as a white powder when in contact with a cool body.
The specific gravity of arsenic is 5*67.

Arsenic occurs in nature as realgar As 2 S 2 , orpiment
AsgSg, mispickel FeAs + FeS, nickel pyrites NiAs, and
kupfer-nickel or copper-nickel NiAs 2 .

The metal is obtained by heating nickel pyrites, mis-
pickel, etc., in closed retorts, when the arsenic is expelled,
and sublimed in condensing chambers.

Arsenic enters into the composition of some alloys,

42 METAL-COLOURING AND BRONZING part

such as shot metal, its general effect being to harden, and
render the alloys brittle, and more fusible. Its com-
pounds are used in medicine, and in glass-making.

Ikon Group — Iron, Chromium, Manganese, Nickel,

and Cobalt

§ 26. Iron. — Malleable iron is of a grayish -white
colour, having a granular, crystalline, or fibrous fracture,
according to the mode of treatment. When rolled or
hammered hot, the iron becomes fibrous, but continued
cold hammering induces a crystalline or granular structure,
making it hard and brittle. The nature of the fractured
surface varies also with the manner in which the iron has
been broken, for specimens broken by progressively in-
creasing stresses invariably show a fibrous structure, whilst
the same specimen broken by a sudden blow may be
crystalline. The presence of impurities generally tends to
impart a granular or crystalline fracture, and makes the
iron less malleable. When impurities, such as sulphur
and arsenic, render the metal unworkable at a red heat, it
is said to be hot- or red-short On the other hand, some
substances, such as phosphorus, cause iron to crack when
hammered cold; it is then termed cold-short. The
specific gravity of iron is about 7*8. Its fusing point is
said to be about 1600° C. ; but before melting it assumes
a pasty form, at which point two pieces may be joined
together by welding. To ensure a good weld the surfaces
must be clean and the metal at a white heat. In order
to dissolve any scale the smith adds a little sand, which
unites with the oxide and forms a fusible silicate. The
presence of any foreign bodies, such as carbon, silicon,

i PROPERTIES OF IRON 43

sulphur, phosphorus, copper, oxygen, etc., increases the
difficulty of welding. Iron possesses considerable malle-
ability, ductility, and tenacity. Its tensile strength
ranges from 17 to 25 tons per square inch, but this, like
all the other physical properties, is modified by the pres-
ence of impurities, which tend to make it harder, more
fusible, and brittle. When iron is heated to dazzling
whiteness, it burns, forming the black oxide FegO^ the
iron becoming friable and brittle, and is then termed
"burnt iron." Iron may be magnetised by bringing it
in contact with, or near to, a magnet, but it loses its
magnetism when the exciting magnet is withdrawn. Its
specific heat is '1137; its conductivity about 120, silver
being taken as 1000. Its electric resistance is 5*8 times
that of pure copper. When iron is exposed to moist air
it readily rusts or oxidises, so that it is often coated with
some substance to prevent this action, such as tinning, gal-
vanising, and painting. Professor Barff preserves iron from
rusting by exposing it at a red heat to superheated steam,
which imparts to it a coating of the black oxide Fe 3 4 .

Iron may be exposed to dry air for an indefinite period
without alteration, but in the presence of moisture a layer
of rust Fe 2 O s is formed. The oxidation is accelerated by
the presence of carbonic acid, which is always present in
the air, a carbonate of iron being formed. This rapidly
absorbs a further portion of water and oxygen from the
air, and in this way the rusting is slowly conveyed to the
centre of the mass of iron. The layer of oxide and car-
bonate is electro-negative with regard to iron, so that a
galvanic action is set up, causing decomposition of the
water. This electrical condition still further augments
the liability of iron to rust.

44 METAL-COLOURING AND BRONZING part

When iron is strongly heated in contact with air or
oxygen its surface is rapidly coated with a scale of black
oxide Fe 3 4 , which peels off when struck with a hammer.

Chromium is a comparatively rare metal, which only
occurs in nature in combination with other elements, the
chief ore being chrome ironstone FeO, Cr 2 3 . Chromium
or its oxides form the colouring matter of several minerals ;
the green colour of the emerald, for example, is due to
chromium oxide. The metal is obtained by the reduction
of its oxide or chloride, or by the electrolysis of its
chlorides, when chromium separates out in brittle glisten-
ing scales. It is tin-white in colour, having a specific
gravity of 6 '8. The fused metal is said to be as hard as
corundum; it melts with more difficulty than platinum,
and is only slowly oxidised when heated in air. It is
used in the form of an alloy with iron and carbon, forming
a hard, white, and brilliant steel, much esteemed for
special purposes.

Manganese. — The pure metal, obtained by the reduc-
tion of its oxide, is a gray or reddish- white body, hard,
and brittle ; its specific gravity is about 8 ; it oxidises
more readily than iron, and must therefore be excluded
from air by keeping it under rock-oil, or in sealed vessels*
Its chief use is in the formation of alloys with iron, steel,
and copper. It is not used in the unalloyed state. Com-
pounds of this metal are very widely distributed in
nature; one of the most common is pyrolusite, or black
oxide of manganese Mn0 2 .

Nickel. — This is a brilliant white, malleable, ductile,
weldable, and very tenacious metal, with a melting point
only a little below that of iron, but the presence of carbon
and other impurities considerably lower its fusing point

I PROPERTIES OF NICKEL 45

Its specific gravity is 8'8 ; it is magnetic like iron, but in
a less degree. It does not readily oxidise in air at ordinary
temperatures, but when heated the monoxide NiO is
formed. It readily unites with sulphur, forming nickel
sulphide NiS, which is brass-yellow in colour ; and with
arsenic forming nickel arsenide NiAs.

Nickel is found in commerce in the form of dull gray
cakes or cubes, and by melting these at a high tempera-
ture a compact, silver -white metal is obtained. The
malleability of nickel allows of its being fashioned into
various articles, which possess great lustre, hardness, and
durability. These properties render it valuable for coat-
ing base metals by the process of electro-plating, especially
as it is little liable to oxidation.

Commercial nickel was formerly very impure, due to
the presence of carbon and other bodies, which make it
hard and brittle. Dr. Fleitmann and other metallurgists
have devised simple and effective means of refining and
toughening nickel, which are now largely practised.
Fleitmann adds to. the melted metal minute quantities of
magnesium in several charges, and well stirs each time a
dose is added. One ounce of magnesium is sufficient for
refining 60 lbs. of impure nickel The magnesium is sup-
posed to reduce the occluded carbonic oxide CO forming
magnesia, and to cause the carbon to separate out as
graphite. Nickel unites readily with most metals forming
alloys, some of which are of great commercial utility.
The most important of these is German silver. Nickel
occurs in nature as kupfer-nickel or copper-nickel NiAs,
which is a copper-red coloured mineral, with a metallic
lustre. As nickel pyrites NiS, which is brass-yellow in
colour. As nickel-glance, which is a variable compound

46 METAL-COLOURING AND BRONZING pabt

of nickel, arsenic, and sulphur. As garnierite, which is a
hydrated silicate of nickel, iron, and magnesium.

Cobalt. — This metal resembles nickel in appearance
and properties, and is generally associated with it in
nature. Cobalt is a white metal, highly malleable,
ductile, and tenacious; its specific gravity is 8*5; it is
magnetic like nickel ; almost unalterable in air at ordinary
temperatures, but oxidises when heated, and at a high
temperature burns with a red flame. It is seldom used in
the metallic state, but its compounds are largely employed
in pigments. It unites with arsenic to form iron-gray,
fusible, and brittle compounds.

The principal ores are smaltine CoAs, cobalt glance
Co 2 AsS, and cobalt bloom (Co 3 As0 4 , 4H 2 0).

Zinc Group — Zinc, Cadmium, and Magnesium

§ 27. Zinc, commonly known by the name of
"spelter" when in the cast state, is a white metal, with
a bluish shade, and bright metallic lustre. Ordinary
zinc is hard and brittle, and when fractured exhibits a
highly crystalline structure. When pure it is malleable
at the ordinary temperature, while commercial cast-zinc
is brittle; the latter, however, becomes malleable and
ductile if heated to a temperature of 100° to 150° C, but
beyond that point it again becomes brittle. Its specific
gravity in the cast state is 6*9, which may be increased to
7*15 by rolling or forging; it contracts but slightly on
cooling from the liquid state, and is thus well adapted for
castings. The castings made at a high temperature are
brittle and crystalline ; but when cast near the solidifying
point are more malleable. Zinc melts at 412° C, and boils

i PROPERTIES OF ZINC 47

at 1040° C. At a red heat in air it rapidly oxidises,
and burns with a greenish-white flame, forming zinc oxide
ZnO ; if raised to a bright red heat in a closed vessel it
may be readily distilled. When rolled zinc is exposed to
air and moisture a gray film of suboxide is formed, which
preserves the metal from further oxidation. Ordinary
zinc readily dissolves in dilute hydrochloric and sulphuric
acids, while the pure metal is unaffected; both kinds
dissolve in nitric acid and in alkalies. Zinc displaces
silver, gold, platinum, bismuth, antimony, tin, mercury,
and lead from their solutions. The chief impurities of
the commercial metal are iron, lead, and arsenic.

Zinc and sulphur do not readily unite, but when a
mixture of finely divided zinc and sulphur is projected
into a red-hot crucible, some zinc sulphide ZnS is formed.
It is also formed by heating zinc with cinnabar HgS.

Zinc forms compounds with phosphorus and arsenic,
when these bodies are heated with it, having a metallic
lustre and somewhat vitreous fracture.

The chief ores of zinc are — The oxide ZnO called
zincite or red oxide of zinc, which is white when pure, but
generally red from the presence of oxide of manganese.
The sulphide ZnS, known as "blende" and "black jack,"
is the principal source of the metal, and generally black
or yellowish-black in colour, but sometimes it has a
reddish tint from the presence of galena ; when pure it is
white, and contains 67*03 per cent of zinc. The car-
bonate ZnCO s , called calamine, is also an important
source of zinc. Zinc forms with other metals a most
important class of alloys, such as brass, German silver,
etc. It is used in the form of sheets, worked into
a variety of shapes; it protects iron from rusting, as

48 METAL-OOLOURING AND BRONZING part

in galvanised-iron ; it forms the electro-positive element in
many batteries; and in the form of fine dust it is
obtained in large quantities mixed with zinc oxide, and
forms a valuable reducing agent.

Cadmium. — In the process of zinc extraction it was
observed that a volatile vapour, in some cases, was dis-
tilled off with the first portions of zinc ; this was found to
be the metal cadmium. It possesses a tin- white colour, has
a fibrous structure, and takes a high polish. It is harder
than tin. malleable, ductile, and readily volatile. It has
a density of 8*6 ; it melts at about 320° C, and boils at
860° C. Its vapour is of a dark yellow colour, with a
disagreeable odour. Like tin it emits a crackling sound
when bent. It is used in alloys to produce a fusible
metal, which melts below 100° C. ; and an amalgam of
cadmium is employed as a stopping for teeth, such amalgam
being soft when first prepared, but soon becomes hard.

Magnesium. — This metal possesses a brilliant white
colour, but soon tarnishes when exposed to moist air, due
to the formation of magnesium oxide. It is stated to
possess great tensile strength, being nearly equal to that
of aluminium bronze. Its specific gravity is 1 *74. At a
temperature of 450° C. it can be rolled and worked into a
variety of forms. Screws and threads made of this
metal are sharper and more exact than those made of
aluminium. When ignited in a flame it burns with a
dazzling white light, which is said to have been seen
at sea from a distance of 28 miles. This light is
used for purposes of photography. Magnesium occurs
abundantly in nature in combination with other elements
forming compounds, such as magnesite MgC0 3 , dolomite
MgCa(C0 8 ) 2 , etc.

PROPERTIES OF ALUMINIUM 49

Aluminium

§ 28. Aluminium, — With the exception of oxygen and
silicon, this is the most widely distributed of the elements,
and contained in the largest quantity in the solid crust of
the earth. It occurs in a variety of forms as oxide, but
more generally in combination with other metals, such
as zinc, iron, magnesium, etc., forming aluminates;
as silicate in all clays, and as fluoride in cryolite
(6NaF, p

Aluminium is a white metal which takes a fine polish.
It has no taste or odour. It is soft, very malleable, and
ductile, with an elasticity and tenacity about equal to
that of silver. Its specific gravity is 2*5, which is
increased by hammering; it melts at a temperature a
little below that of antimony, and is not volatile when
strongly heated out of contact with air. Its conductivity
for heat and electricity is said to be very high; but,
according to Mr. Roby, it is very low, and diminishes the
conductivity of copper considerably when alloyed with it.
It does not oxidise in air or combine with sulphur ; it is
insoluble in cold nitric acid; sulphuric acid has no action on
it ; but hydrochloric acid and alkalies dissolve it readily.

Aluminium is valuable for making articles where
lightness is an important feature ; this, combined with its
lustre, unalterability in air and sulphuretted hydrogen,
non-poisonous properties, and ease of working, gives it a
widespread interest. It is, however, in its alloys that its
greatest value appears. In some cases it imparts strength,
in others it modifies the colour, while in others it pro-
motes soundness in castings.

50 METAL-COLOURING AND BRONZING part

Alkaline-earthy Metals

§ 29. The term " earth " was formerly used to denote
those bodies which are insoluble or but slightly soluble in
water, and unaltered by exposure to a high temperature.
Some of these were found to have an alkaline reaction,
and to easily neutralise acids ; hence the term " alkaline
earth." These oxides — viz. baryta, strontia, lime, and
magnesia — were found to be composed of metals in com-
bination with oxygen.

Barium is a pale yellow metal, malleable, and fusible
at a red heat. It rapidly tarnishes in air, and burns
brilliantly at a red heat, forming barium oxide. Its
melting point, according to Frey, is above that of cast-
iron. It decomposes water rapidly at the ordinary
temperature. Its specific gravity is 3 '75.

Strontium is similar to barium in colour; it is
malleable, fusible at a red heat, quickly oxidises on
exposure to air, burns brilliantly in air when heated,
and violently decomposes water. Its specific gravity
is 2-54.

Calcium is a yellow metal, tenacious and malleable;
it melts at a red heat, oxidises in air, and burns when
heated ; it decomposes water rapidly. Its specific gravity
is 1-58.

The alkaline-earthy metals, although their compounds
are widely distributed, do not occur in nature in the
metallic state, and the isolated metals have little applica-
tion in the arts, on account of their easy oxidation.
They may be useful in removing oxygen from other
metals and their alloys.

PROPERTIES OF THE ALKALIES 51

Alkali Metals— Sodium, Potassium, Lithium, etc.

§ 30. The word "alkali" was originally used as the
name of a soluble salt obtained from the ashes of sea-
plants, and is now applied to a well-defined class of bodies
having the following properties : — They turn red litmus
blue, completely neutralise acids, are soluble in water, and
their solutions exert a caustic action upon animal matter.
The alkalies proper are the oxides of sodium, potassium,
lithium, rubidium, and caesium. To these is added the
hypothetical metal ammonium NH 4 , which is called the
volatile alkali in contradistinction to potash and soda.
The metals of the alkalies are soft, readily fusible, volatile
bodies, easily oxidised on exposure to air ; and they
rapidly decompose water at ordinary temperatures.

Sodium. — This metal melts at 96° C. and volatilises,
forming a dark blue vapour. It rapidly oxidises in air,
and when strongly heated burns with a yellow light. It
decomposes water rapidly at ordinary temperatures. It is
a silver- white metal, with a specific gravity of *97.
Sodium is used for the preparation of aluminium, mag-
nesium, boron, and silicon. As an amalgam it is used in
the extraction of gold, and in the laboratory as a reducing
agent. It occurs very abundantly in nature in a state of
combination, in the forms of chloride, nitrate, borate,
carbonate, and silicate.

Potassium. — This element is very similar to sodium in
appearance and properties. It is a silver-white, lustrous
metal, having a specific gravity of '87 ; it is brittle at
0° C, but at 15° C. it becomes soft, malleable, and weld-
able; it melts at 62*5° C, forming a liquid like mercury

52 METAL- COLOURING AND BRONZING part

in appearance ; at a red heat it boils, emitting a green-
coloured vapour. It has a strong affinity for oxygen, and
decomposes water, with evolution of great heat. It is
used for similar purposes to those of sodium, and occurs
abundantly in nature in analogous forms.

Lithium. — This is a widely diffused element, being
found in many micas and felspars, in the ashes of
plants, and in sea-water. It has the colour and lustre
of silver, is soft and weldable, melts at 180° C, is
volatile at a high temperature, burning with a white
flame, and rapidly oxidises in contact with air at ordinary
temperatures ; its specific gravity is '58, and it is, there-
fore, the lightest of all solid and liquid bodies.

Rubidium and Caesium. — These rare metals so closely
resemble potassium that they cannot be distinguished
from that metal by many of the ordinary tests. Their
presence is detected by means of spectrum analysis.

Nature op Alloys

§ 31. When two or more metals are caused per-
manently to unite, the resulting mixture is termed an
alloy. When mercury is an essential constituent, the
mixture is then termed an amalgam. The general
method of effecting combination is by the agency of heat,
but with certain soft metals true alloys may be formed by
subjecting the constituents to considerable pressure, even
at the ordinary temperature.

Alloys were doubtless first discovered by the metallur-
gical treatment of mixed ores, from the simultaneous
reduction of which alloys would be formed ; or in some
cases, as in ores of gold and silver, naturally formed alloys

I NATURE OF ALLOYS 58

would be obtained by a simple melting process. The direct
preparation of alloys by the simple melting together of the
constituent metals has been enormously developed in
modern times, and the attention which mixed metals are
now receiving by chemists is far greater than in any
period of history.

Comparatively few of the metals possess properties
such as render them suitable to be employed alone by the
manufacturer; but most of them have an important
application in the form of alloys. Even among the
metals which can be used independently, it is often found
expedient to add portions of other metals, to improve or
otherwise modify their physical properties. Thus gold is
hardened, and made to resist wear and tear, as well as its
cost lowered, by the addition of copper ; silver is likewise
hardened by alloying it with copper; and the bronze
coinage is formed of an alloy of copper, zinc, and tin for
similar reasons.

The purposes for which metals are alloyed are as
various as the uses of the metals themselves, but, as a
rule, the combination is employed to harden, render more
fusible, alter the colour, or to reduce the cost of produc-
tion. Thus the class of alloys known as solders, which
are used for joining the several parts of a body or bodies
together, are formed so as to possess melting points below
that of the articles to be soldered. The well-known class
of alloys termed " brass " furnishes a good illustration of
the effect of alloying in producing different shades of
colour. These bodies are composed of the metals copper
and zinc in varying proportions, the colour depending to a
great extent on the quantity of copper present. When
the copper predominates the colour is yellow or reddish,

64 METAL-COLOURING AND BRONZING part

when the two metals exist in equal proportions the colour
is still yellow; beyond this, when the zinc is in excess,
the colour gets white, or bluish-white, resembling impure
zinc. Nickel is added to brass to whiten it, forming
German silver.

Again, some metals, such as copper, can only with
difficulty be made to produce sound castings; and the
metal is too tough to be conveniently wrought in the
lathe or with the file, but when alloyed with zinc or tin,
good castings can be readily obtained, and rolled, turned,
or filed with considerable facility. In some cases the
tensile strength of a metal is enormously increased by the
addition of another metal, sometimes in very small pro-
portions ; the various bronzes may be cited as examples.
The addition of a second metal is often a source of weak-
ness, as in the case of adding antimony to lead. One
might be led to consider that the alloying of two malleable
metals would produce a malleable alloy, and while in many
cases this undoubtedly is so, there are others in which the
opposite is the fact. Thus lead added to gold in very
small quantity makes the gold exceedingly brittle and
weak.

The specific gravity of an alloy nearly always differs
from the mean specific gravities of the constituents, some-
times being greater and sometimes less. When the
density is increased it shows that contraction has occurred,
and chemical combination has probably taken place
between the components. This is the case with bronze
rich in copper, while with similar alloys rich in tin ex-
pansion occurs, the specific gravity being less than the
mean of the two metals. One of the greatest difficulties
connected with the subject of alloying is the tendency of

I NATURE OF ALLOYS 55

the constituents to separate on cooling according to their
specific gravities. As a rule, it is more difficult to alloy
three or four metals than two metals, especially when the
components differ widely in fusibility, unless the combina-
tion forms a true chemical compound. The mixture is
promoted by constant agitation when the body is in the
liquid condition, and by pouring the metal into the mould
at the lowest possible temperature consistent with the
proper degree of liquidity.

Most metals are capable, to some extent, of existing in
a state of chemical combination with each other, but, as a
general rule, they are united by feeble affinities, for it is
necessary, in order to produce energetic union, that the
constituents should exhibit great dissimilarity in properties.
It is probable that the metals do unite in definite propor-
tions, but it is difficult to obtain these compounds in a
separate condition, since they dissolve in all proportions in
the melted metals, and do not generally differ so widely in
their melting points from the metals they may be mixed
with, as to be separated by crystallisation in a definite
condition. For these reasons it has been questioned
whether alloys are true chemical compounds. Definite
compounds do, however, exist in definite proportions by
weight in the native as well as the artificial state. Such
is the case with mercury and silver, which are found
crystallised together in the proportion of one atom of
silver to two or three atoms of mercury. A good illustra-
tion of chemical combination between two metals is seen
in the alloy of copper and tin, which may be represented
by the formula SnCug, containing 38*4 parts of tin and
61*6 parts of copper. It is distinguished by its peculiar
colour, its homogeneity after repeated fusions, its brittle-

56 METAL-COLOURING AND BRONZING part

ness, and by having a greater density than any other alloy
of these metals.

As a general rule, it may be stated that all metals
which unite with oxygen to form only bases, have a strong
tendency to unite with metals, some of whose oxides
possess an acid character; and those metals which are
allied in regard to basicity or acidity, have little affinity
for each other. Thus sodium and potassium, although
miscible in various proportions, exhibit little or no tend-
ency to unite in definite quantities ; and the same is true
of antimony and tin. On the other hand, copper and tin
form very stable alloys. In some instances, as in the case of
the metals lead and zinc, only very unequal portions can be
made to unite, the main bulk of the metals separating on
cooling; the lead retaining 1*6 per cent of zinc, and the
zinc retaining 1*2 per cent of lead. In all cases of mixed
metals there is not that total and complete alteration in
physical characters which is the distinctive feature of
chemical action between a metal and a non-metal. Alloys
still possess the metallic character, but there is frequently
a considerable evolution of heat evinced; the melting
points, as in fusible metal, are considerably lowered ; the
mean density is increased, and the colour and other
physical properties are considerably modified.

Most often, however, as already indicated, alloys seem
to be mixtures of definite compounds with an excess of
one or other metal, and the separation of their components
from each other is generally easily effected by simple
means. Thus an alloy of lead and copper may be largely
separated by exposing the mixture to a temperature a
little above that of the fusing point of lead, when the
latter metal liquates out, leaving behind a porous mass of

I NATURE OF ALLOYS 57

copper, containing a little lead. Also in remelting brass
a considerable quantity of zinc is lost by volatilisation,
When silver is amalgamated with mercury the amalgam is
dissolved in an excess of mercury, which excess may be
removed by simple pressure; and the remaining portion
of mercury is completely separated by the agency of heat.
Many metals combine together when melted, and only
remain in union within certain ranges of temperature, as
shown by the wide differences between their melting and
solidifying points.

NATURE AND OBJECT OF METAL - COLOURING

OR BRONZING

§ 32. The object of bronzing metals is partly to beautify
the exterior by imparting to it a coating or surface film
of colour, which the common metals do not naturally
possess after a short exposure to atmospheric influence ;
and partly to protect the surface from further discolora-
tion, produced by the combined action of air, moisture,
and various gases, with which the air often becomes
adulterated, especially in large towns and places where
coal-gas is largely burnt.

During the process of natural atmospheric metal-colour-
ing, which may extend over years and even ages, the
change is very gradual and by no means uniform in its
extent, for one part of a body is usually affected more than
another, or one portion may be corroded while another por-
tion remains unaltered, doubtless depending on the irregular
condition of its surface ; for, as the author has repeatedly
experienced in practice, it is absolutely essential that every
part of the exterior should be in exactly the same physical
condition for a uniform tint to prevail all over. Metals,
with few exceptions, become altered on their surfaces by
continued exposure to the atmosphere, and it is to such
alterable metals that the process of bronzing is chiefly ap-

pakt i OBJECT OF BRONZING 59

plied. The metals which combine directly with oxygen
gas are termed " base " metals, and the product of their
oxidation is an oxide. This is the kind of change which
most frequently occurs on exposure to air, producing the
well-known substance commonly called rust, and for this
reason it is generally necessary to coat them with some
protecting film in order to protect them from the action
of the atmosphere. Some of the base metals are more
easily corroded than others, and those metals least affected
in air are often employed to form a protective covering on
those metals which are most easily affected. For this
reason iron is coated with tin or zinc, or electro-plated with
nickel. Brass and its various other alloys are also coated
with silver, and sometimes with gold for the same reason.

The element copper, both in the pure and in the alloyed
state, is the metal par excellence which lends itself most
effectually to the production of beautiful bronze tints.
This is in a great part due to its red colour, which may be
altered into a great variety of tones and half-tones by
various solutions, chiefly solutions in which its own salts
are present as essential ingredients. It is well known that
articles of copper and its alloys, after a long exposure to
the action of the air, acquire a beautiful brown or green
colour, which considerably contributes to their handsome
appearance. This colour is known as jErugo nobilis (noble
rust) or " patina." The green colour is due to the forma-
tion of a basic carbonate of copper, falsely termed verdi-
gris, and is often associated with brown and blackish tints,
especially on very old bronze medals and statues. No
artificial colouring can compare in beauty and durability
with the natural patina.

In order to obtain a coating similar to genuine patina,

60 METAL-COLOURING AND BRONZING pabt

it is recommended to pursue, as nearly as possible, the
same course by which the latter is naturally formed. By
the action of air and moisture which contain minute quan-
tities of salts in solution, the copper is attacked, and in
course of time the basic carbonate of copper is formed
by the agency of carbonic acid. This is often associated
with copper sulphide, which accounts for the dark tones
alluded to above.

It will thus be seen that the natural coloration of
bronze is a chemical process produced by gradual chemical
changes, resulting in a complete alteration of the surface,
and if a similar effect is to be produced on the surface of
metals by artificial means, a chemical process must be re-
sorted to.

But metals are often coloured by mechanical means,
such as painting, lacquering, and varnishing. This is not
bronzing in the original sense of the word ; and however
highly a metal may be coloured by such means, it always
lacks that peculiarly beautiful effect generated by chemical
agency, and has a more or less common look in proportion
to the effacement of the metallic character.

Chemical action is often promoted by the influence of
electricity, and such a change may be termed "electro-
chemical" Very beautiful results in the coloration of
metals may be achieved by this agency, either by a chemi-
cal change of the surface, or by the electro-deposition of
one metal on another. Bronzing or metal-colouring may
therefore be considered under the following heads, viz. —

§ 33. I. Chemical Metal-colouxing, produced by a
corrosion of the surface by chemical agents. In this
method the surface film of metal is changed into a
chemical compound, having a colour peculiar to itself

I MECHANICAL METAL-COLOURING 61

under definite conditions, but which may be modified by
various circumstances, such as the nature and colour of
the metal which forms the base, the temperature at which
the change occurs, and the thickness of the layer of the
compound itself. In fact, the same chemical compound
may exist in different modifications of colour, varying,
as in the case of cuprous oxide, from bright red to dark
brown.

But the change occurring on the surface of metals by
chemical bronzing is often the result of complicated
chemical changes, which produce more than one compound,
each of which exercises an influence on the colour accord-
ing to the relative amount of each constituent. Thus in
oxidising copper, for example, both its oxides may be
formed, the cupric oxide which is black and cuprous oxide
which is red, each colour modifying the other in tone.

§ 34. II. Electro-chemical Metal-colouring. — This
may be brought about by coating a metal, wholly or in
part, with a very thin film of another metal, the shade of
colour of which may be modified by the temperature and
strength of the electric current employed. Nobili's figures
or electro-chromes are illustrations of electrical metal-
colouring. All methods of deposition, whether by simple
immersion, or by immersion and contact with another
metal, will be considered under this head.

§ 35. III. Mechanical Metal -colouring, such as
coating a metal by the superposition of paint, lacquer,
varnish, or bronze powders, by means of varnish or other
adhesive material on its surface, but which coating does
not enter into chemical combination with the metal or in
any way alter its properties. There are other means, of a
purely mechanical character, by which the colour of a metal

62 METAL-COLOURING AND BRONZING part

may be altered. If, for example, we look along the surface
of a plate of gold at a great angle it appears very brilliant,
but almost white in colour, but if we view it at a small
angle the natural yellow colour appears. It may be still
further developed and enriched by repeated reflections at
a small angle of incidence. It is for this reason that
chased gold and " granulated " gold appear of a far richer
colour than burnished gold; and by so shaping the
grooves or lines of chasing upon any piece of coloured
metal that repeated reflections at small angles of incidence
occur in them very beautiful tones of colour may be
produced.

Some metals which lack distinctive colour under
ordinary conditions may be thus made to develop it.
Yet the colour so produced not only changes in purity and
brightness as it becomes enriched, but its hue is also
modified. Thus copper may be made to yield ultimately
a nearly pure or monochromatic light by repeated re-
flections. The colour is more decided, it is purer; but
there is less light. 1 In fact, anything which alters the
evenness, or breaks the continuity of the surface of a
metal will not only modify its colour, but impart different
shades of colour in different parts. This method of treat-
ment is chiefly applicable to the noble metals, since the
common metals soon change when exposed to atmospheric
influences, and the effect is gradually destroyed.

§ 36. Up to about seventy years ago the only artificial
bronze employed for copper and its alloys seems to have
been the various shades of antique green, to which the
name patina was exclusively applied. But since that
time the meaning of this word has been expanded so as to

1 Colour, by A. M. Church, p. 160. Cassell and Co.

I PROGRESS OF METAL-COLOURING 63

apply to all colours produced on metals by way of
bronzing.

About the year 1828 the now celebrated Florentine
bronze was introduced by a Frenchman named Lafleur,
and soon became very popular. The colour has remained
in favour to the present day in consequence of the variety
of agreeable brown shades which can be produced on
copper and its alloys by means of iron oxide and other
agents, causing the formation of a mixture of cuprous
and cupric oxides on the surface. Following this
many new bronzes were introduced in this country
and soon acquired considerable notoriety. It is a cause
of great regret that this art has been allowed to de-
teriorate of late years in England, and that such a
valuable industry has passed into the hands of foreigners.
In 1833 the Germans began to apply metal-colouring to
articles made of cast zinc, which was soon taken up in
France, and the refined taste of the French soon enabled
them to adapt the principles of metal-colouring, both
chemical and mechanical, to all metals, and to become the
acknowledged authorities on the subject. They combined
the two methods so as to produce a variety of colours on
the same article, such as the original greens with the
Florentine browns. Also they made the groundwork
black or dark coloured, with yellows, greens, and browns,
on the embossed parts, the latter being bronzed by means
of varnish and bronze powders. During the last fifty years
great strides have been made both by the French and
Germans, and although the productions of some of the early
workers have never been surpassed for beauty of tone and
perfection of finish, the art has now a much wider and more
extensive application. The number of solutions available for

64 METAL-COLOURING AND BRONZING part

bronzing is largely increased, with a consequently extended
variety and shades of colour. Moreover, the number of
alloys has been enormously extended of late years, so that
the art bronzer has a much greater variety of materials at
command on which to exercise his skill.

Mention has already been made of the admirable skill
displayed by the Japanese in combining different coloured
alloys, as well as in the production of bronzing effects on
the surface by means of different pickling solutions. This
nation and the French and Germans are pre-eminently the
masters of the world in the art of metal-colouring. Their
styles are different, each has its own particular modes of
treatment, and in consequence of this it is impossible to
say which occupies the first position. Undoubtedly the
French and Germans are without equals in Europe, and
with regard to chemical metal-colouring they probably sur-
pass the Japanese, especially as their ordinary chemical
knowledge enormously exceeds that of the Japanese. The
Viennese among European nations have attained great
excellence in the colouring of metals, and come very near
to the French in artistic merit and scientific knowledge of
the subject.

§ 37. Metal-colouring is essentially associated with art,
and no workman can beautify a metal without he possesses
some artistic taste. It has, unfortunately for us, become
the idea among many manufacturers that bronzing is a
very simple mechanical operation and can be left to an
ordinary dipper to accomplish, who may not have learned
the art, and who may not have the remotest idea of the
nature and harmony of colour. It is quite true that the
cheap work, coloured in monochrome, does not require
much knowledge and skill, and cheap labour can be

I NATURE OF BRONZING 65

obtained to do it. But there is as much difference between
true artistic bronzing and mere dipping, as there is
between whitewashing a wall and painting a meritorious
picture. Other nations have grasped this difference and
take care to provide skilled labour, and until we do the
same we must continue to send our best work to a foreign
country to be executed.

We do not sufficiently realise how exceedingly beautiful
a common metal can be made to appear with little expense
as regards materials, if only tastefully and suitably coloured.
An effect can be produced by bronzing on a metal peculiarly
its own, and which cannot be produced on any other material
so as to give the same satisfaction, or to have anything near
the same appearance. The suitable colouring of a metal
then is not a matter of subordinate importance but of the
highest significance if we are to get from it the utmost
pleasure and profit it is capable of yielding

The superiority of French workmanship is doubtless due
to art and scientific training, the combination of theory and
practice, so that while a change is taking place, or a particular
solution is selected, the operative has an idea of the effect the
desired colour will have on the final result, and a know-
ledge of the chemical change requisite to produce that result.
Every year adds to the sum of human knowledge with
regard to chemical change, and therefore to the possibili-
ties of an art which is so essentially a chemical one.

No mere recipe will be sufficient to enable a person to pro-
duce a given colour, as each metal not only has an individu-
ality of its own, but is modified, sometimes to an enormous
extent, by even small traces of an impurity, and an alloy of
two or more metals may vary in its action with a given solu-
tion according to the proportion of the constituents present,

66 METAL-COLOURING AND BRONZING part

I have seen varieties of brass, differing but slightly in com-
position, behave quite differently in the same solution, and
the same metal will often take a different shade of colour
in the cast state to that in the rolled or stamped state.

The art bronzer should therefore, in addition to his
knowledge of the principles of colouring, have a fair know-
ledge of chemical and metallurgical principles, especially
of alloys. Some solutions after serving their purpose as
bronzing reagents may contain valuable ingredients which
will pay for their recovery, and, with a knowledge of
chemistry on the part of the operator, may be profitably
extracted. Some solutions depend for their success on
the purity of the chemicals employed as well as on the
composition of the metal required to be bronzed, and it
will be a great assistance if the bronzer is capable of test-
ing such solutions and metals as to their degree of purity.

The author, in some experiments on bronzing brass
articles of different makers in a certain solution, found
that the alloy from one firm gave a very beautiful colour,
varying from light gray to black, according to the length
of the time of immersion, while a similar kind of metal from
another source acted quite differently. Great importance
therefore is attached to a thorough knowledge of the
nature and composition of the materials operated upon
on the part of the workman, if the best results, which a
given combination is capable of yielding, are to be secured.

Again, many failures result from a disregard of
time and temperature during the bronzing operation.
Some liquids work best at the ordinary temperature for
the production of a given shade of colour, but in the
majority of cases solutions are best worked hot. Some
require to be used at a boiling temperature, while in

I NATURE OF BRONZING 67

others better results may be obtained at a temperature
of 60° to 70° C. Most metals act differently in the
cast state to what they do when stamped or rolled, and
here a metallurgical knowledge is of great value.

Lastly, a knowledge of the principles and practice of
electro-metallurgy is of the greatest importance to the
art bronzer. In fact, very limited results can be achieved
without recourse to this valuable art. As already re-
marked, a given bronzing liquid affects different metals in
different ways as regards colour, and by depositing copper,
brass, nickel, silver, etc., on different parts of an object a
great variety of colours may be produced when such a
complex metallic surface is subjected to the influence of a
bronzing solution. The groundwork may thus be made to
assume a dark or light tone, and each portion may be in-
duced to accept that particular tone which is in most
perfect harmony with its prominence in the design.

It should also be borne in mind that a metallic article
is not like canvas or paper, which has to be completely
covered with paint of all colours from the brightest scarlet
to the dullest gray, but a material which must always assert
its peculiar metallic character, so that there is never any
doubt as to its real nature. It is true, that a bronzed metallic
article is not coloured so that one may say whether it is
made of copper or brass or spelter, but the colours must
always allow the beholder to say without hesitation that
the base is a metal and not wood or any other material.

The above remarks are not completely applicable to
the precious metals. Gold, for example, apart from its
monetary value, is an exceedingly beautiful metal as
regards colour and lustre, and is not altered by exposure
to the atmosphere, nor by sulphur, so that bronzing of

68 METAL-COLOURING AND BRONZING part

gold is entirely opposed to the nature of the metal, and
therefore scarcely a legitimate process. There should
never be any doubt about a gold article whether it is
gold, and to make it look like a base metal would be
quite opposed to artistic taste. Slight differences of tone
may be obtained by alloying gold with other metals, and
the intensity of the reflection enables jewellers to use
these coloured varieties with the best effects.

Another argument why gold should not be debased
has already been mentioned — viz. that gold has above
every other metal the property of harmonising with, and
setting off, all ordinary colours, whether metals or pig-
ments. Two instances may be noted — the gilt frame of
a picture and the gold threads in embroidery. Gold, in
fact, is removed from the series of ordinary paints and
dyes by its great lustre, and so combines into agreeable
assortments with all colours, even with those with which
yellow and orange pigments do not associate well. In a
picture frame this peculiarity of intense reflection prevents
its yellow colour from interfering with the similar hues
of the picture, while its colour, being luminous and
" near," gives the idea of some degree of distance to the
picture itself. 1 Silver differs from gold in the fact that
it is tarnished by sulphur and sulphur compounds,
although it is not affected by pure air. Hence, silver
may very fittingly be bronzed, or, as it is erroneously
termed, " oxidised." In many cases the exceeding white-
ness of pure silver forms too violent a contrast to other
metals, and the general effect is greatly improved by
toning down this great lustre by one of the so-called
oxidising processes. Yet the silver should still convey
1 Colour, by A. H. Church, p. 168,

NATURE OF BRONZING 69

the impression that it is silver and not lead or some other
base metal.

§ 38. It is of the utmost importance then that a
bronzer should have clear conceptions of the principles
laid down in the preceding pages, so as to treat each
metal or combination of metals in a manner for which
it is best adapted. Some persons have the impression
that the colour of a liquid imparts its own colour to the
metal, like fabrics receive their colour in dyeing. That
this is not so will soon be evident after a few experiments
on colouring metals. The colour of the solution em-
ployed is no criterion whatever as to the colour the metal
immersed in it will receive.

It should be borne in mind that the surface colour
the metal will assume will depend on the power of the
solution to effect a chemical change, and produce a
coloured compound of the metal with other elements,
such as oxygen, sulphur, chlorine, etc. The colour will
depend on the condition, uniformity, and cleanness of the
surface, and not entirely upon the cleanness of the solution
in which it is immersed. It is not a matter of indifference,
as already mentioned, whether the article to be coloured is
made of cast or rolled metal, or whether it is an electro
deposit, as in the case of copper, for example, for each physi-
cal condition has a great influence in modifying the colour.
In most cases, the smoother and more regular the continuity
of the surface, the more uniform will the colour be.

The natural physical condition of a metal appears, in
most cases, to be crystalline or semi-crystalline, but by
mechanical treatment this structure may be so altered as
to confer on the metal new properties. It may become
granular, fibrous, or vitreous in structure, and when this

70 METAL-COLOURING AND BRONZING part

alteration extends to the fundamental parts or molecules,
the metal may be so changed as to act like an entirely
different body. Cast zinc, silver, gold, and nickel have
their crystalline condition destroyed by rolling, while
tin, lead, and brass may be less altered. On the other
hand, many chemical agents, as well as mechanical forces,
have power of changing fibrous or granular structure
back to the crystalline condition.

All these conditions therefore influence the power of
a given solution to bring about a certain change in the
chemical colouring of metals. It is of importance to
consider the size of an article as well as the degree of
dilution of the bronzing agent with reference to the
time required to effect a chemical change. Two things
may be considered — a short time under the influence of
a strong solution, or a longer time under the influence
of a weak solution. The first condition is preferable,
if the desired colour can be obtained in a uniform layer,
but the second condition is found to be generally most
effective. The temperature at which the bronzing is
effected has also to be considered. If the operation can
be performed in the cold, even at the sacrifice of longer
time, it is far more convenient and desirable, especially
with large articles. There is always a difficulty in obtain-
ing vessels large enough to hold articles of great size
when the solutions have to be boiled or strongly heated.

Moreover, many bronzing solutions have a strong
corrosive influence on the materials of which the vessels
are usually made, and this destructive power increases
with a rise in temperature. Copper vessels are generally
best for hot solutions. A metallic vessel, such as
iron, will decompose some solutions, and enamelled iron

I NATURE OF BRONZING 71

pots soon wear away when in constant or frequent use.
If, therefore, the bronzing can be done at the ordinary
temperature, earthenware vessels may be employed, which
are the best whenever their use is admissible.

§ 39. It is advisable to use simple salts for solutions,
if they are capable of producing the desired effect, as a
mixture of two or more salts, although often necessary,
will not long remain uniform, and after several articles
have been bronzed will begin to produce a lighter tone
of colour. Of course this is not always a disadvantage,
as in some cases the lighter colour may be preferable to
a darker one, and consequently some solutions are more
valuable after they have been worked some time. In
all cases, whether a solution contains a single salt or a
mixture of several salts, it will become exhausted after a
time and require renewing by the addition of fresh salts.

It is generally better to dissolve a salt before adding it
to an old solution than to add it in the solid state, or con-
siderable time may be lost while waiting for it to completely
dissolve. It often happens that a solid salt added to an
old solution will fall to the bottom and become covered
with the insoluble mud, which will prevent its being dis-
solved unless the solution is constantly agitated and heated ;
sometimes it is necessary to raise the whole bulk of solu-
tion to the boiling point before complete solution occurs,
while the salt itself may be fairly soluble in a compara-
tively small quantity of clean boiling water.

As previously stated, the number of colours a metal
is of itself capable of assuming will largely depend on
the colour of its various compounds with oxygen, sulphur,
etc., and it will greatly assist the operator in his work
if he knows the number and nature of the different

72 METAL-COLOURING AND BRONZING part i

coloured compounds which a given metal forms, and the
ordinary chemical means of producing them. Take iron,
for example; we know that there is a black and a red
oxide of iron, each being produced by oxidation — one at
a high temperature and the other at a low temperature.
We also know that the red oxide is modified in colour tone,
from a deep red through various shades of reddish-brown to
brown, and finally to a dark brownish-black, by varying
degrees of heat, and by a partial deoxidation, if another
body is present which has a strong affinity for oxygen.

Copper also forms two different oxides — viz. red and
black oxides, according to the degree of oxidation, and
this power of being coloured by oxidation is continued
in a modified degree in many of its alloys. On the other
hand, the metal zinc has few available coloured compounds,
and a direct bronzing of zinc is therefore difficult by
any chemical process, and it becomes advisable to asso-
ciate it with a metal, such as copper, which possesses
coloured compounds, either by alloying or by electro-
deposition, in order to impart to it a bronze tone.

But other conditions are necessary for bronzing besides
the production of coloured compounds on the surface. The
colours should be beautiful and artistic, firm, uniform, and
consistent with the metallic character. Many beautiful
colours are obtained in bronzing solutions which, if the
deposits were firmly adherent to the metal, would be very
valuable, but unfortunately they are swilled off by the
wash-water to the great disappointment of the experi-
menter. Another annoyance which one often encounters
is the non- uniform character of the coloured surface.
But this, in many cases, can be remedied by the scratch-
brush and polishing bob, and by another immersion.

PAET II

PRELIMINARY TREATMENT OF METALS

FOR COLOURING

A. CLEANING

§ 40. In the first place, it is absolutely essential that any
article or portion of an article to be bronzed by chemical
means should be perfectly clean, as any dirt, grease, or
oxide stains, although invisible to the naked eye before
the body is placed in the pickling solution, will be at once
revealed after the process, showing up as ugly stains.
After cleaning, the article should not be touched with the
hands, or exposed to air any longer than necessary before
being suspended in the bronzing solution, or the surface
will be altered and the subsequent bronzing defective.

The article should be cleansed from dirt by washing
with water and brushing with fine sand, pumice, whit-
ing, etc., so as to remove all matter simply adhering to the
surface. Grease and fatty matter as well as lacquer on
old work may be best removed by boiling in a hot solu-
tion of caustic potash or soda, contained in a cast-iron
pot. The articles should be suspended by means of wire
previous to immersion to avoid handling as much as

74 METAL-COLOURING AND BRONZING pabt

possible. After boiling for some time they should be
removed, and if not perfectly clean it may be necessary to
scour with fine sand, swill in water, and again suspend. in
the solution.

Small articles may be freed from grease by dipping
in benzine or paraffin. The best plan is to have three
vessels containing the cleansing liquid. The first is used
for dipping the articles so as to remove the greater
portion of the grease by frequent dipping in and taking
out to examine. The second dip is more pure and is
used for removing the remainder of the dirt. The third
or clean dip is used for removing the last traces that may
not have been completely removed in the second bath.
The article after removal from the third bath is left in the
air to dry. The first and second dipping liquid may very
conveniently be paraffin, and the third benzine. It should
be observed that these liquids are very inflammable, and
therefore must be removed from the vicinity of a naked
light, especially in the case of benzine.

Large and bulky articles, such as copper, brass, iron,
and bronze goods, are best cleansed from grease in boiling
potash or soda solution. One lb. 6i potash to 1 gallon of
water is a convenient quantity to make a strong solution.
It should be borne in mind that in boiling this solution the
water only evaporates, leaving the remainder in a more
concentrated state, so that the solution should be frequently
made up with water, thus keeping the solution at a con-
stant amount.

Zinc and tin articles may also be cleansed in potash
solution, but care is required, as these metals are some-
what soluble in this liquid, and the fine lines of the
pattern are liable to be removed. The best plan to adopt

II CLEANING 75

in the case of zinc is to allow it to remain only a short
time in the solution, then to take it out and scrub it with
a brush which has been dipped in fine wet sand. Care
must be taken to thoroughly swill the cleaned articles in
two or three, lots of wash-water so as to remove every
trace of potash, especially with zinc and tin articles. More-
over, the articles must be kept immersed in perfectly clean
water until they are ready for the acid dip and bronzing
bath, as they will tarnish if exposed to the air. Caustic
potash or soda solution lasts a long time, and when it is
partially exhausted, may be renewed by adding fresh
portions of the solid salt.

Another liquid in constant use for cleansing purposes
is a solution of potassium cyanide. Potash and soda
solution themselves act as bronzing liquids in some cases,
and most metals are coloured more or less when immersed
in them, especially when the solutions have been in use for
some time. This is not always a defect, but occasionally
it is important that the articles should leave the dipping
liquid perfectly uncoloured, and then a solution of potas-
sium cyanide is valuable. About 1 lb. of good potassium
cyanide dissolved in 1 gallon of water makes a solution of
convenient strength.

Besides dirt and grease which may be removed by the
methods just enumerated, articles are often coated with a
film of some firmly adherent chemical compound which
can only be removed by means of an acid, and therefore is
not. soluble in water or the liquids already described.

§41. (1) Copper and its Alloys. — Copper, brass,
bronze, etc., become oxidised in ordinary moist air, and, in
consequence of the simultaneous presence of carbonic acid,
become gradually converted into carbonates. In fact, the

76 METAL-COLOURING AND BRONZING PAM

brownish -black to bluish -green deposit, often seen on
copper, brass, and bronze goods, is a mixture of oxide
and carbonate of copper, mixed with oxygen compounds
of zinc or tin respectively when the copper is present as
an alloy of these metals. Sulphur compounds are often
formed on the surface, when the above metals are exposed
for some time to the atmosphere of large towns or rooms
where coal-gas is burned.

These films may be removed by immersion in suitable
acid dips. For this purpose a series of liquids is used :
pickle or spent aquafortis is very generally employed for a
preliminary dip. The articles are allowed to remain in it
until the scale of oxide has disappeared, leaving, after
rinsing, a uniform metallic lustre. Dipping in old aqua-
fortis is recommended for two reasons : it economises the
cost of new acid, and, as its action is slow, it prevents the
too rapid corrosion of the cleansed copper during the time
of the solution of the protoxide. A dipping liquid con-
sisting of a mixture of 64 parts commercial sulphuric aoid,
32 parts of aquafortis, 1 part hydrochloric acid, and 64
parts water is also used.

§ 42. Copper, brass, bronze, German silver, etc., are often
cleaned by heating them to dull redness, and then plung-
ing into dilute sulphuric acid. (Those having solder upon
them are not heated thus; neither are articles of cast-
bronze, because they would be liable to crack.) They are
then soaked in old aquafortis, until, after rinsing, they
look uniformly metallic; they may then be dipped in
strong aquafortis for a few seconds and swilled. The
straw-coloured aquafortis acts the best; the white
variety acts too feebly, and the red too strongly. It is
best to use the dips cold, and to have a considerable bulk

n CLEANING 77

of Hquid to prevent them becoming too hot by the
immersion of the heated metal.

In diluting strong sulphuric acid with water a consider-
able amount of heat is generated by the chemical action
which takes place between them, and if the mixing is
made too rapidly the vessel which contains the mixture
is liable to be cracked and the liquid to be projected on
to the operator. The acid should always be poured into
the water and not vice verm.

To dip gilding metal bright. Immerse it in weak aqua-
fortis until there is a black scale formed, then dip it in
strong pickle for a few minutes (N.B. — Strong pickle is
partially exhausted aquafortis; weak pickle is the same
diluted with the washings), then dip it quickly into aqua-
fortis, then into several wash-waters in succession.

There are various mixtures which may be employed
for imparting a bright lustre to brass, German silver, etc.,
by dipping ; the following is one of them : — 1 measure
of nearly exhausted aquafortis, 2 of water, and 6 of
hydrochloric acid ; the articles should be immersed in it
for a few minutes, or until, after washing off the black mud
which entirely covers them, they look bright; they are
then cleaned and dipped again. It is convenient for remov-
ing the sand, etc., which adheres to castings. Large articles
may remain in this bath for twenty to thirty minutes.

§ 43. Dipping in Aquafortis, Common Salt, and Soot.
— Brass and similar articles, after cleaning in pickle, are
rinsed in water, well shaken and drained, then dipped in
a bath consisting of 100 parts nitric acid, 1 part of
common salt, and 1 part of calcined soot. This mixture
attacks the metal with great energy, and therefore it
should only remain in it a few seconds. The volume of

78 METAL-COLOURING AND BRONZING pakt

acid should be twenty times that of the articles immersed
in it to prevent undue heating and too rapid weakening
of the acid. When removed, the articles should be
quickly rinsed in water to prevent the production of
nitrous fumes. They then present a fine lustre varying
from red to golden-yellow and greenish-yellow, according
to the composition of the alloy.

If the metal is not swilled in water after removing from
the acid, there rises on its surface a green froth, and nitrous
vapours are given off which indicate the decomposition of
the acid with which the metal is covered. When the
vapours have disappeared the metal remains dull black,
even after swilling. This last mode of operating, called
blacking by aquafortis, is used by some coiourers to give
a dull dark colour to brass and bronze work. Aquafortis
is spent when its action on copper alloys is too slow, and
when the objects removed from the bath are covered with
a bluish-white film. Such acid is termed " pickle," and is
used for the preliminary cleaning, or for forming what is
termed a whitening bath. Very good aquafortis may
appear too weak and cleanse imperfectly by dipping when
the temperature is too high, or when it is too low, as in
the case of frosty weather.

§ 44. Whitening Bath. — This consists of old aqua-
fortis, sulphuric acid, common salt, and soot. Pour into
a stoneware vessel a certain quantity of old aquafortis, and
add twice the volume of commercial sulphuric acid.
Allow the mixture to stand till the next day. The copper
nitrate of the old aquafortis is converted into copper
sulphate, which crystallises against the sides of the vessel.
Decant the clear liquid into another vessel and add 2 to 3
per cent of common salt, and an equal quantity of calcined

II CLEANING 79

soot This mixture is less active than the acids used for
a bright lustre. The bath may be strengthened when
necessary by the addition of aquafortis and sulphuric
acid.

§ 45. Another dipping liquid may be made with equal
parts of aquafortis and sulphuric acid mixed with forty
times their bulk of watdr and allowed to cool, then adding
a quantity of common salt equal to about one-fifth that of
the strong acid present. Or the following may be used : —

1£ lbs. nitric acid,
2 lbs. sulphuric acid,
10 grains common salt.

To the above ingredients add a mixture of the following
if a dead surface is desired : —

1 lb. nitric acid,
4 lb. strong sulphuric acid,
5 grains common salt,
20 grains zinc sulphate.

The longer the articles remain in this dip the deader will
be the surface. They are then thoroughly swilled and
dried as quickly as possible. Or previous to swilling with
water they may be momentarily dipped in the bright
dipping liquid.

Mr. Aitkin says that "dead dipping" was discovered in
the following way : — In the year 1832 a dipper in the
employ of Mr. David Malins of Birmingham left through-
out the night a quantity of articles in the pickle, and
when he attempted to produce the bright appearance in
the bright dip they presented a dull frosted yellow surface.
Charmed with the effect, certain portions were burnished
and the whole lacquered. Acting on the accidental hint,

80 METAL-COLOURING AND BRONZING part

dead dipping was originated, and has now become the
recognised mode of finish for brass work generally.

Another liquid for dead dipping made may be made
of—

1 volume of a concentrated solution of potassium bichromate,

2 volumes of concentrated hydrochloric acid.

The articles should be left in this solution for some hours,
then well swilled in several wash-waters. If, however,
they are left exposed to the air for some time without
lacquering or further treatment they become coated with
a film of oxide. Dead dipped articles, while waiting to be
bronzed or lacquered, may be kept from oxidising by
immersing in clean water, to which half its volume of
alcohol has been added.

In the case of copper alloys, such as brass, the surface
colour will depend not only on the original composition of
the alloy, but also on the length of time it has been
exposed to the action of the acid. The zinc is oxidised
more rapidly than the copper, so that the effect of dipping
in aquafortis or other oxidising liquid is to increase the
relative quantity of copper on the surface, and to give to
the alloy a richer appearance and a deeper colour. When
it is desired to clean very small articles and not to
appreciably alter the composition, they may be dipped in
a solution of 5 parts potassium cyanide dissolved in 95
parts of water.

If the coloured brass articles show a granular appearance
on the surface after dipping, they should be immersed for
twelve hours in a mixture of 1 volume of nitric acid,
1 volume of sulphuric acid, and 8 volumes of water.
The grayish-black deposit is washed off with water, leaving

II CLEANING 81

an agreeable moir6 appearance. The articles are next
immersed in one of the bright dips above described, then
passed through a weak solution of caustic soda, or milk of
lime, well washed in water, and dried out in sawdust.

If an article remains too long in the bright dip, after
being made dull in the dead dip, the dead lustre disappears.
If the bath for giving a bright lustre is not available, the
objects, after rinsing, may be rapidly passed through the
dead dip to remove the dulness of the lustre caused by
too long immersion. After long use the compound acids
used for bright dipping will give a dead appearance to
brasswork. For large embossed work a hot bath for dead
lustre is used, composed of — old aquafortis, 4 to 5 parts ;
sulphuric acid, 1 part; zinc sulphate, 8 to 10 per cent.
More zinc sulphate is added when required for increasing
the dulness of the lustre. The lustre, however, after
rinsing the article, and passing.it through the same bath
for one or two seconds, and well swilling, becomes clearer.

For the production of a granular appearance on brass,
etc., a mixture of one part of a saturated solution of potas-
sium bichromate in water, and two parts of concentrated
hydrochloric acid, may be employed. The metal is left for
some hours until the desired granular effect is produced. It
is then removed and well swilled with water. The opera-
tion may be considerably hastened by the aid of an electric
current, attaching the metal article to the positive pole of
the battery and using a brass plate as the cathode. The
liquid for this method may be a very dilute solution of
sulphuric and nitric acids, or of potassium bichromate
and hydrochloric acid.

§ 46. When a dipping liquid becomes nearly exhausted,
or when an article is immersed for too long a time, the

82 METAL-COLOURING AND BRONZING paet

surface assumes a dark blackish - gray appearance, or
becomes patchy in parts, as the metal is not acted upon.

The metal may be restored to its right colour by
dipping it in a solution of zinc chloride, taking out,
heating till it is dry, and washing in water. This method
will also serve for the bright dipping of brass in case it
has only a very thin film of oxide to be removed; the
metal must in this case be dipped, boiled, and well
washed.

Old aquafortis may be revived, to a certain extent, by
the addition of sulphuric acid and common salt; the
sulphuric acid decomposes the copper nitrate in it, and
also the common salt, and sets free nitric and hydrochloric
acids. Crystals of copper sulphate also form at the
bottom of the liquid. All the nitric acid may be utilised
in this way.

For dipping small articles they may be either strung on
wires of the same or similar metal, or put into a per-
forated stoneware basket and then dipped. It is best
for the suspending wires to be of the same material as the
articles, because they are then less liable to cause a stain.
It is less economical, but sometimes necessary to use
baskets of brass or copper wire cloth. Those who have
frequently to cleanse very small articles will find it advan-
tageous to employ a basket of perforated platinum foil,
which, though expensive in the first cost, will be found the
most economical in the end, as it is not acted on by single
acids.

§ 47. (2) Zinc. — When clean zinc is exposed to the air,
even at ordinary temperatures, a thin gray film of sub-
oxide of zinc soon forms on the surface, which protects
the metal beneath from further oxidation. If the metal

ii CLEANING 83

has been exposed for a long time to the atmosphere of a
large town, or to the action of impure water, the film
becomes more firmly adherent to the metal, and is composed
of other bodies than the sub-oxide. When the film is
composed only of oxide and is very thin it is very readily
dissolved in a dilute solution of sulphuric acid (15 to 20
parts water to 1 part of acid).

For the thicker and more complicated film, mentioned
above, a cold mixture of equal parts of sulphuric and
nitric acids is best. As great heat is produced by this
mixture the whole must be cooled before using. The
zinc article to be cleaned should be suspended for a second
or two in this dip by means of a wooden support, then
swilled several times with water so as to remove every
trace of acid. It is a good plan to let water finally run on
the article straight from the tap to , ensure a perfect
cleansing. The zinc should then be bright and clean.

Acid dips for zinc become gradually weaker with use,
since the oxide of zinc is dissolved, and combines with the
acid to form zinc sulphate, or a mixture of zinc sulphate
and nitrate according to the composition of the solution.
Such a liquid will still possess cleansing properties, but
will act much more slowly, and the zinc immersed in it,
instead of coming out bright, will be dull, or crystalline.
The dip may in such a case be renewed by adding a little
concentrated sulphuric acid. If the zinc article has been
coppered and it is required to clean it, it must be dipped
in aquafortis till it becomes black, then dipped in one of
the former solutions.

If it is desired to produce a dull appearance on the
surface of the zinc, it should be first dipped in dilute
sulphuric acid or a mixture of sulphuric and nitric acids

84 METAL-COLOURING AND BRONZING part

which has become nearly exhausted by use with zinc
articles, then put into a bath consisting of zinc nitrate
dissolved in very dilute nitric acid. The zinc nitrate may
easily be prepared by dissolving zinc in nitric acid until
the acid is saturated. Other zinc salts may also be used
when acidified with nitric acid.

A dipping bath which contains copper in solution from
previous operations will not suit for articles which consist
of zinc, tin, lead, antimony, bismuth, or their alloys, as these
metals cause a deposition of copper upon their surface.

For small and medium sized articles the above solutions
answer well, but with larger articles there are difficulties
to contend with in consequence of the great bulk of
solution required for immersion, and the size of vessels,
which must be made of a material capable of resisting the
action of acid liquids. For large and bulky articles a
solution of potassium-ammonium tartrate may be used.
This liquid has a weaker action on zinc than the ones
mentioned before, but dissolves its sub-oxide.

The solution is prepared by dissolving £ lb. of cream
of tartar in a pint and a half of water, adding ammonium
carbonate till all effervescence ceases, then adding another
700 grains for excess. The solution is put on the articles
with a brush, left on some time to act, then well rubbed
in with a sponge, brush, or rag, which has been dipped in a
mixture of whiting and water. Lastly, well wash with
water. If the article is exposed to the air it soon oxidises,
so that it should be plated or coloured as soon as possible
after cleaning. Small articles may be kept from oxidising
by immersion in a solution of potassium -ammonium
tartrate, but large articles, if they cannot be immersed,
should be rubbed with a clean cloth till dry.

ii CLEANING 86

§ 48. (3) Silver. — Silver is not oxidised like base metals
by contact with moist air, but in the presence of sulphuretted
hydrogen is readily coated with a film of silver sulphide,
varying from yellow to black, according to the thickness of
the film. Very often the tarnish assumes iridescent colours.
This film of sulphide may be removed in several ways.
Adhering dirt and grease is displaced as described for
copper and its alloys.

Tarnish may be removed by immersing the silver articles
in dilute sulphuric acid (1:5), or by boiling in a solution of
1 part cream of tartar and 2 parts common salt. Silver
sulphide is readily soluble in potassium cyanide, and in
sodium thiosulphate, so that when a tarnished silver article
is rubbed with a cloth which has been dipped in a solution
of either of these salts it is rendered perfectly clean. A
5 per cent solution of potassium cyanide and a 30
per cent solution of the thiosulphate is a convenient
strength for the purpose.

Small articles may be immersed in a saturated solution
of borax, in contact with a piece of zinc, when the surface
after a time becomes very clean. The dull surface may be
made bright with the scratch -brush, to be afterwards
described.

§ 49. (4) Iron and Steel. — Articles of iron and steel
after the removal of adherent dirt and grease, in the same
manner as that described for copper, etc., may be cleaned
by immersion in a mixture of lamp-black and concentrated
nitric acid, washing with water, dipping in a soda solution,
then well swilling with water, and drying out in sawdust.

For cleaning iron articles generally a cold mixture of
about twenty measures of water, and one of sulphuric acid,
is frequently used ; but a better liquid is composed of one

86 METAL-COLOURING AND BRONZING part

gallon of water, one pound of sulphuric acid, with one or
two ounces of zinc dissolved in it; to this is added
half a pound of nitric acid. This mixture leaves the iron
quite bright, whereas dilute sulphuric acid alone leaves it
black, or of a different appearance at the edges. It should
be scoured with sharp sand and brushed with a steel
scratch-brush.

For glassy patches upon cast iron (which usually con-
sist of iron silicate) hydrofluoric acid is used ; it is kept
in a bottle of gutta-percha closed by a bung of india-
rubber ; it must not be allowed to come in contact with
glass vessels, nor must the mouth of the bottle be left
open. The fumes from it are extremely dangerous to
inhale. If a drop of it falls on the hand it should be
thoroughly washed off at once, as it produces ulcers, and
causes great pain after a few hours.

Articles of iron and steel which have been cleaned
in acids, and the adhering acids washed away with water,
may be protected from rusting by continued immersion in
lime water, a solution of caustic soda, or water containing
any caustic alkali, until required.

Articles of polished steel are cleaned in a moderately
strong solution of potassium bisulphate. The article is
immersed in the solution in contact with a piece of clean
zinc. The zinc decomposes the solution with the libera-
tion of hydrogen gas, and the steel is allowed to remain
in the bath until the oxide of iron or rust is removed. Steel
may also be cleaned in a 20 per cent solution of hydro-
chloric acid.

§ 50. (5) Lead, Tin, and their Alloys. — These metals
are cleaned to remove dirt and grease, as with other metals,
by means of caustic alkali solution, and brushing with

II MECHANICAL TREATMENT 87

sand, etc. After another dip and well swilling in clean
water they are ready for the colouring process without
resorting to an acid dip.

§ 51. (6) Aluminium. — Articles of aluminium are
cleaned in very dilute solution of potash, when the sur-
face assumes a bright appearance ; wash well with warm
water and dry with a warm cloth. Aluminium alloys are
treated like copper alloys.

B. MECHANICAL TREATMENT

§ 52. If the articles are not to be bronzed while in a
crude state, which is rarely advisable, the mechanical
treatment consists of imparting to them a more uniform
surface by scratch-brushing, or a smoother and more
lustrous one by grinding and polishing. But scratch-
brushing is not only a part of the preparation of metals
for bronzing, since it is employed alternately with dipping
in the process of bringing out the desired tone of colour.
After the first dip in the bronzing solution, a uniform tint
is rarely attained, but after scratch-brushing one or more
times with alternate immersion in the solution a much
better result is achieved.

Scratch-Brushing. — Scratch-brushing may be achieved
either by hand or by means of a lathe. A scratch-brush
is merely a bundle of fine and hard brass or steel wires,
about 6 or 8 inches long, according to the nature of
the metal to be operated upon. The wires are of various
degrees of fineness, and are also annealed to different
degrees of hardness, to suit the various kinds of work.

Scratch-brushes must be carefully used, and their wires
kept in good order. When they become bent they have

$8 METAL-COLOURING AND BRONZING part

to be straightened by drawing the brush, held in a slanting
position, several times over a sharp grater such as is used
for grating nutmeg Jsor they may be beaten with a wooden
mallet upon a small, :block held between the knees so as
not to produce a dead stroke. By this means the wires
become disentangled and straightened out.

Scratch-brushing by hand is a very slow and tedious
operation, and only adopted for delicate work, or to get
into crevices which cannot be touched by a revolving
brush. The revolving scratch -brush is most generally
used. It is much more rapid in its action and more
effective. For very delicate objects scratch-brushes are
made of spun glass, the fibres of which are very fine and
elastic.

Scratch-brushes, if kept too long in water, become hard.
When greasy they are cleaned in caustic potash solution.
Oxides are removed from brass wire by means of aqua-
fortis. This latter operation may be used for diminishing
the size of the wires and making them smoother.

For making a good hand scratch-brush, choose a bundle
or coil of brass wire of the proper thickness, and wrap a
good string tightly around it for about two-thirds of its
intended length, say about 8 inches long, then with a
sharp steel chisel cut the bundle of wire close to the string
at one end, and at 2 inches from the other end of the
string wrapping. Dip the end closed by the string into a
neutral solution of zinc chloride and plunge it into a molten
alloy of tin and lead (soft solder), which solders all the
wires and prevents their separation and injury to the hand
of the operator. The brush is then fixed to a thin wooden
handle. Very small scratch -brushes are necessary for
reaching small parts and cavities. An old scratch-brush,

ii SCRATCH-BRUSHING 89

the wires of which have been bent in every direction, is
useful for rubbing the insides of certain articles such as
vases.

The ordinary circular scratch-brush consists of a round
wooden frame, into the periphery of which is placed
bundles of wires at regular intervals. A hole is left at
the centre for fixing it on to the revolving spindle of the
lathe.

For cleansing purposes a circular scratch-brush, which
the workman can readily refurnish with new bundles of
wire, is preferable. It consists of a round iron disk about
one -tenth of an inch thick and about 6 or 7 inches
in diameter, provided in the centre with a hole for fixing
on the spindle of the lathe. At the distance of from one-
fifth to one-fourth of an inch from the periphery of the
disk, holes of about one-tenth of an inch are drilled at a
distance of an eighth of an inch apart. Through these
holes are drawn bundles of wire about 4 inches long,
so that they project an equal distance on both sides. Then
the bundles are bent towards the periphery, and on each
side of the iron disk a wooden disk one quarter of an inch
thick is placed. The periphery of the wooden disk, on the
side next to the iron disk, should be turned semi-annular,
so that the wooden disks, when secured to the spindle, press
very lightly on the wire bundles, and the latter remain
very mobile.

When the compound brush is fixed on the lathe and
made to revolve rapidly, the bundles of wire, in conse-
quence of centrifugal force, stand rigid, but being mobile
will give way under strong pressure without breaking off.
It should be remembered that only the sharp points of the
wires are effective, so that by putting too much pressure

90 METAL-COLOURING AND BRONZING pari

on the brush the wires are bent without securing any
advantage as regards smoothing.

Scratch-brushing is seldom done dry; the brush and
work must be constantly wetted with a liquid which
carries away the impurities and makes the wires not too
harsh. The liquids employed to assist the brushing are
various ; such as water and vinegar, or sour wine, stale
beer, etc., or solutions of cream of tartar or alum. A
solution of sodium carbonate may also be used.

Lathe brushes are mounted on the spindle of the lathe,
and over the revolving brush is fixed a wooden box, lined
with zinc or lead, and open only at the front, which
prevents the liquid splashing on the workman. On the
top of this box is a reservoir for holding the liquid for
wetting the brush, or it may be contained in a tank fixed
to the wall, or in any convenient position above the latter.
In the top of the box and floor of the reservoir is fixed a
small pipe terminating in a tap, which is fixed over the
centre of the brush, by this means the stream of liquid
may be regulated to the desired amount. In the bottom
of the box is another hole connected with a pipe to carry
off the waste liquid. As this may be used over again
several times it is usual to have a pipe of flexible material,
and dipping into a large stone jar, from which the top
reservoir may be filled as often as necessary. In working
the lathe the top of the brush is made to revolve towards
the operator, who holds the objects to be brushed at the
bottom. After scratch-brushing every trace of the lubri-
cating liquid must be washed away before placing or
replacing the objects in the bronzing bath.

§ 53. Grinding and Polishing. — Wherever possible it is
advisable to have articles or parts of articles highly polished

II POLISHING 91

before bronzing. But polishing imparts some grease and
dirt which would prevent the bodies taking the bronze
uniformly, so that they have to be dipped in potash or
other cleansing liquid and swilled before putting into the
bronzing solution. Polishing is generally done at a lathe
set in motion by steam power.

When it is necessary to remove a very rough surface
the objects are ground on an emery wheel. This is a
wooden wheel covered with a thick strip of leather on its
edge, which is coated with emery powder of different
degrees of fineness. A much better plan, and one which
is coming into general use, is to have these wheels or
" bobs," as they are termed, made entirely of leather and
faced with emery powder. The bobs have holes in the
centre by which they are fixed on the lathe.

Polishing bobs are made from walrus or hippopotamus
hide in a great variety of sizes to suit all kinds of work;
some are made from bull-neck leather, felt, etc. Three
different grades of emery are generally used — No. 60 to 80
for preparatory grinding ; No. 120 for fine grinding; and
flour emery for imparting a finish. The bobs thus coated
are termed respectively, " roughing," " medium," and fine
bobs. In most cases a circular revolving bristle brush may
be substituted for the last grinding, the articles being
moistened with a paste of oil and fine emery.

Care must be taken not to execute the brushing or the
fine grinding in the same direction as the previous grinding,
but in a direction at right angles to it. The coating of
the roughing wheel with emery is effected by applying
glue and then rolling it in the dry emery powder. When
the first coat is dry a second is applied, and finally a third.
The wheel is then dried in a warm place. Before use the

92 METAL-COLOURING AND BRONZING part

revolving bob is greased with tallow and the inequalities
removed by pressing a smooth stone against it. The
rough grinding is generally done dry, but in fine grinding
the objects are often moistened with a mixture of oil,
tallow, and emery, of the same degree of fineness as that
on the bob.

A polishing lathe consists essentially of an iron frame or
support with two sockets or bearings at the top, in which
works the revolving spindle which carries the brushes. A
usual form of spindle has two pulleys between the bearings
arranged at its centre (fast and loose) and the two free ends
screwed taper-wise so as to fit various sized holes in the
bobs, and to keep them always rigid, since the motion tends
to make them travel towards the wider parts of the screws,
and the more the pressure the tighter they become. Such
a lathe may be driven by power or by manual labour.
The former is usually employed, and for large work is
absolutely necessary. The spindle is made to revolve at
very high speeds. The lathe just described is arranged
for two bobs, one at each end of the spindle, so that two
men can work at the same lathe at the same time.

§ 54. Polishing as distinguished from grinding is
employed to impart a highly smooth surface and brilliant
lustre to goods before plating and bronzing, and also in
some cases to give a final finish after these operations. The
bobs are made of a large number of circular pieces of felt
or calico from 9 to 12 inches in diameter placed together,
having a hole in the centre to fit the spindle of the lathe
on which the bob has to be fixed. A circular piece of stout
leather or metal about 3 or 4 inches diameter, with a
similar hole at the centre, is fixed to each side of the bob ;
these serve to bind the pieces firmly together by driving

ii POLISHING 93

four pins through the whole, each pin having a head on one
side and riveted on the other. These bobs vary in width
from 3 to 5 inches. It is very important that a bob
should be properly centred, otherwise it will not revolve
circularly.

When a new bob is made it is necessary to dress
the rough edges of the disks of cloth by making the
bob revolve on a lathe and holding a sharp knife against
the edge in a fixed position ; this also helps to make the
balance true by shearing off any inequalities.

Two of these bobs are employed to produce a final
polish on brass and metal generally. The first one is used
with oil and powdered rottenstone, or the bob may be
greased with a little tallow and then " tripoli " rubbed on.
This is termed the " grease " bob. The second one is
known as the rouge or finishing bob. A piece of lime is
first held against the revolving bob and then a bar of
rouge, when the articles may be finally polished. The
lime prevents the rouge sticking in parts and enables the
metal to be uniformly brightened. Some polishers use a
paste of powdered rouge mixed with water, and a very
little oil instead of the bar rouge, which is a mixture of
rouge, melted wax, and a little tallow, cast into moulds
with the use of strong pressure.

paet in

CHEMICAL METAL-COLOURING

§ 55. By this term is understood the production of a
thin but firm and compact coating on metals by chemical
means. As already stated, the metals differ in colour
among themselves, and a gfeat variety of shades may be
obtained by suitable alloying, so that the deposition of a
metal or alloy upon another metal is, strictly speaking,
metal - colouring ; but if the whole subject of electro-
deposition of metals were to be included in the present
work, it would go beyond the intention of the author and
the legitimate scope of the subject, especially as there are
several excellent manuals of electro-metallurgy already in
existence, from which abundant information relating to
that branch of art may be obtained.

But a work on metal - colouring would be very in-
complete without embracing certain methods of metal-
deposition, because the two subjects are so intimately
associated that one cannot be treated without some
reference to the other. It frequently happens that
electrical action is an important aid to the production
of a certain colour, and in all cases where such an aid is
useful it will be fully described. For example, a very thin

part in COLOURING OF COPPER 95

coating of nickel on a brass article will modify the colour
when it is put into a bronzing solution.

COLOURING OF COPPER

§ 56. The colours ordinarily produced on copper articles
by means of bronzing agents chiefly depend on the forma-
tion of oxygen and sulphur compounds, by which the
surface is changed into cuprous oxide CugO, cupric oxide
CuO, or cupric sulphide CuS respectively.

Cuprous oxide is capable of existing in different modi-
fications as regards colour, producing various shades, from
bright red to brownish-black, according to its physical
condition and the thickness of the deposited film, which,
as is well known, alters the particular shade of colour.
Cupric oxide, according to its thickness, varies from brown
to brownish-black. Whatever the natural colour of the
compound deposited or produced on a metal, the tone will
be modified by the colour and physical condition of the
base, except when the film of the deposit is of such a
thickness as to completely hide the base, or of such an
opaque character as to completely obscure it.

Bronzes, like pigments, vary as to opacity and trans-
parency, and it will readily be understood that a transparent
or semi-transparent film on a metal such as brass will
produce a very different effect to a similar film imposed on
copper. Advantage is therefore taken of these facts to
produce different colours with the same chemical reagent.
In like manner sulphur compounds may be made to
assume various shades of colour when deposited on copper
and its alloys, varying from a pale brownish-yellow to
brown and brownish-black, and also from grayish-black to

96 METAL-COLOURING AND BRONZING part

a blue-black. If now we employ a combination of these
bodies (oxides and sulphides) it will at once be understood
that a still greater variety of tints may be obtained.
Copper therefore may be coloured from a pale bronze tone
to a deep brownish -black or deep bluish -black by the
above-mentioned agents.

Copper is also coloured by the action of basic copper
carbonate, or basic copper chloride, by which the antique
patina is produced in different shades of green. There is
no other metal which, by the agency of its own compounds,
can be so variously and so easily coloured as copper if the
conditions of its combination with oxygen, sulphur, car-
bonic acid, and chlorine are properly understood. Copper
is gradually oxidised, even at the ordinary 'temperature,
either in dry or in moist air, if carbonic acid is present in
sufficient quantity. If carbonic acid is absent, and the
temperature of the air is suitably raised, or if the copper
is heated, then combination of copper and oxygen occurs.

If copper is heated to redness it becomes first pale
yellow, then red, then violet, by the formation of cuprous
oxide Cu 2 0, and finally receives a black film, which on
the outside is cupric oxide CuO, but on the inner surface,
which is in contact with the copper, a deep red film of
cuprous oxide is produced. When a certain thickness is
attained, this scale falls off, especially if the heated copper
is plunged into cold water. Copper can therefore be
coloured by heating, due to the formation of one or both
of the above oxides.

§ 57. Buchner states that by suitably operating on
copper, so as to produce layers of oxide of different degrees
of thickness, the well-known colours of thin plates are
exhibited in definite order, each colour varying with the

in COLOURING OF COPPER 97

wave-length of the light. Steel and cast iron, when
treated in a similar manner, show the colours yellow, red,
violet, and blue to sea-green. Five bands of colours are
produced. In the first band are pale, clear, and dark
yellows, light blue and sea-green. In the other bands
the colours are nearly the same. In the last colour scale,
produced at the highest temperature, red and green are
distinctly brought out. These colour bands are attributed
to interference, as in the case of Newton's rings, which are
formed when a lens of small curvature is pressed on a
plane parallel plate. It is important for the action in
this instance that the colour bands exhibit different
breadths. Experience has shown that a colour which
appears lighter in the air is more permanent, and furnishes
a wider band in the colour scale.

The colour depends on the degree of temperature and
on the duration of the operation, so that with a high
temperature and a short exposure the same results may be
achieved as by using a lower temperature and a long
exposure. The latter, however, is generally preferable,
as the film of oxide is more firm and durable. There
seems to be a certain limit of temperature, which cannot
be exceeded, for the production of every shade of colour.

But air contains bodies other than oxygen, and when
articles of copper or its alloys are exposed to moist air the
carbonic acid and oxygen both combine to form basic
copper carbonate (often incorrectly called verdigris),
termed antique bronze. In general it may be stated that
the colours will be more beautiful the longer the articles
have been exposed to atmospheric influences.

Concerning the antique bronze two different kinds
exist — (a) The one produced on copper in air, as men-

98 METAL-COLOURING AND BRONZING part

tioned above, and having a bluish-green colour; (5) the
other produced on copper articles which have been buried I
in the earth in contact with saline bodies, by which a
yellowish-green patina or incrustation is formed, consisting
generally of basic copper chloride, especially when the
copper has been deeply buried.

Copper articles may be coloured easily and rapidly by
heating with certain compounds, such as iron oxide.
They may also be bronzed quickly by immersion in certain
acid, alkaline, and neutral solutions, such as copper sul-
phate, copper nitrate, etc. The colouring will also in
these cases depend on the temperature, strength of the
solution, and time of exposure or immersion.

VARIOUS SHADES OF BROWN COLOUR ON

COPPER

COLOURING COPPER BY DRY METHODS

§ 58. This method of colouring copper is largely
employed, the general view being that oxygen contain-
ing bodies, when applied to copper in the form of a paste,
and the whole heated, effect a chemical change whereby
some of the oxygen of the applied oxide is given up to
the copper with the production of the lower oxide of
copper, which, as is well known, has a red colour. 1 It has
been stated before that this oxide is capable of existing
in a variety of modifications as regards colour, so that
numerous shades may be obtained by its production,
varying from bright red to dark brown.

Jt should be remembered* what has been said with
regard to the necessity of having the articles to be bronzed

1 See § 71, p. 112.

in COLOURING OF COPPER 99

scrupulously clean and as highly polished as possible,
because on this necessary polish will the uniformity of the
colour largely depend.

§ 59. Treatment with Iron Oxide at the ordinary Tem-
perature. — (a) Keichardt states that when highly polished
copper articles are rubbed for some time with a piece of
leather and very finely powdered red iron oxide a high
lustre and a dark brown colour result, due to the chemical
action occurring between the iron oxide and copper. One
part of oxygen leaves the iron oxide and goes to the
copper, forming cuprous oxide, which, by the constant
rubbing, passes into the pores of the metal. That this
action occurs is open to very considerable doubt. I have
not been able to produce the effect in this way. Possibly
by uninterrupted rubbing for a considerable period of
time a moderate change might take place, but no one
would trouble to expend such enormous labour for such
an insignificant result when the same end may be achieved
much more effectually with comparatively little labour,
and in a small fraction of the time, by help of a moderate
amount of heat. It may be taken as settled that no good
result has ever been achieved by the action of oxide of
iron on copper at the ordinary temperature, as the author
has proved by a long series of experiments, in some of
which the bodies were allowed to remain in contact with
each other for ten to twelve days with but very little
change taking place. All the experiments were quite
barren of good results, the only consolation being found
in the assurance that no satisfactory bronzing is possible
by this means; and it is hoped the experience may be
useful in preventing others wasting valuable time in the
effort to attain a good colour by this means.

100 METAL-COLOURING AND BRONZING pabt

(b) The following method among others was adopted
in the above-mentioned experiments. Five parts of the
red oxide were thoroughly incorporated with 8 parts of
graphite, and the mixture made into a paste with a little
alcohol : —

5 parts red iron oxide (FeaOa).
8 ,, graphite.

A thick layer of this paste was put upon the copper
articles, which were allowed to remain untouched for
from one to twelve days. The paste was then removed
with a woollen cloth and the copper well brushed with
a moderately soft brush. The experiments were complete
failures.

(c) The experiments were then tried with the addition
of heat. The articles were covered with the paste of iron
oxide, graphite, and alcohol, and allowed to remain on a
hot iron plate at about the temperature of a lacquering
stove for a time varying from five minutes to an hour or
more, and then the paste removed and the object brushed
as before. The articles were coloured very unequally, and
only after repeated operations with the same article could
any thing like uniformity be achieved.

(d) By well rubbing copper with wax or fat and heating,
then rubbing it off with a rag, and well brushing, a nut-
brown colour is said to be obtained. Here again a great
many repetitions of the process on the same metal were
tried, but with no good results. Doubtless, if any one
cared to repeat the process a sufficient number of times
some sort of colour would be obtained, depending on the
nature and continuance of the heating. At the best the
method is only suitable for small articles, and these are

in COLOURING OF COPPER 101

just the things which can be done better and more rapidly
in other ways. 1

(e) The following process has also been recommended for
giving a uniform colour on copper : — Warm the copper to
about 100° C, i.e. to the temperature of boiling water, rub
on it a little moist rouge, and brush with a plate brush
until it receives a uniform colour. Repeat the process
until the desired colour is attained. The iron oxide is
removed from the surface of the copper by pouring over it
a saturated solution of copper acetate, and the metal is
then dried by rubbing with a woollen cloth. This process
is very defective, as it requires an enormous expenditure
of labour for very little result, and is open to the same
objection as the preceding one.

(/) A good result is obtained in the following way,
according to Langbein, by which shades varying from
light to dark brown may be obtained : — Take two parts
of red iron oxide and one part of blacklead, make them
into a paste with water or alcohol, paint the article with
the mixture, place it in an oven or on a hot plate. Remove
the baked powder when thoroughly dry and well brush
the metal as before described. The darker the desired
colour the longer must the metal be heated and the higher
must be the temperature. If the colour is not deep enough,
or, what is very likely, not completely uniform, the opera-
tion must be repeated. The metal should be finally rubbed
with a little alcohol and brushed with a soft brush till
dry. A finish may be given by brushing with a soft brush
which has been previously rubbed on pure wax. The oxi-
dising paste must not be too thin, especially when the
higher temperatures are employed. It is always safer to
heat for some time at about 100° to 150° C. than to

1 See Appendix.

102 METAL-COLOURING AND BRONZING part

employ a higher temperature, as the higher the tempera-
ture the less uniform will the colour be likely to be.

(g) The following experiments were performed to test
the value of this method : — (a) A piece sheet copper, (6)
an electro-coppered ash-tray, (c) an article made of com-
mercial cast copper, were covered with a thick paste
and heated on the plate of a lacquering stove for some
time. They were then cleaned with a soft brush and
alcohol, then finished with a waxed brush; (a) became
shellac-brown in colour, (6) chocolate-brown, (c) a lighter
brown. The operation was repeated several times. Electro-
lytic copper takes the colour readily under the above treat-
ment, cast copper less readily, and sheet copper very
slowly. As a rule, the more quickly the desired shade of
colour appears with moderate heating the more beautiful
it is.

§ 60. A bronzing paste is said to be made with a
mixture of copper tartrate, red iron oxide, bone-ash, and
water. The article to be bronzed must be coated with a
thick layer of the paste, then heated over a fire till dry,
and brushed with a soft brush exactly as directed in the
previous method. The operations must be repeated to
obtain a uniform colouring. The proportions recom-
mended are —

1 part of red iron oxide.

2 parts of a mixture of the other ingredients.

The heating must be done very uniformly or the metal
will be of a darker colour in the parts most strongly
heated. In our experiments electro -deposited copper
received a dark brown colour, which became darker with a
repetition of the process. Cast copper took a light brown.
With sheet copper no good result was obtained. The

in COLOURING OF COPPER 103

method is very inferior to the preceding one, and cannot
be recommended.

§ 61. English Method. — Red iron oxide is made into
a paste with water to the consistence of cream, and this is
painted over the article with an ordinary paint-brush. It is
then heated over a fire, or, better still, in a muffle, or on a
hot iron plate at a moderate temperature, until the paste
is thoroughly dry. After allowing to cool the powder is
removed with a soft brush (a painter's dust-brush) and
then, if not a finished article, it is beaten or hammered.
Or the copper article with its coating of iron oxide is held
over a wood fire into which some pieces of coal are placed.
By means of the smoke a deposit forms on the article, and
when this soot begins to be dissipated by the heat the
metal is considered to be sufficiently heated. It is very
important to keep it at a uniform temperature, and not to
allow it to get too hot, or the bronzing will appear patchy,
and the metal require to be thoroughly cleaned again and
the operation repeated.

On testing this method with a stamped - brass tray
which had previously been electro-coppered a good nut-
brown bronze colour was obtained. Another similar tray,
with a much thinner coating of copper submitted to the
process, gave unsatisfactory results. In succeeding experi-
ments with stamped and cast work, with good coatings of
copper, results equally good with the first were obtained.
The cast work took a paler shade of colour. Some were
coloured a deep gold colour, others were of a brick-red
shade. These results were probably due to insufficient
heating.

From these experiments it may be inferred that red
iron oxide is quite sufficient to effect the desired change

104 METAL-COLOURING AND BRONZING pakt

of colour without the addition of graphite and other
bodies. They also showed that the article must be
thoroughly clean and polished as highly as possible before
the application of the bronzing paste, that a sufficiently
high temperature is required, and that all parts of the
object must be uniformly heated. As a general rule two
or three repetitions with scratch-brushing between is much
more effectual than one operation alone. They prove what
we have frequently observed with immersion processes also,
that a too thin coating of copper is soon corroded and
removed, leaving a surface of brass in places and vitiating
the result. This method is one of the very best for pro-
ducing a dark brown colour on electrotypes. Haldane
states that in preparing medals for the Melbourne Exhibi-
tion, a rich chocolate colour was obtained by the addition
of a little copper acetate, mixed with an alkaline sulphide,
to the ordinary iron oxide bronzing paste by which a film
of mixed copper oxide and sulphide was produced. 1

§ 62. Chinese Method. — According to several authorities
the Chinese use a mixture of —

2 parts copper acetate.
2 „ cinnabar (HgS).
5 ,, ammonium chloride.
5 ,, alum.

The salts must be finely powdered and made into a
paste with water or vinegar. This is pasted over the
copper article in the usual way, and then heated for some
time over a fire or on a hot plate. In case of large
hollow vessels glowing coals are placed inside to raise
them to the requisite temperature. After cooling, the
vessel is well washed, dried, a fresh coating put on, and

1 See Appendix.

in COLOURING OP COPPER 105

the operation repeated until the desired colour is obtained.
The addition of some copper sulphate is said to make the
colour darker, and borax to make it more yellow.

As the result of a number of experiments with this
mixture in the above proportions we found that the mercury
sulphide was decomposed, producing a black deposit which
could not be completely removed, and on washing off the
powder before heating, the copper was observed in every
case to be coated with a deposit of mercury. The metal
was eaten away so much that the design was partly
destroyed and no useful colour was attained. Therefore
the method, using the ingredients in the proportions men-
tioned, is quite worthless. It was thought that the
failure was due to excess of mercury sulphide, but on
making the mixture very dilute, and also by reducing the
quantity of mercury sulphide to one- tenth, the same un-
satisfactory results were obtained.

§ 63. The following mixture is recommended by Lang-
bein for the production of a "brown colour : —

1 part potassium nitrate.

1 ,, sodium chloride.

2 parts ammonium chloride.
1 part of liquid ammonia.

95 parts of vinegar.

The solution is painted on to the article, allowed to dry, and
the article then vigorously brushed. Or the brushing may
commence when the article is still wet and continued until
it is dry.

No good results were achieved by the author with
this mixture, although the experiments were repeated
several times, and the proportion of the ingredients were
varied, as well as the methods of applying them to the

106 METAL-COLOURING AND BRONZING pari

work. Electro-deposited copper, cast copper, and sheet
copper were immersed in the solution, and, without rinsing,
were put on a hot plate, and left there until a brown
colour appeared ; but the surface was disfigured with black
patches. The processes were repeated seven or eight
times without any better results. Simple pickling had
no appreciable effect. The best colour was obtained by
dipping in the solution, and then heating on a hot plate
without previous swilling. No good colour was secured
by heating the article before immersing in the solution.
When the work was pickled and heated on the hot
plate, then reimmersed in the solution, the brown colour
produced by the heating disappeared. Scratch-brushing
does not appear to facilitate the process.

Langbein states that a warmer tone is produced by the
method adopted at the Paris Mint, which is as follows : —
Powder and mix intimately equal parts of copper acetate
and ammonium chloride. Take a large tablespoonful of
this mixture, boil it with water in a copper kettle for
about twenty minutes, and pour off the clear fluid.
To give copper a bronze-like colour with this fluid, pour
part of it into a copper pan, place the objects separately
in it upon pieces of wood or glass so that they do not
touch each other, or come in contact with the copper pan,
and boil them in the liquid for a quarter of an hour.
Then take the objects from the solution, rub them dry
with a linen cloth and brush them with a waxed brush to
finish. This would more correctly be described under the
head of wet methods, but it has been thought advisable to
introduce it here to show the uselessness of a complicated
mixture, when a much simpler one is quite as effectual.
Tn the author's experiments with this method, copper

in COLOURING OF COPPER 107

articles were coloured terra-cotta. On heating they

assumed a deep reddish-brown. Much better methods

are given under the head of Wet Colouring.

§ 64. Dienst states that a very beautiful brown colour,

unaltered by moderate heat, is obtained from the following

mixture : —

1 part by weight of neutral copper acetate.
1 ,, ,, „ ammonium chloride.
250 parts „ „ water.

The above salts are dissolved in the water, and the copper
articles are painted with the solution. They are then
heated over a coal fire until the green colour turns to a
copper colour; then besmeared again with the solution,
which should, however, be more dilute than in the first
coating; heated again over a fire and the operation re-
peated as often as necessary till a uniform colour of the
desired dark brown shade is obtained.

In testing this method a light brown colour was ob-
tained, but not very uniform or of a good tone. The
colour also blackens by the action of sunlight. '

It is stated that if the copper to be bronzed has been
tinned the tin does not go into the solution, but after five
or six repetitions the copper receives a brassy colour,
which, after three or more repetitions of smearing and
heating, changes to a splendid yellow. If it is desired to
produce a brown colour the solution must not be painted
on while hot, and with twenty to twenty-five repetitions
a pale brown colour is obtained. When the desired colour
is reached the metal is washed in clean water, dried quickly,
and polished with waxed brush. The bronzed metal is
then held over a low fire to make the colour deeper and
more durable.

108 METAL-COLOURING AND BRONZING part

§ 65. A very beautiful and pleasing colour of a light
brown shade may be quickly produced by a mixture of —

1 part copper sulphate.
1 ,, zinc chloride.
1 „ water.

The above forms a paste which is applied to the article,
and allowed to dry on it. It is then well washed with
water, when a uniform colour is obtained. This would be
one of the most valuable colours if it were permanent,
but unfortunately it is changed by the action of light to a
dark green, almost black. This change also occurs when
the bronze is coated with a film of transparent lacquer, and
although we have tried several methods of preventing the
change, no suitable remedy has yet been discovered.

§ 66. The following mixture forms an excellent bronz-
ing solution, and may be used for copper articles with or
without subsequent heating : — Take

120 grains of copper sulphate.
i pint of water.

Boil the water until the copper sulphate is dissolved,
neutralise the solution with sodium hydrate, then add 150
grains of red iron oxide. Now immerse the copper articles
in the turbid solution, take out and heat on hot plate.
Kepeat the operation until a reddish-brown colour with a
violet tint is obtained. The violet tint can only be
observed in full daylight. Electro-deposited copper re-
ceived a full and rich reddish-brown colour, and cast
copper a lighter brown. These beautiful colours become
darker in tone by more heating.

This solution gives good results both on copper and brass
by immersion only, without the addition of heat, but the

in COLOURING OF COPPER 109

colour is not of that deep rich tone which is secured by subse-
quent heating. The best effects are obtained after the process
has been repeated once or twice. The results achieved by
heating over a charcoal fire were found to be most effective.
§ 67. A solution similar to the former one, with the
omission of iron oxide, produces very good colouring
effects, but the colours are paler in tone. Take

120 grains of copper sulphate.
9 fluid ounces of water.

Boil and add strong sodium hydrate solution until a pre
cipitate sufficient to render the solution turbid is formed.
Immerse the articles and treat them as in former method.
§ 68. Another bronzing paste which is said to give very
good colours, of a pale to dark brown shade, is made by
mixing basic copper sulphate with water and coating the
article with a thick layer, then heating as usual. The mix-
ture for testing the method was made in the following way :
— (a) 100 grains of copper sulphate were dissolved in water
and copper hydrate precipitated by the addition of a
solution of sodium hydrate ; (6) the copper hydrate preci-
pitate was filtered off and mixed with 100 grains of copper
sulphate and made into a paste with water. The paste
was thickly smeared over the work, which was then heated
over a charcoal fire. Electro-deposited copper received a
light brown or dark chestnut-brown colour according to
the degree of heat. Better effects were obtained by repeat-
ing the process once or twice. The residue left on the
work after heating is somewhat difficult to remove, and it
is difficult to obtain a perfectly uniform colour over the
whole surface. The method given in § 66 is much pre-
ferable to this one.

110 METAL-COLOURING AND BRONZING pabt

§ 69. Walcker recommends the following mixture which
has been tried by the author with the results recorded
below : —

Ammonium carbonate . . . 300 grains.
Copper sulphate . . . . 150 „
Vinegar 5J fluid ounces.

Evaporate the solution to dryness, and then add —

Vinegar 5J fluid ounces.

Oxalic acid 4 grains.

Ammonium chloride . . . 16 ,,

The mixture is boiled and the residue filtered off.
The article is warmed and the solution painted on the
work. Then boiling water is poured over it, and the
surface rubbed, first with an oiled pad, then with a dry
pad.

(a) Brass ash-tray, oiled and wiped dry, as recommended above.

(b) ,, „ dried in sawdust only.
(e) Coppered ,, oiled and wiped dry.
(d) „ ,, dried in sawdust only.

The above trays were immersed in the filtrate only.

It was found that immersion was much better than
painting on the solution. The wiping with oil removes
some of the bronze colour, unless done very softly indeed.

The colour produced on brass is distinctly yellowish-green,
while the colour imparted to copper is similar to that
produced by copper cloride (§ 73), and blackens by exposure
to light.

(a) darkens slightly in diffused daylight; (6) appears
not to change ; (c) darkens slowly, probably because the
bronze has been largely wiped off by the oiled rag ; (d)
darkens very quickly in daylight.

in COLOURING OF COPPER 111

The residue left on the filter was rubbed into a paste
with a few drops of the filtrate, and painted on the work
to be bronzed, which was then heated on a lacquering
stove. After the paste was thoroughly dried it was
washed off with boiling water. Brass received the same
colour as by the immersion method, but the colour was
not quite so uniform. Copper assumed a deep brown
colour, which became darker on exposure to light. This
process might do for work that is too large for the
immersion method.

§ 70. A method similar to the above has also been
recommended, using a mixture of—

Ammonium chloride . . . 460 grains.
Potassium oxalate .... 150 ,,
Vinegar 9 fluid ounces.

The above salts are to be dissolved in the vinegar, and
the metal to be immersed in it, or the solution painted on
the warmed article, using a soft brush. It may also be
rubbed on with a soft linen rag.

We found that the colour produced on copper after the
lapse of a considerable time was the same as that given by
copper chloride solution. The liquid is quite inopera-
tive at first, but it becomes green after a time by dis-
solving some of the copper, so that it would save time
and trouble by adding some soluble copper salt at the
commencement. Kubbing the solution on to the work
appears to hasten the reaction, but the method is very
tedious. But dipping the work in the solution, draining
off the excess, drying on stove, rinsing in water, and
drying out in sawdust, is a quicker method than rubbing
only with wet and dry pads respectively. Neither of the

112 METAL-COLOURING AND BRONZING part

methods is, however, effective till the solution becomes
charged with copper, as stated above.

§ 71. From observations made during the various
experiments on bronzing with a paste of oxidising material,
such as iron oxide, the author felt convinced that the
usual explanation was not the correct one. Most
authorities state that when copper is covered with a paste
of red iron oxide and moderately heated (say to 150° C),
that the red oxide Fe 2 3 changes to the black oxide
Fe 8 4 , giving up oxygen which unites with the copper,
forming the red copper oxide CugO. For this reason
graphite is usually mixed with the iron oxide to effect the
reduction.

It is well known that red iron oxide prepared by
decomposing iron oxalate is converted into the black
oxide by heating to 350° C. in a current of hydrogen.
Carbon monoxide will produce the same effect, but
requires considerable time. At a higher temperature
solid carbon can effect the same change, but the action
is exceedingly slow. Hence it may safely be inferred that
no oxidation of copper by iron oxide at the temperature
admissible for bronzing can take place.

In order to show that the oxidation of copper, when
coated with a paste, does not proceed from the paste
itself but from the oxygen of the air, a paste was made
with finely powdered fluor-spar and water, with which a
coppered article was covered in the usual way. This was
then heated till the paste was well dried (about fifteen
minutes), then it was washed off and the copper was
found to be darkened. The article was scratch-brushed,
and again covered with the paste and reheated. When
the paste was removed a beautiful brown colour was

in COLOURING OF COPPER 113

found on the surface exactly as when the iron oxide paste
was used. This shows that the paste simply acts
mechanically in limiting the supply of oxygen to the
copper, with the consequent formation of the red oxide,
or a mixture of the red and black oxides, in which the
former predominates.

LIGHT BROWN COLOURS ON COPPER
Colouring Copper by the Wet Way

§ 72. This method of colouring metals presents many
advantages over the dry way, both as regards economy of
time and uniformity of results, since the articles are com-
pletely immersed in the same medium, practically at the
same temperature in all parts, and equally exposed to the
same influences, always provided that the metal is made
scrupulously clean in every part before being suspended in
the bronzing solution.

Any inequalities as regards the physical condition of
the plain parts will not be removed, but rather intensified
by the operation, so that such parts should be highly
polished and scratch-brushed to remove this unevenness
before the final cleansing. With every precaution it often
happens that inequalities are revealed after the first dip,
but these are generally removed by a good scratch-brush-
ing after well swilling in water.

It might be supposed that the stronger the solution
the more effectual it would be in effecting a desired
change of colour, and in some cases this is true, since the
chemical action is retarded by diluting the solution. But
there are other considerations to be kept in view. It

114 METAL-COLOURING AND BRONZING paet

must be understood that a bronzing solution is, in its very
nature, corrosive, and if the corrosive action is too rapid,
not only is the colour patchy, but the metal is pitted
with holes, due to the unequal action of the salts on the
surface of the metal or alloy. Moreover, when a solution
is too concentrated the salts crystallise on the surface of
the suspended article and become baked on, as it were,
by the heat, and make it exceedingly difficult to remove
them without much scratching, which destroys the coloured
surface, and necessitates beginning the operation over
again.

It may be taken as a rule that dilute solutions are
necessary, not only to prevent undue corrosion of the
metal, but also to keep the salt in solution by the excess
of the solvent, which is generally water. Strong solutions
give a deeper tone than weaker ones, so that for the
lighter tones of colour dilute solutions are necessary.
It is sometimes necessary to pour the solution over an
object when the latter is too large to be suspended in the
bath, and in such a case it is still more important not to
have the liquid too strong, as the tendency of the salt to
crystallise on the work is greater in consequence of the
cooling effect of the air and of the article. In such cases
a dilute solution poured over the body several times is
much better than a saturated solution poured over it fewer
times. In bronzing, as in many other things, " slow and
sure" is an excellent motto. Of course the solution
may be made so dilute as to have no effect on the metal,
in which case more of the salt must be dissolved in
it; or a solution may have been used so much as to
become exhausted, in which case a fresh liquid should be
prepared.

in WET COLOURING 115

The beginner, however, should satisfy himself that the
solution is exhausted before throwing it away. It
frequently happens, especially when boiling solutions are
used, that the salt is deposited at the bottom of the
vessel in consequence of the evaporation of the water
or other solvent, a sufficient amount of that liquid not
being present to keep the bronzing agent in solution.
In such a case more water should be added and the
contents of the vessel well stirred while the liquid is
kept simmering.

When a solution is being made or an old one being
revived care should be taken to see that all soluble
matter is dissolved, and that the liquid is allowed to
stand a sufficient length of time afterwards to enable the
insoluble suspended matter to settle, when the clear
liquid should be poured into a clean vessel for use. It is
sometimes necessary to resort to filtration, as certain
ingredients are mixed together so as to form an insoluble
precipitate, which requires to be removed from the
soluble portion. Sometimes the precipitate is the sub-
stance required for use, but more often it is the solution
which is most valuable. In either case they may be
separated by filtration.

Several methods are adopted for separations of this
kind. A bag of very fine muslin or calico is often
sufficient, but the best way, when the precipitate is very
fine, is to use a funnel, in which is placed white blotting-
paper or filter-paper, which is specially prepared for the
purpose, and may be obtained at chemists' shops.

Vinegar, which may be considered as impure acetic
acid, is a solvent frequently used in making bronzing
solutions. But commercial vinegar is often contaminated

116 METAL-COLOURING AND BRONZING part

with hydrochloric and sulphuric acids, and in the presence
of these bodies bronzing often becomes impossible, or
only partially successful, so that vinegar free from these
ingredients should, if possible, be used. If mineral acids
are added to vinegar, even in small quantity, an equivalent
quantity of acetic acid is displaced. One part of sulphuric
acid added to one thousand parts of vinegar is lawful, and
is added with the idea that its presence prevents putrefac-
tion, although it has been proved that the notion is
erroneous. The presence of hydrochloric acid is rarer.
English proof vinegar contains 6 per cent of acetic acid.
It is better to work with a dilute solution of acetic acid
containing 5 or 6 per cent of- pure acetic acid, and
thus avoid the defects arising from the use of adulterated
vinegar.

In workshops and places where bronzing is conducted
on a large scale the solutions should be placed and used
under a hood connected with a chimney, so as to carry off
the acid and other fumes. This is important for two
reasons — (a) from a sanitary point of view, the fumes
from many solutions being injurious when inhaled by the
workman ; (b) any finished work standing in the shop is
very liable to be affected, and specks produced on the
surface.

It is also necessary to work with clean vessels. When
a vessel which has been used for a certain solution is re-
quired for one of another kind, much inconvenience and
disappointment will be avoided by taking care to remove
every trace of dirt from it before using it to prepare or
to hold another solution.

If common water is used it should be examined to see
that it is of good quality and free from many impurities

hi WET COLOURING 117

with which ordinary water is often contaminated. Spring
and well water often contain considerable quantities of
lime, magnesia, common salt, iron salts, etc., the presence
of which may cause various kinds of separations in the
bronzing baths. On the other hand, river water is
frequently impregnated with so much organic matter that
its employment without purification cannot be recom-
mended.

Rain water, or better, distilled water, is the best for
the preparation of solutions. However, rain water
collected from metal roofs and gutters exposed to various
fumes arising from works in large manufacturing towns
should only be used with caution. Bain water should be
caught in vessels of earthenware or wood free from tannin,
and filtered. Where river or well water has to be used,
thorough boiling and filtering before use is generally
necessary in order to separate the carbonates of alkaline
earths held in solution.

Another important factor is the purity of the chemicals
employed for bronzing. Many failures arise from inatten-
tion to this point Attention has already been drawn to
the necessity of purity in vinegar, and many instances
might be cited of imperfect results in consequence of using
impure salts. Take caustic potash for example, it is
likely to be contaminated with sulphuric and hydrochloric
acids among other impurities. Ammonia may contain
chloride, sulphate, and carbonate of ammonium. Acetates
are liable to contain sulphates, etc.

§ 73. Before proceeding to give definite instructions for
the preparation and mode of applying bronzing solutions
suitable for copper goods, it will be well to discuss the
chemical changes likely to occur in the various processes,

118 METAL-COLOURING AND BRONZING part

for it is as important for the operator to know what to
avoid as to know what to search for.

It has been already explained that copper forms two
compounds with oxygen, viz. Cu 2 0, which is red, and is
termed cuprous oxide ; and CuO, which is black, and is
termed cupric oxide. Also copper forms two compounds
with' chlorine, viz. cuprous chloride ChigClg, and cupric
chloride CuC^ of various shades from white to brown.
The same metal unites also with sulphur forming two
compounds varying from brown to a deep bluish-black,
termed respectively cuprous sulphide Cu 2 S, and cupric
sulphide CuS. There is also a pentasulphide, said to be
formed by the action of potassium pentasulphide on
copper sulphate, and having the formula CuS 5 . Certain
salts of copper, such as the carbonate and acetate, produce
colours varying from pale to deep greens and bluish-
greens.

Carlemann observed that a plate of copper exposed to
chlorine gas till it becomes yellow, blue, red, yellow, and
finally blue again, or treated with a solution of cuprous
chloride in hydrochloric acid till it assumes a light brown
tint, becomes so sensitive to light that photographs may
be taken upon it, the parts on which the light falls turning
black. The photographs, if kept in the dark, remain dis-
tinct for months, but ultimately fade.

According to Priwoznik, a solution of cuprous chloride
does not produce this effect unless it has been partly con-
verted into cupric chloride by the action of the air. He
finds also that the best way to make a copper-plate sensitive
to light is to dip it in a neutral and only slightly diluted
solution of cupric chloride, and thinks it very likely that
the sensitiveness is due to a thin film of cuprous chloride,

in WET COLOURING 119

this compound in the moist state being known to become
coloured by the action of light.

Alloys of copper with silver, manganese, and zinc, may
also be rendered sensitive to light by similar treatment,
but in a lower degree than pure copper ; in fact, this differ-
ence of behaviour may be used as a ready means of testing
the purity of copper. 1

The action referred to in the above paragraph had been
observed by the author before his attention had been
directed to the work of the experimenters there mentioned,
and in a large number of experiments with various solu-
tions, in which a soluble chloride was present, the same
light brown colour was obtained which invariably blackened
on exposure to daylight, although gaslight appears to have
no action.

Many unsuccessful attempts have also been made to
fix the colour, which is one of the most beautiful and
valuable, from an artistic point of view, in the whole
range of metal colouring. Brass does not receive a light
brown but a brownish to slaty drab, and with some solu-
tions a dark neutral green. If, after swilling and drying
out in sawdust, the coloured brass is heated to about
130° C, the surface film appears to fuse and form the
beautiful light brown shade characteristic of coloured
copper. If the heat be continued the colour disappears,
and the brass is coated with a uniform deposit of copper.
Also, if the brass article is removed from the source of
heat as soon as the glaze appears, the light brown colour
remains, and is but very slowly changed when exposed to
daylight. In some cases no change occurs. If, however, the
article be scratch-brushed a coating of copper is revealed.

1 Ding. pol. J., ccxxi. 38.

120 METAL-COLOURING AND BRONZING fart

In this way brass may be as completely and uniformly
coppered as by electro-deposition, but with this difference,
that in the former case the coating of copper is exceed-
ingly thin, while in the latter the thickness may be made
to vary with the length of time it is exposed to the voltaic
action. A brass article may be coppered by means of a
solution of ferric chloride without any copper salt being
present, but the result is not nearly so good. If, after
bronzing a brass article in any of the before-mentioned
solutions, it is scratch-brushed without heating, no copper-
ing is seen, but simply a brass surface.

§ 74. Priwoznik makes the following observations con-
cerning the action of various acids and salts on copper : —

When copper is immersed for a short time in a boiling
solution of ammonium chloride, or in acetic acid, little or no
action occurs, but after continued immersion a change occurs.
When placed in a boiling solution of copper acetate it
takes after a time a poor pale colour, but the solution is
quite useless for effecting a good brown bronze suitable
for such articles as medals. Hence neither of the above-
mentioned liquids alone is suitable for bronzing. When
ammonium chloride and copper acetate are mixed together
a bluish -green deposit of hydrated copper oxy chloride
(CuCl 2 , 3CuO, 4^0) is produced.

Copper oxychloride cannot be used alone, but when
ammonium acetate and acetic acid are added the oxy-
chloride partly dissolves, and the solution formed may be
used for bronzing copper. The insoluble residue should
be filtered off and the clear solution only employed. It is
better to dilute it with water and not to use it in the
concentrated state.

For making a suitable bronzing solution with the last-

m WET COLOURING 121

mentioned materials Priwoznik dissolves in 110 cubic
centimetres (4 ounces) of vinegar about 17 grammes
(260 grains) of precipitated copper oxychloride, by means
of heat As soon as the greater part of the excess of
acetic acid is driven off, as shown by the crystalline
deposit of green copper acetate on the surface, the solu-
tion is diluted with 4300 cubic centimetres (152 ounces)
of water, then 12*7 grammes (190 grains) of ammonium
chloride is added, and after a time the undissolved
portion is filtered off. If the solution has not been
boiled the filtrate will become muddy and prevent a
uniform deposit. The liquid should contain about '118
per cent of copper.

The colouring is effected chiefly by the formation of a
film of cuprous oxide. This may be illustrated by the
following experiments. Moisten a bronzed medal with
a very dilute solution of sulphuric acid, and the moistened
part will be blackened, because the cuprous oxide is
changed to cupric oxide. If a portion of an alkaline
solution of copper containing grape sugar is heated a
precipitate of cuprous oxide is obtained. When this
precipitate is collected, dried, and heated in a porcelain
crucible, it becomes first brown, and then black, in conse-
quence of its conversion to cupric oxide by means of the
oxygen of- the air. The same thing occurs when copper
articles which have been bronzed with cuprous oxide are
heated on an iron plate.

Reference has been made to the effect of light in
producing changes in a bronze from a light to a darker
tone. The most complete and most rapid change appears
to occur on copper which has been bronzed in a copper
chloride solution. From this it may be inferred that

122 METAL-COLOURING AND BRONZING part

metallic copper acts on cupric chloride, forming a thin

film of cuprous chloride, as shown by the following

equation : —

Cu+CuCl 2 =Cu a Cl9.

On exposure to light, even when the atmosphere has
been excluded by a film of transparent lacquer, some
change occurs which produces a dark coloured compound.
It may be that the ChigClg is decomposed by the light
into CuClg. By heating the copper which has been
blackened by light to a temperature of 130° C. the
original colour is restored.

Not only does copper undergo these actinic changes
by the action of chlorine, but also its alloys, such as brass,
though in a less degree. Some solutions containing chlor-
ine impart to brass a beautiful greenish-yellow colour,
which is but feebly susceptible to light, or only changes
after a considerable time. The colour imparted will also
depend on the composition of the metal. Pure copper
will be coloured in a different manner to commercial copper,
which may contain from 1 to 2 per cent of impurities,
and the colour produced on brass will not only depend on
the amount of copper present, but also on the impurities,
i.e. on bodies other than copper and zinc, such as lead for
example.

A vessel containing caustic potash or soda is generally
kept boiling in a bronzing shop in order to remove grease
from work before colouring. In time this becomes im-
pregnated with metallic and other matters which have
been dissolved or mechanically removed during the
cleaning, and these foreign bodies, together with the
potash itself, continue to discolour the work after the
grease has been removed.

m WET COLOURING 123

This coating is frequently iridescent, and becomes very
firmly adherent to the metal, making it very difficult to
be removed. In some cases this does not interfere with
the subsequent colouring, but in others it does, in which
case the work should have a previous dip in a solution
of potassium cyanide. Also after articles have been
dipped in aquafortis and then left for some time in the
swill water a similar coloured and iridescent sheen will
appear, but often darker than with potash.

An identical effect was produced as follows : — A box
lid was dipped bright, rinsed in water, and placed on the
top of a lacquering stove at the maximum temperature.
The surface soon tarnished (became darker), then in a
few minutes assumed a brassy hue, after which it be-
came darker again, and iridescent brown and red films
extended over the entire surface of the article, which was
then removed from the stove and allowed to cool. A
still longer exposure on the hot plate produced a dark
reddish-brown, the relief parts remaining more or less
bright

By pickling in a strong boiling potash solution before
heating upon the stove, the surface of an ordinary cast
copper article was uniformly tarnished. When the same
was heated in the manner described above, the entire
surface became uniformly coloured. Another lid of cast
copper which had been pickling in a solution of potassium
permanganate for two days was uniformly tarnished, and
on similarly heating on the stove assumed a darker
reddish -brown colour. A brass figure pickled in aqua-
fortis, swilled, and heated on the stove, did not assume
any pleasing effect. A coppered cast -brass figure of a
child heated on the stove for half an hour was covered

124 METAL-COLOURING AND BRONZING part

with iridescent green and crimson tints upon a brassy-
brown ground. A coppered ash-tray simply became
tarnished without showing any pleasing colour tints.

From these experiments it may be concluded that the
effects in each case were due to the same cause, viz.
oxidation of the metal by the combined means of the
oxidising dip and the oxygen of the atmosphese.

§ 75. The following solutions are given to show the
effect of chlorides or mixtures, in which a soluble chloride
is one of the essential constituents : —

Cupric chloride 400 grains.

Water ...... 4 ounces.

Dissolve the copper chloride in the water, raise to the
boiling point and immerse the previously well cleaned
copper articles for a few minutes. If the copper is pure,
or nearly pure, a very beautiful rich reddish-brown colour
will be obtained, but with ordinary commercial copper
containing 1 to 2 per cent of impurities the colour
obtained is nearly black.

On heating pure copper, after colouring in this solu-
tion, a fusion appears to occur upon the surface of the
heated metal, which assumes a paler colour than before
heating. These colours, with or without heating, blacken
when exposed to daylight, although gaslight has no
influence in changing the colour. It was thought at
first that this action was due to oxidation by the air,
but the same reaction occurred after a coloured article
had been coated with transparent lacquer. The bluish-
black colour produced by this actinic reaction is a very
excellent colour, and we have used it very effectively in
bronzing a certain part of an object which was required

in COLOURING OF COPPER 125

to be black while the main portion was bronzed a
lighter colour. This solution may therefore be very
useful in such cases, or where an article is required to be
finally coated with a bright black colour.

A number of experiments were performed in the
author's laboratory to discover the cause of the above-
mentioned change, and to find some means of arresting
it so as to secure the most beautiful colour produced by
copper chloride. We found that with copper bronzed in
copper chloride the coating was a chloride, or a chloride
mixed with some other compound of copper, for on
treating the coloured body with ammonia till the coating
was dissolved, and analysing the solution obtained,
abundant evidence of chlorine was observed. Plates of
copper were bronzed in this solution, and used for
printing from photographic negatives. The pictures
obtained were swilled with very dilute ammonia water so
as to fix them, but they gradually faded.

A series of experiments were performed to ascertain
whether the colour was due to a deposit superimposed
on the copper, or due to corrosion of the surface of the
article. It was found that the copper loses weight. The
following results will show the amount of loss in each
case: —

(a) Weight of coppered ash-tray .... 30*721
„ ,, ,, after bronzing . 30*587

Loss = '134

(6) Weight of brass ash-tray .... 13*1413
,, „ „ after bronzing . . 12*8893

Loss = '2520

126 METAL-COLOURING AND BRONZING pabi

„ „ ,, after heating on stove 30*579

Loss = *008

(d) Weight of brass ash-tray after bronzing . . 12*889

„ ,, ,, after heating on stove 12*899

Gain = *010

(«) Weight of bronzed and heated brass ash-tray 12*8995

,, „ after scratch-brushing 12*8700

Loss = -0295

(/) Weight of bronzed and heated coppered ash-
tray 30*5795

Weight of bronzed and heated coppered ash-
tray after scratch-brushing . . . 30*5380

Loss = '0415

Summary of the above results —

Copper 80*721

,, after bronzing 80*587

„ „ heating 80*579

„ ,, scratch-brushing .... 30*538

Brass 13*1413

,, after bronzing ...... 12*8895

,, ,, heating 12*899

,, ,, scratch-brushing .... 12*870

The following results were obtained with ash-trays
bronzed in a solution of definite strength. One part by
weight of copper chloride was dissolved in four parts of
water : —

in COLOURING OF COPPER 127

Coppered ash-tray

after bronzing .

»»

tt tt

heating

,, „ scratch-brushing

Brass ash-tray

„ after bronzing .

it ,, heating .

scratch-brushing

9} II

23*5115
23*4830
23*4800
23 4445

127445
12-5805
12-5840
12-5470

§ 76. Similar colours to those produced on brass and
copper by copper chloride may be obtained when ferric
chloride is also added. The following were tried : —

Cupric chloride 30 grains.

Ferric chloride . . . . . 30 ,,
Water 5 ounces.

The work required scratch-brushing and re-immersing
to secure uniformity. Another solution was made with
half the quantity of water, but with the same result as
before. Another series of experiments was tried with a
solution of

Cupric chloride 250 grains.

Ferric chloride ..... 250 ,,
Water 10 ounces.

A coppered ash-tray received a reddish-brown colour as
usual, and a casting of ordinary copper acquired a chestnut-
brown. In this case the darker colour was doubtless due
to the zinc, which is added to the copper in small quantity
so as to produce sound castings. 1

We have often observed with other solutions that while
pure copper, such as is used for electrical work, receives a
light brown colour, ordinary commercial copper turns

1 See author's work on Mixed Metals t p. 75. Macmillan and Co.

128 METAL-COLOURING AND BRONZING pari

almost black. The above solution was found to work
best at a temperature of 50° to 60° C. It should be
remembered that ferric chloride freely dissolves copper
and brass when heated with them. Brass which has been
bronzed in this solution has its coating decomposed by
heat with the formation of a coating of copper, as in the
case before mentioned, with copper chloride alone.

§ 77. Experiments were performed with a mixture of
copper sulphate and ferric chloride. Although in this
mixture no copper chloride is added at first, it is formed
when the above salts are dissolved in water. The follow-
ing proportions were employed : —

Copper sulphate (saturated solution), 16 ounces.
Ferric chloride (1 of salt to 4 of water), 1 ounce.

(a) Stamped brass tray, electro-coppered . . light brown.

(b) „ ,, ,, heated after

pickling lighter brown.

In each case the surface blackened on exposure to day-
light. If the work after being darkened by daylight is
heated on a stove the black disappears, and the original
colour, slightly altered in tone, is reproduced. This is
again blackened on exposure to light, but the black colour
has a blue tint as distinguished from the jet-black pro-
duced by the first exposure.

§ 78. The following solution was tried : —

Copper sulphate solution (1 of salt to 4 of water) 4 ounces.
Common salt 400 grains.

(a) Stamped ash-tray, electro-coppered . brown colour, then

white.

(b) Caat copper-plate .... greenish-gray.

in COLOURING OF COPPER 129

The brown colour of (a) turned white in the solution after
a time, and after a day or two became green. The
greenish-gray colour of (6) also turned to green in the
same time. The above solution appears to be unsuitable
for bronzing. If the metal after turning white is swilled
and dried out in sawdust the colour is partly removed.

§ 79. Wuttig states that the usual antique bronze colour
may be imparted to copper by a mixture of ammonium
chloride, cream of tartar, common salt, and copper nitrate
dissolved in water. He says " the pickle produces in a
short time a very durable coating of oxide. The cream of
tartar may be replaced by vinegar and the copper salt
omitted after the copper has been once stained. With
too much common salt the metal becomes yellower, with
too little a bluish tint prevails." The following solution
was employed by the author : —

Ammonium chloride .
Cream of tartar (pure)
Common salt
Copper nitrate .
Water

(a) Electro-coppered brass ash-tray
(&) » i) »

8 grains.

24 „

48 „

480 „

5 fluid ounces.

rich orange colour.

a >>

(a) was simply pickled in the hot solution; (6) was.
pickled in the hot solution, swilled, dried out in sawdust,
and heated on a hot plate, when it assumed a light nut-
brown. The effects were the same as described under the
head of copper chloride solution. Both blackened on
exposure to daylight.

A similar solution to the above, but with the omission
of the common salt, was tried, and although the same

180 METAL-COLOURING AND BRONZING pakt

effects were eventually obtained they took a much longer
time to produce.

§ 80. The action of copper acetate in combination with
a soluble chloride is said to give good results as a bronzing
agent for copper. The following solution was tested : —

Copper acetate . . . . 180 grains.
Ammonium chloride . . 60 ,,

Water 20 fluid ounces.

Some small brass ash-trays were electro-coppered and
immersed in the hot solution. Those which had a thin
coating of copper assumed a darker colour than those which
had a strong coating.

(a) Ash-tray well coppered . . . light red ochre colour.
(6) ,, with thin coating of copper brown, darker than (a).

\W >> >i i> it »i

{d) Brass ash-tray .... yellowish-brown.

The tray marked (c) was coppered by pickling in copper
chloride solution and then heating on the stove, by which
method the coating is always exceedingly thin but per-
fectly uniform. All these colours became darker on
exposure to daylight.

§ 81. Copper acetate and ammonium chloride dis-
solved in vinegar and water, and containing red iron
oxide in suspension, has been recommended for bronzing.
The following proportions were tried : —

Copper acetate 180 grains.

Iron oxide 180 ,,

Ammonium chloride . . . . 60 ,,

Vinegar 4 fluid ounces.

Water ,,♦,,, 16 „

Ill

COLOURING OP COPPER

131

(a) Coppered ash-tray

bright light brown,
light red ochre.

(6) was pickled in the hot solution, dried out in sawdust,
and Ideated on the lacquering stove, when it assumed the
above-mentioned colour ; (a) was simply pickled, swilled,
and dried out.

The following solution was also tried : —

Copper acetate .
Iron oxide
Ammonium chloride
Water

180 grains.
180 „

60 „

20 fluid ounces.

The results were much the same as in the former solution,
but the colours were darker and with a less brilliant
lustre. The bronzed trays were exposed to full daylight
during a clear February morning. One tray, which was
simply pickled, darkened a little; another, which was
heated after pickling, appeared to be unchanged. They
would both gradually blacken if exposed to the direct rays
of the sun in summer.

§ 82. A solution composed of copper sulphate and
copper chloride and water gave the following results : —

Copper sulphate
,, chloride
Water

400 grains.
400 „
4 fluid ounces.

light orange,
brownish-black.

(a) Electro-deposited copper
(6) High conductivity copper
(e) Commercial copper

The metals were pickled in the hot solution, swilled, and
dried out in sawdust, when they had the above appearance.
They all became black after exposure to daylight. On

132

METAL-COLOURING AND BRONZING

PAKT

repeating the experiments, but with a subsequent heating
after pickling, the colours were darker, but the surfaces
not uniform.

§ 83. Coppered ash-trays were immersed in a hot
solution composed of the following ingredients :—

Copper chloride and ammonium chlor-
ide (CuCl 2 , 2AmCl) .
Water

500 grains.
5 ounces.

The trays assumed a beautiful reddish -brown colour,
which soon turned to black on exposure to daylight. On
strongly heating the trays, after bronzing, until the
surface blackens, and then allowing to cool, a fine
purplish-red colour appears, but the coating rubs off easily
and exposes a copper surface. By coating the surface with
transparent lacquer (zapon) without previously touching,
the red colour becomes fixed, but does not appear so
dense after the lacquering process as before.

A series of experiments with this solution showed that
it works better cold than when hot. A freshly prepared
solution imparts a richer colour to electro -deposited
copper than it does after some time of working.

§ 84. The following complex mixture is given by
Haldane for bronzing copper. The proportions given
below were tried—

Copper acetate

5 grains.

,, sulphate .

. • 20 „

Common salt

■ . 7 „

Sulphur

' ' U »

Acetic acid .

10 cubic centimetres,

Water

5 fluid ounces.

in COLOURING OF COPPER 133

A coppered ash-tray immersed in the hot solution received
a terra -cotta colour, which afterwards became nearly
black on exposure to daylight. A plate of commercial
copper acquired a steely-copper bronze.

During the working of this solution a black precipitate
of copper sulphide separates out, causing the solution to
become paler in colour. This precipitate clings to the
edges of the work and blackens it, but not very much.
No advantage was obtained by heating the work previous
to immersion.

§ 85. A solution, consisting of copper chloride and tar-
taric acid dissolved in water, and sodium hydrate added,
has been recommended for bronzing. The following pro-
portions were used, with the results given below : —

Copper chloride solution . 1 fluid ounce (1 of salt to

4 of water).

Tartaric acid ... 75 grains dissolved in

water.

Sodium hydrate solution . 1J fluid ounces (1 of

alkali to 5 of water).

(a) Coppered ash-tray . . yellowish-brown colour,

(ft) A copper electrotype .

>> )»

Both darkened on exposure to daylight. A brass tray
immersed in the hot solution first assumed a golden
yellow, then deep gold, nut-brown, and yellowish-brown.
If immersed longer the ordinary brass colour reappears.

§ 86. According to Bottger, copper can be bronzed a
yellowish-brown colour by immersion in a solution of
ammonium nitrate and potassium chloride. By heating
the bronzed article after swilling and drying, it assumes
a dark reddish-brown colour.

134 METAL-COLOURING AND BRONZING pah*

A saturated solution of the above salts in equal pro-
portions was tried by the author and used boiling.
Copper articles were boiled in the solution, even until the
salts began to crystallise upon them, without any perceptible
change occurring on the copper. The work was also
immersed in the solution, and dried upon the hot stove
without previous rinsing, but no useful effect was pro-
duced. The work was then rubbed with the concentrated
solution by means of a cloth, without any noticeable
change of colour. The solution appears to be quite
worthless as a bronzing agent either for copper or brass.

§ 87. Kayser recommends the following solution : —

AmmoDia 20 grammes.

Acetio acid in slight excess of the ammonia.
Ammonium chloride . . . 10 ,,

Water 1 litre.

By painting this solution on to the work, allowing it
to dry, and then rubbing, the copper assumes a brown
tone. The operation must be repeated several times.
This solution was tested in the following ways : —
The work was pickled during a long time in the hot
solution, dried gently on the hot stove, and then rubbed.
The operation was repeated several times, according to
Kayser's instructions, without any useful effect After a
considerable time of working the solution became green
from the presence of dissolved copper, and then acted
slightly on the work.

§ 88. The following solution has been recommended
for bronzing copper, but we found it to be practically
useless both for copper and brass : —

186

METAL-COLOURING AND BRONZING

PART

Copper nitrate, neutral solution containing 100 salt

900 water.

II.

M M II

ii 200 „

800 „

III.

Calcium chloride „ „

ti 100 „

900 „

IV.

ii ii ii

ii 200 „

800 „

Mercury chloride „ „

t> 60 „

950 „

VI.

Tin chloride „ „

ii 100 „

900 „

Mix equal parts of I. or II. with III. or IV. according as
a darker or lighter shade of colour is required. That good
colours may be obtained by the above recipes is shown by
the results of the preceding experiments with similar
solutions in which a soluble chloride is present, but they
suffer from the same great defect, viz. the alterability on
exposure to light, causing them to assume very dark and
irregular shades of colour.

§ 91. The various solutions which have been described
in the preceding pages owe their virtue chiefly to the
formation of a chemical compound, such as an oxide, a
chloride, etc., and the deposition, or rather formation of
the same on the surface of the copper immersed in it.

It has also been proved that in every case where a
chloride was employed that the colour deepened consider-
ably in tone by exposure to daylight, which greatly limits
the usefulness of such solutions. There is one chloride,
however, which is not subject to variation by the action of
light, but the shade is blackish, and altogether different to
the beautiful light browns produced by certain other soluble
chlorides. This difference is due to the deposition of a
thin film of metal on the copper instead of a metallic
compound. The following results have been obtained by
the use of platinum chloride solutions : —

Thoroughly clean the work in the usual way, and
then immerse it in a solution of

Platinum chloride
Water

1 grain.
500 grains.

in COLOURING OF COPPER 137

When the article has become tarnished all over then
transfer it to a stronger solution containing

Platinum chloride .... 2£ grains.

Water , 500 „

Allow the work to remain in the solution until the requisite
depth of colour has been attained. Buchner recommends to
previously cleanse the work by pickling it in a solution of
cream of tartar, but this does not appear to be of any ad-
vantage if the articles are properly cleaned in the usual way.

(a) Coppered ash-tray, after scratch-brushing . light steel-gray,

with a reddish
tint.

(ft) „ „ „ . light steel-gray,

with a reddish
tint, but more

opaque.

(d) Brass tray ,, „ . lighter than (c).

The tray (a) was done in the weaker solution only, and
scratch-brushed. The tray (6) was done in both solutions
as directed above. The colouring in each case is due to
the deposition of a thin dark film of metallic platinum.
The stronger solution gives a more dense deposit than the
first one, and scratch-brushing in each case renders the
colour paler.

Moreover, as the above is a case of deposition of
platinum by simple immersion, one would not expect the
colour on brass to differ much from that produced on
copper, and that such is the fact is proved by the result
of experiment (d). Any difference observed must be due
to the coating of platinum being so thin as to be
semi-transparent, and allow the underlying metal to be
seen somewhat through it ; this is notably the case with

138 METAL-COLOURING AND BRONZING pabt

the platinised copper tray (a), which was well scratch-
brushed after immersion in the bronzing solution.

Leaving the work too long in the solution is wasteful,
as, beyond a certain limit, all the platinum deposited is in
pulverulent state, and easily rubs off, either in drying
out in sawdust, or under the action of the scratch-brush.
Heating the work after bronzing in a platinum solution
does not appear to improve it, and if the article is made
too hot the uniformity of the colour is destroyed.

The following proportions were also tried with the
results recorded below : —

Platinum chloride 1 part.

Water 5 parts.

Copper and brass work were painted with this solution
with the production of a dark gray deposit. On evapo-
rating the solution to dryness and redissolving the residue
in alcohol the following results were obtained : —

(a) Brass tray . . black at front, platinated at back.
(6) Coppered tray . black.

The tray (a) was scratch-brushed at back only ; the tray
(b) was not scratch-brushed. It appears from these experi-
ments that if a black colour is desired the platinum chloride
should be dissolved in alcohol. The deposit is more firmly
adherent to brass than to copper.

§ 92. The following solutions, although containing
ammonium chloride, are chiefly effective on account of
the sulphur compound with which it is associated. The
colours obtained are darker than with chlorides alone,
since the sulphides tend to produce copper sulphide, which
is black, but the presence of the chloride modifies the dark
tone, producing a compound colour of a reddish tint

in COLOURING OF COPPER 139

Potassium sulphide .... 30 grains.

Ammonium chloride . . . 90 ,,

Water 5 fluid ounces.

If the work to be coloured is large and unsuitable for the
immersion process then the solution may be applied with a
sponge, and the work allowed to stand exposed to the air
from half an hour to an hour until dry, and then scratch-
brushed. The operation will require to be repeated one
or more times to get a satisfactory shade of colour. When
this is secured, finish by brushing with a waxed brush.
The following results were obtained from experiments with
the above solution : —

(a) Coppered ash-tray . . reddish-brown colour.

(6) „ „ . . rich purplish-brown.

(«) „ ,, • u purplish-gray.

(d) „ „ . . ,, light purplish-red.

The tray (a) was done as described above by sponging
on the solution ; (6), (c), and (d) were immersed for varying
lengths of time in the solution and became coloured much
more rapidly, especially with a hot solution. This solution
is not suitable for brass in consequence of the great length
of time required to produce a useful colour, which varies
from a coppery-red to brown. The solution acts very
vigorously on copper, the colour passing from light
crimson through various shades of purple, red, purplish-
gray to plumbago-black. If the bronzing has produced
too dark a shade of colour, it may be made lighter by
scratch-brushing.

This solution soon becomes exhausted, as after about
half an hour's working its action is very slow, even upon
copper. The effectiveness of the above mixture is doubt-
less due to the formation of ammonium sulphide, which

140 METAL-COLOURING AND BRONZING pabt

produces copper sulphide upon the metal. When the
liquid is freshly prepared and warm, the well-known
odour of sulphuretted hydrogen is perceived, but when it
begins to work somewhat tardily, the odour perceived is
more like water in which flowers of sulphur is sus-
pended.

Another solution and stronger than the former was
tried, using the following proportions : —

Potassium sulphide . • • 150 grains.

Ammonium chloride . • . 2000 ,,

Water 5 fluid ounces.

The work was immersed in the solution —

(a) Coppered ash-tray

m „

(d) „ „

(<0 ii »»

light purple-red.
darker „
bluish-black,
plumbago-black,
jet-black.

It will be observed that this solution only differs from the
preceding one in density, containing five times as much
salt dissolved in the same amount of water.

(a), (6), and (c) were all done in the warm solution, and
each one was patchy in appearance when removed, but
became uniform and lighter in colour on scratch-brushing ;
(d) and (e) were done in the cold solution ; (d) was simply
dried out in sawdust and brushed with a soft brush ; (e)
was simply rinsed in alcohol and then coated with the
transparent lacquer termed " zapon."

§ 93. Copper nitrate forms a very useful medium for
bronzing copper articles, in consequence of its oxidising
nature, varying with the strength of the solutions employed.

in COLOURING OF COPPER 141

When the amount of copper nitrate is small, and the quan-
tity of water very large, then merely a stain is produced
by the immersion of brass or copper in it ; but with stronger
solutions electro-deposited copper is coloured a beautiful
light chestnut-brown.

A very slight amount of impurity present in the copper
is sufficient to alter the tone of colour. A solution, just
sufficiently strong to effect the change on copper mentioned
above, will turn an article of ordinary commercial copper
dark brown, almost black. Even with samples of com-
mercial copper of approximate purity the colour is not
perfectly uniform all over the surface, and is generally iri-
descent, instead of a good solid colour, such as is obtained
on pure copper.

A solution of the following strength is recommended : —

Copper nitrate 800 grains.

Water 6 ounces.

High conductivity copper received a purplish-chocolate
colour when immersed in the hot solution; commercial
copper assumed a blackish stain ; and brass a dark sage-
green. On heating after immersion the work became
irregular in colour and patchy in appearance.

A saturated solution of copper nitrate was next tried,
with the following results : —

(a) A coppered ash-tray, heated on the lacquering stove, and im-

mersed in the solution assumed a chestnut-brown.

(b) A coppered tray, immersed without previous heating, received

a chocolate, inclining to purple colour.

turned to an Indian red colour on the high relief parts,
and umber-brown on the groundwork.

142 METAL-COLOURING AND BRONZING pakt

A black may be obtained with this solution, both upon
brass and copper, by immersing the articles in the solution
and transferring them to the hot stove without rinsing.
If the work is covered with an inverted glass beaker, it
will be observed to dry of a light green colour, with the
production of brown fumes. After a little time, the green
colour changes to a dead black, which, on brushing with
a soft brush, assumes a uniform and bright appearance.

§ 94. A copper nitrate solution containing 15 per cent
of the salt, to which sufficient potash was added to produce
a slight precipitate, gave the following results : —

(a) Coppered ash-tray . . . shellac-brown.

(b) Commercial sheet-copper . ,, but paler, with

iridescent patches.

Similar solutions to the above, but containing 10 and
20 per cent of salt respectively, gave similar results.
Ordinary sheet-copper appears to change more rapidly in
each of the three solutions than electro-deposited copper, but
the colour is not nearly so rich in tone. Electro-deposited
copper, after bronzing in copper nitrate, will withstand a
much more vigorous scratch-brushing without exposing the
bare metal than ordinary copper under similar treatment.

§ 95. The following solution yields very good results,
both for copper and brass : —

Copper nitrate, 20 per cent solution . 3 fluid ounces.
,, acetate, saturated solution . 3 „

A brass tray by immersion assumed a neutral greenish-
brown ; a coppered tray received a light-coloured bronze
of a very pleasing appearance ; a piece of ordinary sheet-
copper became reddish-brown with iridescence.

in COLOURING OF COPPER 148

The following solution acts very similar to the preceding
one, but yields a deeper tone of colour with more of a
reddish hue, and inclining to iridescence : —

Copper nitrate, 20 per cent solution . 3 fluid ounces.
,, acetate, saturated solution . . 6 „

The following proportions were also tried : —

Copper nitrate, 20 per cent solution . 1 fluid ounce.
„ acetate, saturated solution . . 4 fluid ounces.

(a) Coppered tray reddish-brown.

(b) Ordinary sheet-copper ... „ but with

iridescence.

It should be noted that with all solutions containing acetic
acid, or an acetate, there is more or less iridescence, which
in many cases is objectionable on account of the lack of
uniformity it imparts to a plain surface, which is generally
required to be of the same shade throughout. This effect
is most marked in ordinary copper, in consequence of the
impurities it contains. With all solutions containing
copper nitrate as a principal ingredient, the work may be
rendered paler in colour by scratch-brushing.

§ 96. Copper nitrate gives an acid reaction which has a
solvent action on copper and brass, and therefore it is
thought desirable in some cases to neutralise it with am-
monia. When ammonia is added to a soluble copper salt
it first precipitates the hydrated oxide, but on the addition
of more ammonia the precipitate is redissolved. This
solution gave the following results : —

Copper nitrate 400 grains.

Water 1 pint.

Ammonia in sufficient quantity to just redissolve the pre-*
cipitate at first formed.

144 METAL-COLOURING AND BRONZING part

A coppered ash-tray pickled in the cold solution assumed
a dirty chocolate-drab colour. On swilling and drying
out in sawdust the drab colour disappeared, and the sur-
face presented a coppery hue. The result was probably
due to excess of ammonia, which dissolves many copper
deposits as well as copper itself. On heating after pickling,
and then re-immersing when cold in the same solution made
hot, a smoky chocolate colour streaked with black was
produced. A brass tray pickled in the above solution be-
came dark brown ; and when pickled in the hot solution
received a still darker shade of colour. Excess of ammonia
may be removed by boiling the solution for some time.
There appears to be no advantage gained by adding am-
monia to copper nitrate, except in very small quantity.

§ 97. The addition of tartaric acid to copper nitrate and
sodium hydrate is said to produce a good bronzing mixture.
The following proportions were tried : —

Copper nitrate 1 ounce.

Tartaric acid 200 grains.

Sodium hydrate .... 250 ,,

Water 8 fluid ounces.

Very little effect was produced on brass or copper. The
tartaric acid seems to prevent or to considerably retard the
action generated by the solution in § 94. Four drachms of
ammonia were then added to the above solution, which caused
the liquid to work much more satisfactorily and imparted
to a coppered ash-tray a good permanent nut-brown colour.

With respect to black bronzes the Zeitschrifl fur
Instrumentenhunde (X. Jahrg., Mai 1890), says —

" The best result is obtained by the following mixture :
600 grammes of copper nitrate is dissolved in 200 c.c.

in COLOURING OF COPPER 145

of distilled water, and to this solution is added 2*5
grammes of silver nitrate which has been dissolved in
10 c.c. of distilled water.

"The addition of the silver nitrate to the solution
greatly assists in the formation of the proper colour.
More than the above-mentioned quantity, however, is
not advisable. If the solution is too concentrated, some
of the salt will crystallise out, and more distilled water
should be added to redissolve it. The articles to be
bronzed must be perfectly clean, and especially be free
from grease. They may be cleansed by dipping in a
mixture of 1 part common hydrochloric acid and 1 part
water, or a mixture of 2 parts concentrated sulphuric
acid and 7 parts water. In such dilute acids the articles
may be suspended by means of brass wire for five or ten
minutes, then well swilled with water and immersed in
the hot bronzing bath. If the article is too large for
immersion, it may have the solution painted on by means
of a brush. A thin painting of the solution is not
recommended, as it is apt to make the colour patchy.
After the work is taken out of the solution, any collection
of liquid in any one place must be avoided, and the
metal is then left to slowly dry. Too quick drying is
likely to produce spots. In a short time a pale green
deposit appears on the surface. If the process has gone
on satisfactorily the metal can now be put into a hot flame,
when the green colour changes to black. After cooling,
the surface is brushed with a not too hard brush.

"The above method is very useful for copper, brass,
gilding metal, German silver, and the various bronzes.
The method gives a beautiful bronze, but it is likely to
be not quite uniform."

146 METAL-COLOURING AND BRONZING tajlt

LIGHT TO DARK BROWN COLOURS ON COPPER

§ 98. Copper sulphate is a most useful bronzing re-
agent, and is capable of producing some very beautiful
effects. The following proportions have been tried by
the author : —

Commercial copper sulphate • • 400 grains.
Water 2 ounces.

The solution is best used warm, and after two or three
immersions with scratch-brushing, if necessary, a uniform
light brown colour is obtained on pure copper, and a
much darker shade on ordinary copper.

(a) Electro-deposited copper . . terra-cotta brown.

(b) High conductivity ,, . . purplish-chocolate,
(e) Commercial copper • • . dark brown.

The dark brown colour on commercial copper becomes
almost black if the metal contains much impurity.

§ 99. Experiments were next performed with copper
sulphate and ammonia.

Copper sulphate (com.) . . . 200 grains.

Water 5 ounces.

Ammonia in sufficient amount to precipitate and re-
dissolve copper hydrate.

(a) High conductivity copper . . light brown.
(6) Ordinary copper .... purplish-brown,
(e) Brass yellowishrbrown.

It should be remembered that ammonia in excess dis-

Ill

COLOURING OF COPPER

147

solves copper, so that any excess should be removed by
boiling the solution.

§ 100. The following solution produces good results,
but the action is slow. It works much better after
boiling for some time, from which it may be inferred that
a stronger solution is preferable : —

Copper acetate .
,, sulphate
Water

800 grains.
200 „
2} pints.

(a) High conductivity copper

(b) Ordinary copper

brownish pale terra-cotta.
dark purple by long immersion,
copper-coloured bronze.

§ 101. The following experiments were undertaken
with a view to ascertain the cause of the colouring effect,
whether it is due to a deposition of a copper compound
on the metal, or whether the change is due to a corrosion
of the surface, and consequent loss of metal. A solution
was used consisting of

Copper sulphate (com.)
Water

(a) Brass ash-tray

(b) Coppered ash-tray . .

(d) Cast copper-plate (com.)

(e) Coppered ash-tray, strongly coated

4 ounces.
8

pale umber-brown,
brownish terra-cotta.
light reddish-brown,
chestnut-brown,
nut-brown.

Some of the above brass and copper articles were weighed
before and after bronzing, with the following results.: —

Before bronzing . .
After ,, . .

Brass.
206*9 grains.
206-5 „

Copper.
452*15 grains
455-10 „

Loss=

*4 grains.

Gain= 2*95 grains

148 METAL-COLOURING AND BRONZING pari

Brass. Copper.

Before bronzing . . 184 '20 grains. 497*55 grains.

After „ . . 184*07 „ 497*25 „

Losa= 'Ingrains. Loss= *30 grains.

The above experiments would tend to show that copper
receives a deposit at the expense of the solution, although
in the second trial the copper lost weight. Copper
bronzes somewhat slowly in a copper sulphate solution,
while brass bronzes rapidly, especially if after once
pickling in the solution it is swilled and scratch-brushed,
then re-immersed.

If brass is heated on a stove after bronzing it loses its
umber -brown colour and assumes the appearance of a
very pale coinage bronze. When the bronzed copper
article is similarly treated it slightly darkens in tone,
but if strongly heated it passes through a purplish-
chocolate colour to black by the formation of black
copper oxide, which readily peels off and exposes the
copper. It will be seen from these experiments that the
impurities present in commercial copper exert an influence
in modifying the tone of colour.

§ 102. Another solution containing copper sulphate
was found to yield excellent results. The proportions
employed for the experiments were —

Copper sulphate .... 580 grains.
Water 45 fluid ounces.

Boil till the copper salt is dissolved, neutralise with
sodium hydrate, then add 770 grains of red iron oxide.
Immerse the work in the solution for a short time, and
heat to below redness. Well swill in water and repeat

Ill

COLOURING OF COPPER

149

the operation till a satisfactory brownish-red colour is
obtained.

(a) Coppered ash-tray

(b) Ordinary cast copper

(d) Coppered ash-tray

if) „

reddish to dark brown,
lighter brown,
rich reddish-brown,
similar to (a) bat lighter,
similar to (a).

(d) was obtained by immersion only, which accounts for
the lighter tone of colour. By heating, as in the case of
(a), (c), and (/), a darker shade is produced. The process
may have to be repeated several times to get the desired
depth of colour. The colour is much richer and denser
with pure than with impure copper. The best method
of heating is by means of a charcoal fire. The colour is
modified by the length of immersion and by the temperature.
§ 103. The solution referred to in this paragraph may
be useful for some varieties of copper goods, as the
general shade of colour is pleasing in tone, but it is
unsuitable for plain surfaces, in consequence of the
iridescence which accompanies all bronzing in liquids
containing acetic acid, or a soluble acetate.

Copper sulphate

,, acetate
"Water . •
Alum

(a) Coppered ash-tray

• 15 grains.

• • • <hD ,|

• 8 fluid ounces.
• . • 10 grains.

shellac-brown, with bluish sheen.

The above results were obtained by heating on a hot iron
plate after immersion, swilling in water, and drying out
in sawdust. The colours are permanent.

§ 104. The following solutions containing copper sulphate

150

METAIrOOLOURING AND BRONZING

PART

have been recommended for bronzing, but no useful results
were obtained from them by the author's experiments.
They also react on copper and brass very slowly with
regard to colouring : —

1.1

II.

III.

' Copper sulphate .... 1000 grains.

Water 10 fluid ounces.

Then add solution of caustic potash containing 1500 grains

dissolved in 15 fluid ounces of water. Also oxalic acid

200 grains.

Copper sulphate • • . . 250 grains.
Cream of tartar . .... 125 ,,

Water 5 fluid ounces.

Then add to the above a solution of caustic potash contain-
ing 125 grains dissolved in 5 fluid ounces of water.

Copper sulphate
Water

Tartaric acid

Sodium hydrate

240 graius.
2 fluid ounoes.
( 150 grains, dis-
J solved in 1$
v ounces of water.
192 grains, dis-
solved in 2 fluid
ounces of water.

{

A brass tray immersed in No. III. solution only assumes
a brownish -yellow colour, and a coppered tray a brassy
yellow, after leaving in the solution for two days. The
same trays were then kept immersed in a boiling solution
until the latter was so concentrated that it solidified
on cooling.

A fresh solution of the composition No. III. was made,
and a slight excess of ammonia added. It gave the
following results after the articles had been immersed a
considerable time : —

in JAPANESE BRONZE 151

(a) Coppered ash-tray . • • pale shellac-brown.

(6) Brass ash-tray • • . pale greenish-brown.

(d) Copper casting ... „

The results were uncertain and could not be exactly
reproduced.

§ 105. Japanese Bronze. 1 — According to Roberts-
Austen, the Japanese employ a wide range of alloys, many
of which contain silver and gold in varying proportions.
The following are examples : —

Shaktj-

II.

Copper
Silver

• •

94-50
1-55

Copper
Silver

• •

95-77
•08

Gold.

« •

3-73

Gold.

• •

4-16

Lead .

. •

•11

Iron and

arsenic

traces

99-89

100-01

The quantity of gold is very variable, and certain speci-
mens examined by the above authority contained only
1*5 per cent of gold. The next important alloy used by
the Japanese is called " Shibu-ichi," of which the follow-
ing are typical analyses : —

Shibtj-iohi

III. IV.

Copper
Silver
Gold .
Iron .

67-31

32*07

traces

•52

Copper
Silver
Gold .

51-10

48*93

•12

99-90

100-15

There are many varieties of it, but in both these alloys,
1 Jour. Soc of Arts, 26th Oct. 1888.

152

METAL-COLOURING AND BRONZING

PAET

" Shaku-do " and " Shibu-ichi," the point of interest is that
the precious metals are, as it were, sacrificed in order to
produce definite results, gold and silver, when used pure,
being employed very sparingly to heighten the general
effect. In the case of " Shaku-do " the gold appears to
enable the metal to receive a beautiful rich purple coat
or patina when treated with certain pickling solutions,
while " Shibu-ichi " possesses a silver-gray tint of its own,
which, under ordinary atmospheric influences, becomes
very beautiful, and to which the Japanese artists are very
partial.

These are the principal alloys, but there are several
varieties of them, as well as combinations of " Shaku-do "
and " Shibu-ichi " in various proportions, as, for instance,
in the case of " Kiu-shibu-ichi," the composition of which
would correspond to one part of " Shaku-do " rich in gold,
and two parts of " Shibu-ichi " rich in silver.

With regard to pickling solutions Roberts- Austen states
that the following are largely used : —

II.

III.

Verdigris . . 438 grains

87 grains

. 220 grains.

Copper sulphate 292 „

. 437 „

. 540 „

Nitre .

. 87 „ .

i ■ • •

Common salt .

146 „ .

» • • •

Sulphur . . ...

. 233 „

» • • •

Water . . 1 gallon .

* • • i

1 gallon.

Vinegar .

1 gallon

5 fluid drachms.

That most widely employed is No. I. When boiled in
No. III. solution, copper will turn a brownish-red, a
" Shaku-do " becomes purple. Copper containing a small
quantity of antimony gives a shade very different from
that resulting from the pickling of pure copper. It is

m JAPANESE BRONZE 153

strictly accurate to say that each particular shade of colour
is the result of minute quantities of metallic impurity.
The action of these solutions then produces entirely
different effects on pure copper to that produced on the
impure metal.

The following experiments were performed by the
author, using No. I. solution worked hot : —

(a) Coppered ash-tray . . light brown.

(b) Brass ash-tray . . . umber-brown, with bluish sheen.

On the high relief parts the colour was umber-brown, with
a bluish sheen, and a tendency to iridescence, while on the
groundwork the colour was umber-brown, with a tend-
ency towards blue.

(d) Brass casting . . . coppery umber-brown.

Electro-deposited copper bronzed very slowly and cast
copper less slowly. Commercial sheet-copper, containing
99 per cent copper and 1 per cent arsenic, and other im-
purities, bronzed quickly, and the same with sheet-brass,
but cast-brass was only slowly affected. In all the above
cases the articles required to be scratch-brushed and the
operation repeated to secure uniformity. The solution
after a little working yields a pale green precipitate and
a nearly colourless liquid. The colours obtained are per-
manent in the light.

No. I. solution was again tried, but with placing a
piece of zinc in contact with the work.

(a) Coppered ash-tray umber-brown, with incipient blue sheon.

(b) Ordinary cast-copper umber-brown, but darker than (a).

parts, and coppery -brown on the
groundwork.

154

METAL-COLOURING AND BRONZING

PART

Contact with zinc appears to hasten the process on first
immersion. I have observed that the first change pro-
duced on the work in the bronzing solution is, in many
cases, a tarnishing of its surface (oxidation ?).

If, after this tarnishing has occurred, the work is
rinsed in water, scratch-brushed, and re-immersed in the
solution, the tarnishing goes on much more rapidly and
more uniformly than on first immersion, and this second
tarnishing is quickly followed by the colour proper to the
solution and metal concerned in the process. There is a
tendency to the production of iridescent colours, especially
on cast-copper work pickled in this solution, and it is
possible to arrest the process at the moment these colours
appear. If the work is lacquered the colours are per-
manent, not being altered by exposure to daylight, except
that the blue sheen largely disappears.

§ 106. The following solution yielded results as re-
corded below, using a mixture of : —

Copper acetate

,, sulphate
Common salt
Nitre
Water

(a) Coppered ash-tray

[d) Cast-copper

>»

11 grains.
54}
18
11
1 pint

tt
it

ight brown,
ighter brown than (a),
redder ,,

drab-brown.

(a) was immersed in the hot solution, swilled, scratch-
brushed, and re-pickled ; (6) was pickled, then heated on
hot plate, and the immersion repeated ; (c) was pickled,
heated over a Bunsen burner, and again re-immersed in the
solution ; (d) was pickled, scratch-brushed, and re-pickled.

in JAPANESE BRONZE 155

On exposure to daylight (a) became black, (6) and (c)
darkened and showed a tendency to blacken, (d) became
nearly black.

§ 107. The following mixture, which is the same as
the preceding one, with the omission of the common salt,
was then tried : —

Copper acetate . • . . . 11 grains.

„ sulphate .... 64J „

Nitre 11 „

"Water 1 pint.

(a) Coppered ash-tray • . . pale umber-brown, with

yellowish sheen.

(b) Cast-copper .... reddish-umber-brown.

These colours remain permanent on exposure to day-
light, so that the solution is much more suitable as a
bronzing agent than the preceding one, which contains a
soluble chloride, and therefore produces a film which
blackens on exposure to daylight.

§ 108. Instead of using the recipe given in the last
section, similar results of a permanent character may be
obtained from the following solution : —

Copper acetate 28 grains.

,, sulphate .... 135 ,,

Vinegar 40 fluid grains.

Water 20 ounces.

The vinegar employed should be of good quality and free
from sulphuric acid, otherwise it is preferable to use a
6 per cent solution of acetic acid in place of the vinegar.
The following results were obtained : —

156

METAL-COLOURING AND BRONZING

PART

(a) Coppered ash-tray .

(b) Cast-copper ornamental plate

light umber-brown.

yellow amber on the
groundwork, and
yellowish sheen on
the relief parts.

dull red.

(a) and (6) were immersed in the hot solution for some
time, then swilled and scratch-brushed. The operation
was again repeated, and the metals dried out in sawdust
without a previous scratch-brushing; (c) was bronzed,
swilled, and dried out in sawdust, then heated on a hot
plate until it assumed a dull red colour, when it was
scratch-brushed to produce uniformity, re-immersed, swilled,
dried out in sawdust, and brushed with soft brush to finish,
when it appeared as a chestnut-brown.

DARK BROWN TO BLACK COLOURS ON COPPER

§ 109. A dilute solution of ammonium sulphide used
cold yields very beautiful effects ; as shown by the follow-
ing results : —

(a) Coppered ash-tray .

(«) „ „ • •

light nut-brown.
• ,, but darker,
dark brown.

(<*)

(/)

steely brown,
grayish-black,
dead black.

This solution works very well for copper, but it is not
suitable for brass. If, however, brass work is thinly
coated with copper, then this method will produce the
colours enumerated above. The solution works well

in COLOURING OF COPPER 157

either hot or cold, strong or dilute. The colours depend
more upon the manipulation of' the process than upon
either temperature or density. Colours may be obtained,
ranging from a neutral crimson through brown and steel-
gray to black.

(a), (6), (c), and (d) were all scratch-brushed, the lighter
colour on (a) was obtained by a momentary immersion
only, which produces such a transparent film that the
copper can be seen through it, modified, of course, in tone
by the thin coating of copper sulphide; (e) was dried
out in sawdust without scratch-brushing ; (/) was rinsed
in alcohol after immersion in the bronzing solution, and
the alcohol burnt off, leaving a black velvety coating,
which is fixed on the metal by dipping it in zapon. As
long as the film is sufficiently thin a reddish-brown tint
is visible, but when a certain thickness prevails the colour
is black without any trace of red.

This solution may be used for bronzing work which is
too large to immerse in the solution, by moistening it with
a sponge or cloth, then allowing the articles to stand
exposed to the air till they are dry, when they may be
scratch-brushed, and the moistening repeated if the colour
is not deep enough, or the bronzing not uniformly distri-
buted. When the right tint is attained, the articles
should be thoroughly washed, first with warm water, then
with cold water, and finally dried out in sawdust and
brushed with a waxed brush.

§ 110. A similar solution may be formed by dissolving
potassium sulphide (liver of sulphur) in water. The
following were tried : —

I. Potassium sulphide .... £ part.

Water 99J parts.

158 METAL-COLOURING AND BRONZING pakt

A coppered ash-tray assumed a dark steel-gray colour
after immersion in the solution

II. Potassium sulphide .... £ part.

Water 99} parts.

A coppered ash-tray received a reddish-gray colour. The
remarks made with regard to the ammonium sulphide
solution apply also to potassium sulphide. The colour
may be modified in the manipulation of the working of
both solutions. Darker colours can be produced by a
longer immersion, and these again can be further modified
by judicious scratch-brushing.

§ 111. The following solution has been recommended
for producing a reddish-brown colour, which becomes paler
on heating : —

Dissolve 1 part of copper acetate in 16 parts of water,
then add sufficient ammonia to give a deep blue solution,
and add 2 parts of potassium sulphide, 3 parts of ammonia,
and 10 parts of water. The following was used by the
author : —

Copper acetate 60 grains.

Water 2 fluid ounces.

Ammonia, till the solution is blue.

Potassium sulphide .... 120 grains.

Ammonia 3 fluid drachms.

Water 1£ fluid ounces.

This solution gave precisely the same results as with
potassium sulphide and water, so that the other con-
stituents appear to be useless. The reaction on copper
is instantaneous, but brass is simply tarnished.

§ 112. The following solutions containing a mercury

in COLOURING OF COPPER 159

salt are said to yield good results, but the author could
only obtain indifferent colours and cannot recommend the
method : —

Mercury chloride .... 150 grains.
Ammonium nitrate . • • 150 ,,

Water 3} fluid ounces.

Ammonia, till the precipitate formed redissolves.

After pickling the work in the above solution, the operator
is then advised to immerse it in a solution of ammonium
sulphide' or potassium sulphide.

The mercury solution behaves like a quicking solution
as used by electroplaters. Both copper and brass are
instantly coated with mercury when immersed in it.

A brass tray first coated with mercury in this way and
then immersed in the sulphide solution was darkened upon
its surface, but on scratch-brushing the mercury was
partly exposed.

A coppered tray blackened instantly on immersion in
the sulphide bath, but became nearly white on scratch-
brushing. The black surface may be preserved by omit-
ting the scratch-brushing and simply drying on a hot
plate. The black colour on the copper is doubtless due to
a deposit of copper sulphide, and possibly a little mercury
sulphide, for if the black colour was due to mercury sul-
phide only, one would expect the mercury-coated brass to
blacken as readily as the mercury-coated copper, but this
is not the case. Better results can be obtained upon
copper by omitting the mercury solution and using the
sulphide bath only. The effect of these solutions upon
brass is of no practical utility.

Another solution containing a mercury salt consists of—

160 METAL-COLOURING AND BRONZING pabt

Mercury sulphide .... 50 grains.

Potassium ,, .... 50 M

CC Sodium hydrate (20 per cent solution) 1 fluid ounoe.

Water 3 fluid ounces.

The following results were obtained : —

(a) Brass tray . . . dark brown and patchy.
(6) Coppered tray . . gray-black,

The colour on (a) was only obtained with considerable
difficulty, and a second attempt to produce an identical
result failed completely. From this it may be inferred
that brass can only be coloured in this solution when it is
freshly prepared and nearly concentrated. The colour
even then is very poor, and practically worthless.

A copper tray blackens immediately after immersion,
and then presents a fine velvet-black surface which is easily
rubbed off, leaving a gray-black appearance. The jet-
black colour can be preserved by rinsing the article in
alcohol, igniting the latter, and coating the article with
zapon lacquer.

In this case results equally as good can be obtained by
omitting the mercury sulphide and sodium hydrate, and
using only the potassium sulphide and water. It is prob-
able that mercury sulphide has very little influence in
the production of the black colour, and that the effect is
almost entirely due to the potassium sulphide acting on
the copper and producing the black copper sulphide, as
before described.

§ 113. The following solution is recommended for
producing a steel-blue colour on copper : —

in COLOURING OF COPPER 161

Potassium sulphide .... 20 grains.

Sodium chloride . . . . 20 ,,

"Water 1 pint.

A copper ash-tray received a colour which was not a
steel-blue but rather a gray-black. This solution behaves
much the same as potassium sulphide alone without the
sodium chloride.

§ 114. Bottger states that a solution of potassium
sulphantimoniate imparts a beautiful and durable coating
to copper. He was occupied for some time experiment-
ing, so as to produce a coating of sulphide on the surface
of copper vases and similar articles in order to give them
a pleasing appearance, and enable them to resist the
action of the atmosphere. Such a coating we have
already seen can be obtained by means of potassium or
ammonium sulphide. The present solution is said to
impart a brilliant grayish-blue colour, of a shade intermedi-
ate between the gray of platinum and the gray of steel.

The difficulty is to protect the metal completely from
all superficial oxidation on immersing it in a nearly
boiling solution of the sulphantimoniate. If several
articles are suspended at the same time they must not
touch each other, or the sides of the containing vessel.
When the articles have assumed the proper tone, which
takes some time, they are well swilled with water and
dried with a linen cloth.

In case the operator has no potassium sulphanti-
moniate at hand, which decomposes somewhat readily, the
following mode of preparing a similar solution is recom-
mended, and is both simple and economical. Mix
together 4 parts by weight of dry sodium sulphate, 3 parts
of finely powdered antimony sulphide, and 1 part of

162 METAL-COLOURING AND BRONZING part

powdered charcoal, introduce the mixture into a crucible
which has been previously made red-hot, cover with a lid,
and when all action has ceased pour out the mass. Place
in a vessel with sufficient water, add \ part of flowers of
sulphur, boil for some time and filter off the insoluble
residue. The clear liquid, when further diluted with
water, is ready for use.

A solution for testing this substance as a bronzing agent
was made by dissolving 25 grains of potassium sulphide
in 3 fluid ounces of water, and saturating the hot solution
with freshly precipitated antimony sulphide. The follow-
ing results were obtained : —

(a) Brass tray .... rich shellac-brown.

(b) ,, .... „ ,, but paler.

(d) ,, ... steely-brown.

The solution acted on brass very slowly; (a) was pro-
duced in about fifteen to twenty minutes, (6) occupied at
least half an hour and was not so good as (a). The
result in the second case may be due to the solution
having altered in the interval. Both were scratch-
brushed at intervals and re-immersed in the solution.
The tray (b) was not bronzed until after (d) was finished.
The reaction of this solution on copper is almost
instantaneous. The metal assumes various shades from
nut-brown to blue-black, according to the thickness of the
deposited film. If the copper is immersed too long or
the solution used too hot, the coating is not adhesive.
The coloured film produced by this solution is probably a
mixture of copper sulphide and metallic antimony. This
mixture is useful in giving dark tints to copper differing

in COLOURING OF COPPER 163

slightly from those obtained by the agency of potassium or
ammonium sulphide alone, in that the reddish-black tone
obtained in their solutions gives place to a grayer-black
in the sulphantimoniate solution.

§ 115. A dark colour on copper may be obtained by
immersion, or by painting the following liquid on the
articles : —

Arsenic oxide 120 grains.

Hydrochloric acid .... £ fluid ounce.

Sulphuric acid 60 ,, grains.

"Water 8 „ ounces.

The solution works quickly both on copper and brass, but
does not produce a pure black on either ; the deposit of
arsenic has a dark gray colour, which becomes lighter on
scratch-brushing. If copper is dipped momentarily into
the solution it receives a very thin transparent film of
arsenic, which on scratch-brushing presents a pinkish-
gray colour, as on (b). The following results were
obtained : —

(a) Brass ash-tray . • . deposit of arsenic

(b) Coppered ash-tray . . . film of arsenic.

§ 116. The following solution is recommended for
obtaining a deep black colour on copper and its alloys : —

Copper nitrate 100 parts.

Water 100 „

The above salt is dissolved in the water and the article,
if large, is painted with it, if small, the article may be
immersed in the solution. It is then heated over a clear
coal fire and lightly rubbed. The article is next placed

164 METAL-COLOURING AND BRONZING part

in or painted with a solution of the following com-
position : —

Potassium sulphide .... 10 parts.

Water 100 „

Hydrochloric acid . . . . 5 ,,

We have obtained more uniform results by using
a solution about three times more dilute than the
above, viz. : —

Copper nitrate 100 parts.

Water 300 „

Small work can be much more conveniently treated by
immersion in the solution, and after draining off or
shaking off the excess of the solution to heat the work
on a hot plate until the copper salt is decomposed into
the black copper oxide. It would be difficult to heat
large articles upon a hot plate, but a closed muffle
furnace should give better results than an open coal
fire. In any case the heating process should not be
continued longer than is necessary to produce the change
mentioned above.

A good black can be produced upon brass in this way
without the aid of the potassium sulphide solution, which
imparts a bluish tint to the copper oxide. The black
coloration does not appear to adhere so firmly to copper as
it does to brass, and, considering this circumstance, it is
probable that the subsequent immersion of blackened
copper work into the potassium sulphide is an advantage.

§ 117. A great variety of shades of brown may be pro-
duced on copper by a barium sulphide solution, beginning
with a tone only a little darker than copper itself and

m COLOURING OF COPPER 165

passing through crimson to light "brown, and then through
a succession of darker shades until a very dark brown,
nearly black, is reached.

It appears difficult to obtain the crimson colour
uniformly distributed over the surface of the work, so that
this method cannot be recommended for that colour. The
brown colours, however, are very rich in tone, and very
easily produced in almost any required shade, and their
distribution is practically uniform. The bronzed surface
will also stand a severe scratch-brushing, and is, in fact,
improved by that process, but rendered a little lighter in
tone. It is well, therefore, to produce a colour in the
solution a little darker than that required upon the
finished work.

The effect produced upon work having relief parts can
be heightened by rubbing those parts, after bronzing,
with a wet cork which has been dipped in silver sand, so
as to expose the underlying metal, in which condition the
work may be finished. If it is preferred, after treating
the metal in the manner just described, it may be
immersed momentarily in the bronzing solution until
the exposed copper takes a different shade of colour to
that upon the general surface of the work, when the
whole may be scratch-brushed and finished.

If the solution is strong — say 100 grains of barium
sulphide to 1 pint of water — and hot, a black colour
is imparted to copper immediately after immersion, but if
the solution is used cold, this result is only achieved after
some time. A dilute solution, containing say 25 grains of
the sulphide to 1 pint of water, can be more easily con-
trolled, and the process arrested at any required stage of
coloration.

PRODUCTION OF A GREEN BRONZE ON
COPPER AND BRASS

§ 118. Reference has already been made at some length
to the formation of a green patina on copper and its
alloys by the slow and continuous action of air and
moisture, extending over lengthened periods of time. It
is not surprising then that attempts should be made to
imitate the effects of nature by artificial means and to
impart to copper and copper alloys similar beautiful
effects.

This may be brought about in a variety of ways, with
the formation of the so-called antique patina, varying in
tone from dark brownish-black to dark greenish-black, also
from yellowish-green to bluish-green ; and these different
shades being capable of production in different parts of
the same article, a great variety of shades may be gener-
ated. Thus a dark greenish-black may be formed on the
groundwork, with a bluish or yellowish-green on the parts
which are in high relief.

The bluish -green patina is formed by the action of
basic copper carbonate, and the yellowish -green by the
action of basic copper chloride, so that if one wishes to
imitate the natural colouring use must be made of reagents
containing similar ingredients. The action may be re-

Paet HI GREEN BRONZE 167

ferred to the influence of carbonic acid and chlorine respect-
ively upon copper oxide. The slower the action the more
beautiful and lasting will be the effects, so that a very
dilute solution of a copper salt, or a very dilute or weak
acid, acting for a considerable period of time on copper or
a copper alloy, will give better results as a rule than a
more rapid action by means of a stronger solution:

If a weak solution is painted on to the metal by means
of a brush or dabbed on the surface by means of a cloth
or sponge a slight stain is imparted which facilitates the
subsequent colouring. A judicious and thorough scratch-
brushing will then be a great advantage to secure that
uniformity of surface, without which the final effect will
be considerably marred.

The solution must not be too cold or the liquid will
not be uniformly distributed, thus causing an uneven and
patchy appearance ; nor must it be too hot or it will dry
up too quickly and simply leave a loose deposit which
will readily peel off, in consequence of the surface of the
metal not being sufficiently corroded by the acid liquid,
as is the case when the action is more prolonged. A suit-
able temperature is about 20° C. both for the article to be
bronzed and the solution which is to be applied to it.

It is an advantage in many cases to impart to copper
articles a dark tone at first by means of a sulphur com-
pound, such as ammonium or potassium sulphide solution,
which should be very dilute so as to yield only a very
thin coating, for if the sulphide deposit is too thick the
metal will not take the desired green patina. It is desir-
able in some cases to produce different shades of colour in
different parts of the same article — for example, to have
a dark groundwork and green on the relief parts, when

163 METAL-COLOURING AND BRONZING part

the advantage of the primary sulphide deposit will be
apparent.

Instead of using saline solutions to produce the
antique green rapidly, the colour may be obtained by
submitting the articles to the same chemical conditions
and influences as those developed upon them by air and
moisture during a long period of time. The green patina
consists, as already stated, of a hydrated copper carbonate
similar to the natural minerals malachite and azurite, and
it is probable that they are formed in the same manner.
The surface of the metal is first oxidised by the agency of
air and water, and the oxide thus formed slowly combines
with carbonic acid, forming copper carbonate mixed with
copper hydrate.

§ 119. Eisner imitates this natural method by introduc-
ing carbonic acid gas through an aperture into an
earthen vessel, in the bottom of which has previously
been placed a solution of common salt. Then the per-
fectly cleaned article is first plunged into a mixture of
equal parts of vinegar and water, and when every portion
has been perfectly moistened, it is suspended in the vessel
which is filled with the moist carbonic acid. The mouth
of the vessel is then tightly closed and the aperture which
conveyed the carbonic acid plugged up. The article is
suspended by means of a copper wire.

The whole apparatus is allowed to stand for several
weeks, when the object is found to be coated with a
bluish-green patina in the deep parts, and has all the
appearance of a body which has been exposed for many
years to atmospheric influences. This patina afterwards
perfectly resists the action of the air without showing any
signs of change. The longer the process is allowed to

in GREEN BRONZE 169

continue the more beautiful is the patina, and the more it
resembles the natural product. It is necessary to avoid
the employment of too strong a solution of vinegar, for,
although the stronger the vinegar the more rapid the
oxidation, yet the more easily will it be dissolved after-
wards by atmospheric moisture.

If the carbonic acid which is employed contains a very
small proportion of sulphuretted hydrogen (which may be
generated simultaneously with the carbonic acid by adding
a small quantity of iron sulphide to the calcium carbonate
used to generate the carbonic acid) then the surface of the
article quickly assumes a particular brown colour, and
very soon afterwards the green patina begins to form.
The advantage of this primary brown coating has been
previously referred to.

A quicker method than the above is to put the article
in a box, and on one side of it to place a vessel containing
water to produce moist air, and on the other a vessel con-
taining dilute hydrochloric acid, into which, from time to
time, bits of marble or chalk are dropped to generate car-
bonic acid. An antique patina is produced more readily
on copper containing tin or zinc alloyed with it than on
pure copper. Copper containing much arsenic, which has
been bronzed green, will in time turn black, and this is
even the case with copper alloys containing much arsensic,
therefore it is advisable to use as pure metals as possible.

§ 120. Articles of bronze which are exposed to the
atmosphere of a large town soon become discoloured by
the various gases generated by the manufactories and by
the products of decomposition which pass into the air
from the combustion of gas and coaL This imparts to
them a dirty black appearance instead of an artistic bronze

170 METaL^OLOURING AND BRONZING *A*t

colour. The Society for the Encouragement of Arts,
Berlin, in 1864 made an investigation into this subject
and performed a number of comparative experiments with
a view of finding a remedy.

" They first sought to settle the question as to whether
a certain definite composition of the alloy favoured the
formation of a beautiful patina. For this purpose they
placed in different localities some bronze works, remarkable
for a beautiful patina on the surface, the metal of which
had been previously analysed. The analyses proved that
the bronzes had a widely different composition. The pro-
portion of copper varied from 94 to 77 per cent ; that of
tin from 1 to 9 per cent. In others the tin did not
exceed 4 per cent ; while in some cases the amount of tin
was not more than '8 per cent, while it contained up to
19 per cent of zinc. Other metals present were con-
sidered as accidental impurities, such as lead, iron, and
nickeL

"With all this variety in composition each of the
bronzes nevertheless had received a very beautiful green
patina. It was thought possible that the composition
would exercise an influence on the duration of the time
which the bronzes, all other circumstances being equal,
would require to become coated with the patina ; but the
experiments did not leave any doubt on the mind that the
patina was formed on alloys of the most varied com-
position.

"In order to discover what other influences might
determine the formation of the patina,- they placed a
number of bronze busts in a part of Berlin where many
particularly objectionable exhalations were sent into the
atmosphere, and where different statues exist in the neigh-

in GREEN BRONZE 171

- - — ■■ ■"-■ —

bourhood without any trace of a green patina, but were
covered with the dirty black coating before referred to.

" Observing upon several monuments, in the parts most
accessible to the public, and which are liable to be frequently
touched by the hands, that there was not only a green
patina, but a special patina of the most valuable pro-
perties, while the other parts were black and non-uniform.
The commission charged with these researches conjectured
that it was presumable that the greasy matter of the
hands played an important role in the formation of the
green patina. In consequence of this, one of the exposed
busts was syringed with water each day, except when it
rained, so as to keep it quite clean. Besides this, it was
treated with bone oil, once a month, by means of a paint-
brush, and then rubbed with a linen cloth. A second
bust was equally well cleaned every* day, but without any
periodical oiling. A third bust was also washed in the
same way, but treated with oil only twice a year. A
fourth bust, for the sake of comparison, was exposed but
neither washed nor submitted to any treatment. The
result of these trials was to prove in the most unequivocal
manner that the conjecture as to the effect of grease was
correct.

" The bust which had been oiled once a month assumed
a dark green patina, which was declared by connoisseurs
to be very beautiful. That which had been oiled twice a
year had a less beautiful appearance. That which had
been simply washed with water presented none of the
beautiful features acquired by the formation of the patina.
Lastly, that which was left untouched had a dull appear-
ance, and was of a dark colour.

" We may consider it as certain that a bronze article

172 METAW30L0TJRING AND BRONZING vast

which is exposed to the air, and which has been cleaned
once a month, and afterwards rubbed with oil, will become
coated with a beautiful patina.

"With regard to this periodical rubbing with oil, it
would manifestly be unadvisable to treat large monuments
too often, and it was for this reason that the experiments
to determine the effect of treatment twice a year were
performed. The Berlin Commission also tested two new
bronzes which had received a patina by artificial means
with the use of certain chemical reagents, in order to
assure themselves how these would comport themselves by
analagous treatment.

"The manner in which the oil acts in inducing the
formation of the patina is difficult to determine. The
experiments, however, have demonstrated that it is
necessary to avoid excess of oil, and that it should be
rubbed off as far as convenient. If the oil is in excess,
it attracts dust, and the bronze takes a poor appearance.
It cannot be supposed that the minute quantity of oil that
remains forms a chemical combination with the layer of
oxide of the bronze, since bone oil was shown by the
experiments to be as advantageous as olive oil. It is
probable that the minute layer of oil has no other effect
than that of preventing the deposition and adherence of
moisture, which so easily fixes the dust, absorbs gases and
vapours, and in which vegetation is so frequently developed.
In every case in these experiments, whatever the mode of
action, it was abundantly proved that greasy matter con-
tributed to the formation of the patina.

" It is possible that this gradual formation of a patina
may be useful for other work under different circumstances.
We have observed that on bronzes covered with a beautiful

in ANTIQUE PATINA 173

patina, in the points where water has run over them, a pale,
chalky dulness, which with time increases in thickness.
A convenient treatment with oil would probably prevent
the formation of this layer, but it can only be decided by
prolonged experiments.

" This method of the employment of oil for preserving
and beautifying bronze work leads one to hope in the
future the public monuments in large towns will be
clothed with a brilliant patina. This patina, in the
localities where coal is exclusively used as the combus-
tible, will not be a bright green, it will appear deep
coloured, perhaps even blackish, but it will possess all the
other choice properties in the patina, and in particular
the brilliant property of reflection from the surface."

DIFFERENT METHODS OF PRODUCING AN ANTIQUE

PATINA

§ 121. For the production of a yellowish-green bronze
the following solution may be employed : —

Vinegar ...... 1 quart.

Ammonium chloride .... 250. grains.

Common salt . 250 ,,

Liquid ammonia .... £ ounce.

The two salts are first dissolved in the vinegar and the
ammonia added to the solution, when it is ready for use.
Small articles may be immersed in the solution, then re-
moved, and when one part begins to dry, a paint-brush
drawn over it so as to keep all parts uniform. The opera-
tion is then conducted as described for large work. Large
articles, which cannot be dipped in the solution, are coated
with the bronzing liquid by means of a paint-brush. The

174 METAL-COLOURING AND BRONZING pabt

surface of the article must be entirely covered as uniformly
and as rapidly as possible, taking care to keep every part
moist. In a few seconds the solution begins to colour the
body with a green stain, the copper changing in tint, and
a greenish froth begins to form. The workman must now,
without dipping his brush again in the solution, proceed
to spread the colour all over, carefully following the
contour of the ornamental parts.

When the copper has taken a convenient tint, and the
liquid begins to dry and to thicken, the wet parts should
be dried with another brush having long bristles or hairs,
and when this is too wet to use, another, and so on till the
whole is dry. The article is then allowed to rest in a
warm place till the next day, when a second coating is
given in the same manner as the first. The colour now
assumes a deeper tone, and it may be necessary to repeat
the operation several times to get a desired shade. After
allowing the article to remain twenty-four hours after
imparting the last coat, it is finished by well brushing
with a soft brush which has been rubbed on a cake of
white wax. It is always advisable to leave each coating
for an interval of twenty-four hours to act on the metal,
but some operators only allow an interval of six hours.

It sometimes happens that the base or other parts of
an object are of brass, while the other parts are made of
copper ; in such a case the brass parts take a paler tint
than the copper. The tints may be equalised by means of
a little plumbago which is rubbed on the pale parts by
means of a soft short bristled brush. If it is desirable in
any case to quicken the process, the amount of ammonium
chloride and common salt may be doubled for the same
quantity of liquid in giving the second coat.

hi ANTIQUE PATINA 175

§ 122. The following solution may be employed for
producing a yellowish-green patina : —

Ammonium chloride . . . . 155 grains.
Copper acetate ..... 78 ,,

Water 16 ounces.

To test this solution the liquid was painted on the work,
and was found to yield better results than by immersion.
The same precautions were taken as those described in the
previous paragraph, allowing an interval of twenty-four hours
between each application. The following are the results : —

(a) Cast copper . . light brown on the high parts

and bluish-green on ground.

(b) Cast brass . . drab colour on prominent parts

and bluish-green on ground.
(e) Stamped brass . . very patchy, bluish-green only in

parts where liquid was thick.
(d) „ coppered light brown colour in some

parts and bluish - green

patches in others.

Three separate coats were given to each and the colour
became darker and denser after the third coat. Cast work
appears much better adapted for this bronze than stamped
work. The colour, after the first two coats, appeared a
yellowish-green, but after the third application it became
much darker in tone and decidedly bluish-green.

§ 123. A solution of a complicated nature is given by
Buchner as a suitable medium for producing a green
patina. The liquid is recommended to be applied by
means of a brush or a sponge, allowing to dry, and
repeating until the desired colour is reached. It is said
to be best to put the painted articles into a closed chamber
to dry. In the same chamber is placed a vessel containing

176

METAL-COLOURING AND BRONZING

PART

dilute hydrochloric acid and vinegar and some pieces of
marble. By the reaction of these substances on each
other carbonic acid is generated, which favours the forma-
tion of the antique patina. Should the patina become too
blue then the article is to be painted with a solution of
120 parts of ammonium carbonate and 40 parts of ammo-
nium chloride dissolved in 1000 parts of water. The
bronzing solution recommended is —

Ammonium chloride

78 grains.

Cream of tartar . •

78 „

Copper nitrate ....

. 720 „

Hydrochloric acid .

6 drachms.

Water .....

16 fluid ounces.

The above solution was tested by the author in the same
way as in his preceding experiments without obtaining any
useful effects.

[a) Cast copper • • • dirty brown colour, with one

or two small patches of
green after third painting.

dark drab colour, with faint
green shade after third
painting.

dark patchy drab colour,
with faint green shade
after third painting.

terra - cotta patchy colour
but no green patina after
third painting.

From these experiments it may be concluded the solution
is useless by itself for producing a green colour on copper
and brass.

§ 124. The following solution yields fairly good results
both on copper and brass : —

(6) Cast brass

(d) Coppered ash-tray

Ill

ANTIQUE PATINA

177

Ammonium carbonate
Oxalic acid .
Vinegar . •

(a) Cast copper

• • 155 grains.
. • 78 „

16 fluid ounces.

yellowish-green on ground-
work and crevices of the
design, the other parts
were a dark drab, with
pale film of yellowish-
green after the third coat.

same as on cast copper.

>»

»»

(b) Cast brass ....

(d) ,, coppered .

§ 125. The solution referred to in this paragraph yields
a pale green patina on brass and copper much more
uniformly distributed over the surface of the metals than
that produced on the same metals by the solution described
in the former paragraph.

Ammonium chloride .
Common salt .
Ammonia ....
Water ....

The patina on the cast brass and cast copper was about

the same, so that these metals seem to be equally adapted

to receive a light green patina by this solution. A brass

ash-tray was well covered, but a coppered ash-tray had

a reddish-brown colour on the raised parts. In each case

the deep parts had received a denser deposit. Three coats

were given to each article.

§ 126. A very dark bluish-green patina can be obtained

by using a solution of

Ammonium carbonate . . . 1163 grains.

124 grains.
124 „

4£ drachms.
16 fluid ounces.

Sodium chloride
Copper acetate
Cream of tartar
Water .

880
460
380
16 fluid ounces.

»»

>»

178

METAL-COLOURING AND BRONZING

PART

(a) Cast copper

(b) „ brass

(d) „ ,, coppered

bluish-green.

but paler.

»>

and patchy.

»»

The groundwork of (a) was a very dark bluish-green after
the third application of the solution and drying. The
raised parts were brown, with only a shade of green, (b)
was similar to (a) with regard to distribution, but the
blue of the deep parts was of a lighter shade ; (d) was
bluish -green on the groundwork and intricacies, and
reddish-brown on the prominent parts.

§ 127. An excellent yellowish-green patina may be ob-
tained by using the following solution, as shown by the
experiments below : —

Ammonium chloride .

124 grains.

Potassium oxalate

81 „

Water ....

16 fluid ounces.

(a) Cast copper

yellowish-green.

(b) ,, brass ....

• ft

(d) Coppered ash-tray

• »»

(a) was coated a light yellowish-green all over, but denser
in the deep parts ; (6) was very good on the groundwork,
but not so good as (a) on the raised parts; (c) was
yellowish-green on the groundwork, and a brownish-drab
on the prominent parts ; (d) was yellowish-green on the
groundwork and reddish-brown on the raised parts. This
article was somewhat patchy, the others, especially the
cast copper, fairly uniform.

§ 128. The following solution yields results of a uni-
form character, after two or three applications with
subsequent drying : —

in ANTIQUE PATINA 179

Ammonium carbonate . . . 900 grains.

,, chloride • . . 300 ,,

Water ...... 16 ounces.

The solution was painted on the work as in the previous

cases, and it was then allowed to remain twenty-four

hours before the second application, and the same time

between the second and third. The following results

were obtained : —

(a) Cast copper' . bluish-green patina on groundwork.
(6) „ brass . „ „

(d) Coppered tray . „ „

(a) was yellowish -green after the second coating, but
became bluish -green after the third application. The
green of the groundwork was fairly uniform, but the
more prominent parts were brown with a greenish tint.
(6) was scarcely affected by the first coating of the solu-
tion, after the second it turned yellowish-green, and after
the third a bluish-green colour appeared on the ground-
work, but of a lighter shade and less dense than that on
the cast copper ; (c) acted throughout much the same as
(6) j (d) was yellowish-green after the second coating, but
assumed a bluish-green in the deeper parts, with brown
on the raised parts, after the third application of the
solution and drying.

§ 129. The following mixture is equally good with the
last, and acts much in the same way, producing a bluish-
green patina after three applications of the solution and
subsequent drying : —

Ammonium chloride . • • 770 grains.

Alum . . . . . 370 „.

Arsenious oxide .... 160 ,,
Vinegar . . . . , 16 fluid ounces.

180 METAL-COLOURING AND BRONZING pabt

The method of conducting the experiments was the same
as in the previous ones —

(a) Cast copper . . bluish-green and uniform.

(b) „ brass

i» it

(e) Stamped brass . . ,, but patchy.

(<*) » coppered . „ „

This solution is reliable and produced excellent results on
cast work, but is not so good for stamped work

§ 130. A good solution for yielding a yellowish-green
patina on cast copper and cast brass work is composed
of

Copper nitrate . • • 1500 grains.

Sodium chloride • . • 1500 „

Water 16 fluid ounces.

The conditions of its application to the work and the
subsequent treatment are the same as before described.
The following results were obtained : —

(a) Cast copper . . . yellowish-green.

(b) „ brass ... ,,

(d) „ coppered . light brown, with green patches.

(a) was green on the groundwork and brown on the pro-
minences ; (6) was green on the groundwork and brownish-
drab on the upper portions ; (c) was drab in places, the
other parts were green ; (d) was green and reddish-brown
in different parts. The bottle containing the liquid should
be well shook up before applying, the liquid to the work.

§ 131. Buchner recommends the following solutions for
producing a patina on electro-deposited copper : —

I. A 20 per cent copper nitrate solution.
II. A solution of ammonium sulphide,

in ANTIQUE PATINA 181

III. A 20 per cent solution of calcium chloride.

IV. Bromine water (30 parts bromine in 970 parts water).

A. He mixes equal volumes of L, III., and IV. to form
a suitable solution for bronzing, especially for electro-de-
posited copper, which takes different shades according to
the treatment. He then dips a perfectly cleaned figure
in this solution for one or two minutes, removes, and
allows the liquid to dry on the work without swilling,
which gives it a pale bronze tone which in the light is first
greenish, then a yellow bronze tone. After allowing to
dry it is again immersed in the solution, and after the
second drying it assumes a beautiful green patina of a
dead lustre, which may be made more beautiful by brushing
with a waxed brush.

If, after one or two minutes' immersion, the article is
swilled with water and dried by rubbing between sawdust,
it has a browner tone, which somewhat darkens on
exposure to light, becoming at first greenish, then
brownish-black.

B. The tone may be modified by using a mixture con-
sisting of two volumes of the solution No. L, one volume
of the solution No. IV., and one volume of the solution
No. ffl. The greater the proportion of the copper solu-
tion the darker will be the tone. The more bromine and
calcium chloride is used the lighter will be the tone pro-
duced. The coloration depends upon the production of
cuprous oxide in its different modifications, from yellow
to dark brown, along with some cuprous bromide and
cuprous chloride, which undergo darkening by the action
of light.

If the article which has been coated according to either
of the directions given in A or B is, after drying, placed

182 METAL-COLOURING AND BRONZING part

in an atmosphere of sulphuretted hydrogen gas, or in the
vapour of ammonium sulphide, taking care not to have too
saturated an atmosphere, then only the outer surface-film
will be changed into copper sulphide, which will be a
grayish-black. If now the article is immersed in a solu-
tion consisting of two parts of No. I., two parts of No.
IV., and one part of No. III., taken out and allowed to
dry at the ordinary temperature, it will receive a beautiful
greenish-black groundwork like antique bronze, with a
copious patina in the recesses in the lapse of half an hour.
The coloration is so firm that the article may be brushed
with a waxed brush.

It is necessary for success that the preliminary coating
of sulphide should not be too thick. It should be pale
brown to grayish-brown. If it is a dark colour, almost
black, the coating will probably be too thick to take any
green patina at all. Buchner states, with respect to the
foregoing method, that a more beautiful effect, in so
short a time, cannot be obtained by any other method so
far as he himself knows.

§ 132. The following solution is said to yield a good
yellowish-green patina on copper and brass. The appended
results were obtained after three applications, with an
interval of twenty-four hours between for drying : —

Copper nitrate .... 1000 grains.
Zinc chloride .... 1000 ,,
Water 12 fluid ounces.

The above liquid was painted on to the work with a
brush, allowed to remain some time to act on the metal,
but not sufficiently long to allow any part to dry. It was
then wiped off with a brush, so as to leave a thin but uni-

Ill

ANTIQUE PATINA

188

form film of solution) and allowed to stand for a day. The
operation was then repeated.

(a) Cast copper-plate

(b) Cast brass-plate

(d) Coppered stamped tray

very pale greenish tinge.

»»

just stained,
terra-cotta colour.

The plate (6) was only very slightly tinted green after
three applications. Neither (a) nor (6) was of any prac-
tical value, and the method could only be employed when
a considerable number of repetitions is admissible. For
stamped work it appears quite unsuitable.

§ 133. The following solution has been strongly recom-
mended for producing a good yellowish-green antique bronze
on copper, but the author could not get any satisfactory
results, as in the case of other solutions in which a mercury
salt is a constituent The quantities employed were —

Copper nitrate
Zinc sulphate
Mercury chloride
Water .

(a) Cast copper

(b) Cast brass

(d) Coppered

»>

• . . 150 grains.

200 „
200 „
16 fluid ounces,

exceedingly thin and pale green film,
practically unaltered,
coated with mercury,
similar to the cast copper.

The experiments were not continued after the third
repetition.

§ 134. A very good antique green was obtained by the
following mixture : —

Copper carbonate .
Ammonium chloride
Common salt

8 ounces.
1 ounce.
1

»>

184 METAL-COLOURING AND BRONZING part

Cream of tartar 1 ounce.

Copper acetate In

Vinegar 8 fluid ounces.

The above is from an old French recipe, and although the
result is very good there are unnecessary constituents, as
the same effects may be produced by a much simpler
mixture. The proper quantity of copper sulphate fc was
dissolved in water, and sufficient sodium carbonate was
added to precipitate the copper as carbonate. The whole
was allowed to stand until the precipitate settled down,
when it was filtered off, washed, the vinegar added, and
the other substances mixed with it A portion of the
mixture was then applied to the articles with a brush as
usual, and then allowed to rest for twenty-four hours,
when the operation was repeated, and so on a third time.

(a) Cast copper . . . • . bluish-green.

(b) „ brass „

(e) Stamped brass ,,

(d) „ „ coppered ... „

The results were good in each case. After the first appli-
cation there was no apparent change except a tarnishing
of the metal; after the second application there was a
pale yellowish -green film; and after the third a good
bluish-green patina on the surface. The stamped trays,
(a) and (&), were first tarnished by immersion in a solution
of potassium sulphide, and then the first coating of the
present solution was given, but no advantage appeared to
be gained by this preliminary treatment.

When the right degree of green has been obtained, the
article may be finished by rubbing the surface lightly
with a cloth which has been moistened with olive oil,

Ill

ANTIQUE PATINA

185

when the green colour becomes more transparent in the
rubbed parts.

§ 135. According to Kayser, a greenish-brown colour
on copper may be obtained by first staining the surface of
the article with a solution of potassium sulphide, which,
after drying, is moistened with a solution of ammonium
chloride, vinegar, and ammonia. The following proportions
were tried : —

Ammonium chloride

Ammonia

Vinegar

Water.

(a) Cast copper
(jb) Cast brass
(c) Stamped brass

coppered

80 grains.
8 drachms.

to neutralise the ammonia.
16 ounces.

very pale bluish-green.

»»

simply a reddish-brown.

A considerable number of repetitions would be required
to get any useful effect. The process was repeated three
times in the above experiments.

§ 136. The following solution is due to G. Buchner,
and yields very good results, even after the first treatment.
The following proportions were used by the author : —

Copper nitrate .

48 grains.

Ammonium chloride . • ,

. 48 „

Calcium chloride .

48 „

Water

3 fluid ounces.

(a) Cast copper

yellowish-green.

(b) „ brass

j»

>»

(d) „ copper

• »»

All the above assumed the green patina on drying after
the first coating. The colour was uniform all over, and

186 METAL-COLOURING AND BRONZING past hi

appeared as good after the first as after the third
application.

§ 137. The use of bromine water added to a copper
solution has been recommended for imparting an antique
patina, but the author could obtain nothing more than a
stain on brass and copper by either of the two following
solutions : —

(Copper nitrate 96 grains.

Calcium chloride . . • . 96 „

Water 2 ounces.

Bromine water 1 fluid ounce.

{Copper nitrate 96 grains.

Calcium chloride . • . . 48 ,,

Water 1J fluid ounces.

Bromine water J fluid ounce.

It may be stated as a general rule that in all the foregoing
cases where copper nitrate is recommended as a constituent
of a bronzing solution, the blue deposit which forms at
the bottom of vessels, containing old aquafortis or pickle,
which has been used for dipping brass or copper goods,
may be employed, and as in many cases this deposit is
thrown away, it is of course a great saving to employ it
instead of new copper nitrate.

BRONZING OF COPPER ALLOYS

BRASS 1

§ 138. By this term is meant all alloys of which
copper and zinc are the essential and chief constituents,
but more generally the name is confined to those alloys
which have a decidedly yellow colour. Different qualities
of brass are known in commerce by a variety of names
which have given rise to great confusion as to their par-
ticular meaning) seeing that brass sold under the same
fanciful names has a wide difference in properties and
composition, while, on the other hand, many different
names are given to the same alloy in different localities.
Dr. Percy mentions "that the terms tombac, prince's
metal, similor, and Mannheim gold are all represented to
contain 85 per cent copper and 15 per cent zinc." The
alloys termed oreide vary from 90 per cent copper and 10
per cent zinc to 80 per cent copper and 20 per cent zinc.
The term " tombac " is made to answer for alloys containing
from 99 to 70 per cent of copper and 1 to 30 per cent
zinc.

Commercial brass never consists entirely of copper and
zinc, since whatever impurities exist in the separate metals
will also be found in the alloy, the most common of these

1 See author's work on Mixed Metals, p. 80, Macmillan and Co.

188 METAL-COLOURING AND BRONZING part

being lead, iron, arsenic, and tin. It often happens that some
of these are purposely added to produce a certain effect,
as lead, for example, in cast brass. The colour of brass
shows great variations, according to the proportions of the
constituents, ranging from the red of copper at one end
to the bluish- white of zinc at the other, but the change
from red to white is by no means uniform.

Thus, alloys containing 94 to 99 per cent of copper
are red, with only a faint yellow tint ; with 87 to 93 per
cent of copper the colour is reddish-yellow ; from 79 to 86
per cent of copper a yellowish-red tint prevails; below
this, down to 74 per cent of copper, the alloys are yellow ;
with a content of 67 per cent of copper a reddish-yellow
tint is obtained; with 60 to 66 per cent of copper the
colour is a full yellow; with 59 per cent of copper a
reddish colour is obtained ; with 52 per cent of copper the
colour is nearly golden -yellow; with a less amount of
copper than the above the colour of the zinc begins to
overpower the red colour of the copper, the alloys be-
coming more lead-like in appearance as the proportion of
zinc increases.

For rolling into sheets it is essential that the alloying
metals should be practically pure, and that the alloy
should consist only of copper and zinc. One of the most
injurious substances to brass is antimony, which is occa-
sionally found in common copper. Lead, arsenic, tin, and
iron are occasionally present in very small quantities.
The composition of sheet and stamped brass varies from
92 to 65 of copper to 8 to 35 of zinc.

The composition of cast brass varies considerably, and
depends to a large extent on the uses to which the cast
articles are to be put, as also with the particular shade of

Ill

BRASS

189

colour it is desired to obtain. Thus the various alloys for
jewellery having a reddish-yellow colour are composed of
82 to 90 per cent copper and 18 to 10 per cent zinc ; while
those with a full yellow colour contain 60 to 70 per cent
of copper and 40 to 30 per cent of zinc. Lead from 1 to
2 per cent is a usual constituent of cast brass, while iron
and sometimes tin are present as impurities. Very common
cast brass made from old materials, such as scrap and
brass dust, often contains a variety of ingredients besides
copper and zinc. The composition of some French fine
brass castings is given below—

II.

III.

IV.

Copper . .

63-7

64*45

70-90

72-43

Zinc . . .

33-5

82*44

24 05

22*75

Lead

2*5

•25

2-00

1-87

Tin . . .

•3

2*86

8*05

2-95

From the foregoing description of the nature of brass,
which may vary so widely in the amount and nature of
the constituents, it will be easily understood that these
different alloys will not behave the same in the colouring
bath) and that the various shades of colour may differ
according to the composition of the alloys.

Zinc alone is only capable of assuming a very limited
number of shades of colour, and these are generally gray
or black, while copper, on the other hand, is competent to
take on an almost infinite variety of tints from pale yellow
to jet-black. It by no means follows that because a
certain solution colours copper it will also colour brass, as
the zinc may so modify the properties of the copper as to
prevent any notable change taking place. In some cases

190 METAL-COLOURING AND BRONZING pakt

brass articles will assume a beautiful colour in a given
solution which is quite neutral to copper. This may be
due to the galvanic action set up between the copper and
zinc producing greater chemical activity than when the
electrical agency is absent.

It is stated by Buchner that, with respect to the action
of copper-zinc alloys, when the proportion of zinc does not
exceed 50 per cent the colouring action of the copper
predominates, and that copper salts are only precipitated
with difficulty in an acid solution, while alloys with over 50
per cent of zinc partake more of the character of zinc, and
that copper salts are easily precipitated in an acid solution.

The colouring effect on a sample of brass may be
modified by the preliminary cleaning treatment it has
undergone. The acid or alkaline liquids acting unequally
on the components of the alloy will produce a surface of a
different composition to that of the bulk of the metal, so
that two samples of the same alloy will often exhibit
different shades of colour. I have often observed this
effect with specimens which have been bronzed several
times and have had the coloured film dissolved off so as
to try the effect of a different solution on the same
metal.

If brass is placed in a solution of ammonia it becomes
paler in colour after a time by the dissolving of the copper,
but if a sample of the same alloy is treated with hydro-
chloric acid the zinc is largely dissolved, leaving the alloy
richer in copper. Such specimens when subsequently
immersed in certain bronzing solutions will exhibit different
shades of colour.

§ 139. Brass articles may be made to receive all the
colours which can be produced on copper by first coating

ra COLOURING OF BRASS 191

them with a layer of copper by electro-deposition ; or by
wrapping them round with iron wire and placing them in
a very dilute solution of sulphate or nitrate of copper ; or
by immersing them in a solution of copper chloride, and
heating them to a temperature of about 150° C. By
the two last methods the film of copper is exceedingly thin^
and such coppered goods can only be treated in certain
bronzing solutions, as those of a strongly corrosive nature
easily dissolve the copper and produce a very patchy
appearance on the surface of the article. Copper nitrate
solution, for example, is such a liquid, and for a coppered
brass article to receive a good colour the deposit of copper
must be thicker than can be obtained by simple immersion
processes, i.e. it must be deposited by means of an electric
current, when any desired thickness of copper can be
obtained.

Buchner states that a pale copper colour can be
imparted to brass by heating it over a clear charcoal fire,
free from smoke, till it assumes a blackish-brown colour,
then immersing it in a solution of zinc chloride, and
allowing it to boil gently, finally well swilling the article
in clean water. The operation requires repetition.

A golden-yellow colour is imparted to brass by dipping
it in a very dilute neutral solution of copper acetate, which
must not contain any trace of free acid, and kept at a medium
temperature for a few seconds. A golden-yellow to orange
colour may be imparted by immersing polished brass
wares in a mixture of 50 grains of caustic soda, 500 grains
of water, and 100 grains of copper carbonate. The shades
of colour appear in a few minutes. When the right tint
is obtained, wash well with water and dry in saw-
dust.

192 METAL-COLOURING AND BRONZING past

A dark yellow is produced on brass by immersing it
for five minutes in a saturated solution of common salt
containing some free hydrochloric acid, and in which
mixture as much ammonium sulphide has been dissolved
as the liquid is capable of taking up.

§ 140. Dr. Kayser states that an ordinary gold colour
may be imparted to brass in the following manner: —
Dissolve 150 grains of sodium thiosulphate in 300 grains
water, and add 100 grains of antimony chloride solution.
Boil for some time, filter off the red-coloured precipitate,
well wash with water, transfer the precipitate to a vessel
containing 4 pints of hot water, add a saturated solution
of sodium hydrate, and heat till the precipitate is dissolved.
Immerse the brass articles in this solution till the desired
shade of colour is reached. If they remain in it too long
they will become gray.

§ 141. Dr. Schwarz states that a colour from yellow to
brown may be obtained on brass as follows : — Dissolve 50
grains of lead acetate in \ pint of water, and add a
solution of sodium hydrate until the precipitate which
first forms is redissolved, then add 150 grains of red
potassium ferricyanide. When the whole is in solution
immerse the brass goods at the ordinary temperature, when
they assume a golden-yellow colour. By warming the solu-
tion to 40° or 50° C. the yellow colour changes to brown.

§ 142. Dittrich recommends the following method for
obtaining a gold colour on brass. Prepare a solution of

8000 grains distilled water.
30 ,, sodium thiosulphate.
10 ,, lead acetate.

The articles are first covered with silver in a silver-plating

in COLOURING OF BRASS 193

bath, then immersed in the bronzing solution raised to a
temperature of 60° to 70° C. The metal assumes a golden-
yellow colour after two or three minutes, due to the forma-
tion of silver sulphide. The articles are then swilled in
water and dried out in sawdust.

§ 143. The following solutions are given to show the
effects of chlorides on brass in producing colours of a shade
varying from brown to greenish-drab : —

Copper chloride .... 1 part.
Water 1J parts.

A brass ash-tray assumed an olive-green drab colour which,
after exposure to daylight, became very dark chocolate with
evidence of decomposition on the edges, showing a copper-
coloured border, which is probably anhydrous cuprous
chloride. A brass casting also became olive-green drab
in colour after immersion. The results are tabulated
as follows —

{a) Brass tray (stamped) . . . olive-green drab.

(b) ,, ,, ... reddish-brown.

When (a) was heated on the stove it became salmon-coloured,
and after scratch-brushing was found to be well coated
with copper ; (6) was heated on a hot iron plate till it
became nut-brown, and then lacquered. This nut-brown
appears to deliquesce and to rub off under the finger
unless it is lacquered whilst hot. It darkens on exposure
to sunlight.

A series of quantitative experiments were performed to
determine whether the coloration and subsequent copper-
ing were due to a true deposition, or to a corrosion of the
surface with consequent loss of zinc. These experiments

194

METAL-COLOURING AND BRONZING

PART

are recorded in § 75, and proved in each case that there
was a loss in weight of copper. It should be stated that
the coppering reaction appears only to occur during the
heating stage of the process, for if the work is scratch-
brushed without being previously heated, the colour is
rubbed off and the underlying brass is exposed. The
presence of chlorine also appears to be essential to 'the
success of the process, for salts of copper other than the
chloride fail to give the coppering reaction, but any other
soluble copper salt may be used successfully, provided
that a small amount of some other soluble chloride is also
added.

The blackening effect on copper, which has already been
referred to when dealing with the bronzing of that metal
in copper chloride, is much more rapid and deeper in tone
than with brass under the same circumstances.

§ 144. Similar colours to those produced on brass in
copper chloride may be obtained when ferric chloride is
also added. The following solution was used : —

Copper chloride
Ferric chloride
Water

500 grains.
500

20 ounces.

Time of Immersion.

Temperature.

2 minutes

15° C.

>»

70° C.

»>

80° C.

j Colour of coinage, bronze.

it a »

»j i) »>

Do., but paler in tone.

»t

The colour may be deepened by adding a very small
quantity of a soluble sulphide to the above solution.

in COLOURING OF BRASS 195

Scratch-brushing removes the colour almost completely,
although the effect is improved by the scratch-brush if
the articles are subsequently re-immersed in the solution,
then well swilled and dried out in sawdust. The brown
colour became a dark olive-green on exposure to daylight.
Another solution of double the strength of the first one
did not have any great advantage except to yield a slightly
darker tone of colour.

This solution works better at a moderate temperature
than it does at the boiling-point. A brass ash-tray bronzed
in this solution, and after swilling and drying out in saw-
dust, was placed upon a hot plate, when it assumed a
salmon -red colour similar to that produced on copper
when bronzed in the same solution. On scratch-brushing,
a perfect coating of copper was found upon the surface of
the tray.

This tray weighed before bronzing . . . 14*6098 grams.
,, after heating and scratch-brushing . 14*5713 „

Loss= -0385

Ferric chloride freely dissolves copper when hot, but in
the case of brass, the zinc is probably dissolved more
readily than the copper. It should, however, be re-
membered, as stated on a previous page, that the coppering
reaction does not occur during the immersion of the work
in the bronzing solution, but during the subsequent heating,
therefore it must be due to a decomposition of the surface
compound deposited during the immersion.

A solution of ferric chloride alone was then tried, using
the following proportions ; —

196 METAL-COLOURING AND BRONZING part

Ferric chloride 1 part.

Water 4 parts.

(a) Brass ash-tray . . dull and crystalline brass.

(b) ,, ,, . . good coppered surface.

This solution alone appears to be of little value, but as
the liquid dissolves copper, the solution after a time
becomes impregnated with copper chloride, and then acts
as in the preceding method. The tray (a) was not bronzed
at all The tray (b) was done after the solution had been
experimented with for some time on copper articles, and
of course had dissolved a considerable amount of copper.
The coppering effect was produced by pickling and sub-
sequent heating.

§ 145. Experiments were performed with a mixture of
copper sulphate and ferric chloride, using the following
proportions : —

Copper sulphate (saturated solution) 16 ounces.
Ferric chloride .... J ounce.

Water } „

(a) Brass ash-tray .... slaty-green.

(b) ,, ,, after pickling and heating copper surface.

(d) Brass ash-tray .... purple-brown.

(a) and (c) were not sensibly altered on exposure to day-
light ; (b) had a surface of pale medal brown bronze, which
deliquesced on exposure to air, and washed off completely
in water, leaving a nice dead surface ; (d) was pickled and
heated to produce a coppered surface, and then re-pickled,
when it assumed a purple-brown colour, which turned
black on exposure to light.

§ 146. A peculiar bronzing effect on brass, which may
be useful in certain cases, is obtained with a solution of

in COLOURING OF BRASS 197

Copper sulphate

1 oance.
. • • 1 „

(a) Brass ash-tray
(&) Cast brass .

white, inclining to drab,
greenish-gray.

(a) when scratch-brushed after bronzing showed an un-
altered brass surface. When heated on a hot plate after
bronzing it became medal brown in colour, and on con-
tinuing the heat metallic copper appeared on the surface,
varnished with a glaze which deliquesced on exposure to
air. This deposit gave a slightly acid reaction. Brass
pickled in the copper sulphate solution alone, without the
addition of common salt, did not give the copper colour
reaction, which confirms the idea that the reaction is due
to the presence of chlorine. The white bronze of (a) is
partly removed by drying in sawdust, but if the article is
gently dried and lacquered with zapon the coating is
preserved for some time, but ultimately turns green.

§ 147. Wuttig recommends the following ingredients
as a means of producing the ordinary antique bronze
(green). He states that the pickle produces in a very
short time a durable coating of oxide. The cream of
tartar may be replaced by vinegar, and the copper salt
omitted in the second or third repetitions after the metal
has been once stained. The following solution was tested
by us for colouring brass : —

Ammonium chloride ... 8 grains.

Cream of tartar (pure)
Common salt .
Copper nitrate
Water .

24 „
48 „

1 ounce.

5 fluid ounces.

198 METAL-COLOURING AND BRONZING paet

(a) Brass ash-tray olive-green.

(6) ,, „ nut-brown.

(a) was simply pickled in the solution, swilled, and dried
out in sawdust; (b) and (c) were each pickled like (a),
then put on the hot stove and heated until the olive-green
colour disappeared, and was succeeded by a rich medal red
colour ; (c) was finally scratch-brushed.

§ 148. The combined effect of copper acetate and
ammonium chloride on brass was tested with the following
results. Copper acetate alone appears to have very little
action on brass unless concentrated, and the mixture of
acetate and ammonium chloride acts very slowly. This
solution is reputed to have been used at the Paris Mint for
bronzing medals. Some bronzers also add red iron oxide,
which is held in suspension.

Copper acetate
Iron oxide
Ammonium chloride
Water .

(a) Brass tray
(6)

200 grains.
200 „

70 „

20 fluid ouaces.

yellowish-brown.

brown ochre.

bronze coinage colour.

(a) was simply immersed in the hot solution, swilled, and
dried out in sawdust ; (&) and (c) were each pickled in the
hot solution, swilled, dried, and heated on a lacquering
stove for ten minutes. Both acquired the same brown
ochre colour ; (c) was then severely scratch-brushed without
destroying the whole of the colour produced by heating.
They do not appear to alter by exposure to diffused day-
light

in COLOURING OF BRASS 199

The solution was then varied by using sixteen ounces
of water and four ounces of vinegar to the other sub-
stances above-mentioned.

(a) Brass ash-tray . . • yellowish-brown.
(&) ,, , dark brown ochre.

(a) was pickled as before; (b) was pickled and heated
as above ; (c) was scratch-brushed after heating.

§ 149. A solution composed of copper sulphate and
copper chloride dissolved in water may be employed for
bronzing brass and copper, but the slate colour produced
darkens considerably on exposure to daylight, so that the
method cannot be recommended.

Copper sulphate . . • 400 grains.
Copper chloride .... 400 „
Water 1 fluid ounce.

Brass articles were pickled in the hot solution, swilled
and dried out in sawdust when they were coloured as
above. On heating them on a hot plate the colour became
darker.

Similar results were obtained by using the following
solution : —

Copper chloride .... 300 grains.

Ammonium chloride • • . 200 ,,

Water 5 fluid ounces.

§ 150. A good brownish-drab colour may be obtained
on brass by means of a mixture of

Copper sulphate • • 1 part.

Zinc chloride 1 ,,

Water 2 parts.

200 METAL-COLOURING AND BRONZING pabt

The mixture is smeared over the work, which is then
heated till it is dry. Or more water may be used to form
a solution, in which the work may be immersed. The
colour does not appear to change much by exposure to
light By heating the bronzed brass article on a hot stove
in the usual way the surface is coppered.

§ 151. Walcker recommends the following ingredients
for producing a brown colour on copper. It may be used
to produce a yellowish-green colour on brass.

The proportions given below were employed in our
experiments—

Ammonium carbonate . . . 800 grains.
Copper sulphate .... 150 ,,
Vinegar 5J fluid ounces.

The whole was evaporated to dryness, and to the residue
was added

Vinegar 5J fluid ounces.

Oxalic acid 4 grains.

Ammonium chloride . . . 1J ,,

The above was then boiled and the insoluble portion
filtered off. The article, if too large to be immersed in
the solution, should be well warmed and the solution
painted on uniformly with a brush. Boiling water is
then poured over to wash it and to hasten the drying.
When dry the article is rubbed with an oiled pad, and
lastly with a dry pad.

(a) Brass ash-tray . . . oiled and wiped dry.
(p) „ ,, . • dried in sawdust only.

Immersion was found to be much better than rubbing or
painting on the solution. Wiping with an oiled pad

in COLOURING OF BRASS 201

removed some of the bronze, unless it was done very
gently.

The colour produced on (a) and (b) was distinctly
yellowish-green, but the bronze does not appear to change
by exposure to daylight.

The insoluble portion, from which the liquid used in
the above experiments was filtered off, was next rubbed
into a thin paste with a few drops of the above-mentioned
solution, and then rubbed on some fresh articles, which
were heated on a hot plate after swilling and drying.
When dry the paste was well washed off with water.

(d) Coppered ash-tray . . . deep brown.

Neither of them was quite uniform, but that was doubtless
owing to the operation being only performed once. We
infer from our experiments that the method is suitable for
large work that cannot be immersed.

§ 152. The following solutions have been recommended
as bronzing liquids, but no useful results could be
obtained by their use in our experiments. The hot solutions,
as one would naturally expect, dissolved both copper and
brass rapidly.

Copper nitrate .... 1 ounce "I
Hydrochloric acid ... 5 ounces j '**"

Copper nitrate .... J ounce ^

Copper chloride .... 4 ounce Y(b),

Hydrochloric acid ... 4 ounces J

The work is recommended to be pickled in solution (a),
then immersed in the solution (b).

202

METAL-COLOURING AND BRONZING

PABT

(1) Brass ash-tray, solution used cold

pale brassy bronze.

(2) „ „

(a) only, cold

»» i»

(3) „ ,,

(b) only, cold

t$ »i

(4) „ „

(a) and (6), hot

» tt

(5) „ „

(a) only, hot .

»» »»

(6) „ „

(b) only, hot .

»» i>

§ 153. A solution, which is capable of yielding useful
colours, may be employed with the ingredients in the
following proportions : —

Copper chloride . •

• £ ounce.

"Water ....

. • i ..

Tartaric acid . . .

75 grains.

Sodium hydrate

£ ounce.

Water ....

10 ounces

The tartaric acid is used to prevent precipitation of the
copper by the sodium hydrate, and to this is added the
sodium hydrate solution.

(a) Brass ash-tray .

<*) n

i»

pale umber - brown, with

light bluish-green sheen,
pale umber-brown.

The tray (a) was pickled in the hot solution, swilled in
hot water, and dried out in sawdust; (&) was pickled,
heated, and quenched in the hot solution, then scratch-
brushed and re-pickled. Brass in this solution passes
through various shades of colour — gold, deep gold,
umber-brown, nickel-yellow, then brass reappears.

§ 154. The following solution has been recommended,
and it certainly does produce a very good dark green
colour on brass, but unfortunately this turns to a deep
black on exposure to daylight : —

in COLOURING OF BRASS 208

• f — — — —

Copper acetate 50 grains.

„ sulphate .... 200 ,,

Common salt 70 „

Acetic acid solution .... 4 ounces (1 acid

to 16 water).
Water 50 ounces.

A solution similar to the above is said to be used by the
Japanese for bronzing copper and its alloys. It suffers
from the same defect as the former solution with regard
to light changes, but in a much less degree as regards
brass.

Copper acetate 100 grains.

„ sulphate .... 500 „

Common salt 180 ,,

Saltpetre 100 „

Water 10 pints.

(a) Brass ash-tray greenish-drab.

(b) „ „ olive-green drab.

a i>

The tray (b) was immersed for a longer time than (a).
The plate (c) was of a reddish -drab on the embossed
parts, and olive-green drab on the groundwork.

§ 155. The effect of platinum chloride on brass is
different to that of most other chlorides in that it pro-
duces a gray to black colour as shown in the following
results. Two solutions were used, both very dilute, but
one was double the strength of the other : —

(a) Platinum chloride solution, containing *2 per cent of

platinum chloride and 99*8 per cent of water.

(b) Platinum chloride solution, containing *4 per cent of

platinum chloride and 99 '6 per cent of water.

204 METAL-COLOURING AND BRONZING part

The brass must be scrupulously clean and uniformly
polished on the plain parts. The metal was kept in the
weaker solution until it received a stain, then it was
transferred to the stronger solution. The deposit is
largely removed in scratch -brushing and the surface
rendered paler in colour.

As this is a case of platinating by simple immersion
one would only expect the difference between the deposit
on brass and copper to be one of degree. If the work is
left too long in the solution the deposit becomes powdery
and is easily rubbed off. Heating the work after bronz-
ing does not appear to improve it.

A very strong solution of platinum chloride, contain-
ing 5 per cent, gave the following results : —

Articles of brass were made warm, and the solution
was applied to their surfaces by means of a paint-brush.
The result in each case was a dark gray colour.

On evaporating the aqueous solution, which probably
contained some free acid, to dryness, and redissolving
the residue in alcohol, the following results by immersion
were obtained : —

(a) Brass tray ... a coating of reguline platinum.

(b) „ . . . black deposit.

It appears that if a black colour is required it is best to
use an alcoholic solution. The tray (a) was scratch-
brushed after bronzing ; (b) was swilled and dried out in
sawdust. The deposit of platinum seems more firmly
adherent to brass than to copper.

in COLOURING OF BRASS 205

BLACK COLOUR ON BRASS

§ 156. Eisner states that the production of a good
black colour on brass and bronze has excited a consider-
able amount of attention, and many attempts have been
made to secure a good durable colour. It is in great
demand for colouring telescope tubes and similar optical
instruments. He says : " We have many times been con-
sulted concerning this matter, and for that reason have
made some experiments with the following results :—

"To impart a black bronze to brass we make use of
the following substances : — An acid solution of pure silver,
by dissolving pure silver in pure nitric acid ; or a solution
of bismuth nitrate; or of nitric acid alone; or an acid
solution of silver nitrate containing some copper nitrate
(such may be obtained by dissolving an ordinary silver
coin in nitric acid) ; or a solution of copper nitrate alone
may be used.

"According as one or the other of the above sub-
stances is used so the bronzed article will vary in tone.
With copper nitrate it is always very black ; the bismuth
solution gives a particularly deep brown colour ; that of
copper and silver a deep black.

"In every case the bronzed brass acquires a deeper
shade, when the brass after bronzing is placed for some
time in a solution of liver of sulphur, antimony sulphide,
or in a solution of freshly prepared sulphuretted hydro-
gen, or exposed to a moist atmosphere containing that
gas.

" The operation is very simple, and is executed in the
following manner : —

206 METAL-COLOURING AND BRONZING part

"The respective metals are dissolved in pure nitric
acid, taking care to have the acid in excess. Solution
readily takes places without having recourse to the
application of artificial heat. The solution is then
applied to the previously heated articles by means of a
paint-brush, and the heating of the metal is continued
till the surface is quite dry. The solution employed must
not be too concentrated, otherwise the coloured film will
wear off under the action of the brush or rubber in finish-
ing. It is better to dilute the solution considerably with
soft water before applying it to the object, as the thinner
the layer of liquid the firmer will the bronze adhere to
the metal.

" Immediately the bronzing liquid has evaporated from
the surface -of the metal, it is rubbed with a dry leather
or brushed with a dry brush in order to produce uni-
formity of surface. If nitric acid alone is used to colour
the brass, it forms on the surface a layer of copper nitrate,
and there is therefore no need to previously dissolve
copper in nitric acid for this purpose.

"The operation may require to be repeated several
times, and a little experience will make known the most
convenient method of manipulation.

" If the colour is required to be an intense black, the
brass should be treated with the saline solution of liver of
sulphur before mentioned. In about half an hour the
surface will become a deep black, and the operation is
completed by rubbing the surface with a dry leather.

" When the operation has to be repeated several times
with the solution in question, it is necessary that the last
should be very concentrated, otherwise the free acid will
dissolve the bronze already formed. The bronzed surface

in COLOURING OF BRASS 207

should be covered with a solution of ammonium sulphide,
taking care that the bronze is equally affected. It is not
necessary to cover it with an excess of the sulphide
because the brass easily takes a black colour in a little
time by the liberation of sulphuretted hydrogen. If it
is desired to favour the disengagement of this gas a
few drops of hydrochloric acid may be added to the
ammonium sulphide, so that the brass is plunged into a
medium charged with sulphuretted hydrogen.

" Another method for achieving the same result may be
employed by using a solution of one part of neutral tin
chloride, and two parts of gold chloride, which must not
be too dilute. The two liquids are mixed, and then used
for bronzing the brass. In about ten minutes the article
is wiped with a damp linen rag. If the use of a large
excess of free acid has been avoided the surface is a dead
black, and very durable."

§ 157. In order to test the effectiveness of certain re-
agents referred to in the preceding paragraph, and to
discover a good permanent black bronze the following
experiments were performed : —

Potassium sulphide .... 30 grains.
Ammonium chloride . . . . 90 „
Water 2 pints.

The solution was applied with a brush, when the brass
articles were allowed to stand about an hour and then
were scratch-brushed. The operations were repeated
until a satisfactory colour was obtained. Other experi-
ments were performed by immersing some brass articles
in the solution instead of painting the solution on the
work.

208 METAL-COLOURING AND BRONZING part

(a) Brass tray .... tarnished to coppery-red.

(b) „ .... light brown colour (poor).

(a) was done by painting the solution on to it as
described above; (b) was bronzed by immersion in the
hot solution which was found to be much more effective,
especially for copper goods.

This solution reacts very slowly on brass and cannot
be recommended as a bronzing liquid for that metal, as
much better results can be achieved by other solutions.
A much lighter tone is imparted by means of scratch-
brushing. The solution soon becomes exhausted and
requires renewal When freshly prepared and warm, the
solution smells like ammonium sulphide, but after work-
ing some time it smells like water containing flowers of
sulphur in suspension.

The same solution, but with different proportions con-
taining larger quantities of the two salts with the same
quantity of water as before, was found to produce a
darker shade of colour. The following proportions were
used : —

Potassium sulphide ... 15 grains.

Ammonium chloride . . . 200 ,,

Water 5 fluid ounces.

A solution of ammonium sulphide was found to give
results identical with those of potassium sulphide.

§ 158. The following complicated method of prepar-
ing a solution has been recommended, but was found
by the author to be quite unnecessary, as identical
results can be obtained by potassium sulphide and water,
as referred to above, without the use of the other in-
gredients : —

in COLOURING OF BRASS 209

Cream of tartar .... 60 grains.

Copper acetate . . . 60 „

Water 2 fluid ounces.

The solution obtained is then to be treated with liquid
ammonia until the precipitate first formed redissolves.
Then

Potassium sulphide . . 120 grains,
Ammonia .... 3 fluid drachms,

Water 1J ,, ounces,

are added to the above. This mixture is not suitable for
brass, but it acts instantaneously on copper.

§ 159. The following recipe for bronzing may be
classed in the same category as the preceding, but in
addition to it containing unnecessary and expensive
constituents, they are positively damaging to the metal
and serve no useful purpose : —

Mercury chloride . . . 150 grains.
Ammonium nitrate . . . 150 ,,

Water 4 fluid ounces.

Ammonia, till the precipitate redissolves.

After the work has been pickled in the above solution,
it is to be immersed in a dilute solution of ammonium or
potassium sulphide.

The object of the mercury chloride is evidently to
impart a coating of metallic mercury to the metal, as
both copper and brass are almost instantly coated with
mercury when immersed in the first solution.

A brass ash-tray dipped in the two solutions suc-
cessively and scratch-brushed was observed to be simply
amalgamated. The dark colour may be made permanent
by omitting the scratch-brushing, and simply drying out

210 METAL-COLOURING AND BRONZING part

in sawdust. The effect on brass is as useless as in the
preceding methods.

§ 160. The following is another instance of a bronzing
liquid containing unnecessary constituents : —

Mercury sulphide . 50 grains.

Potassium sulphide . 50 ,,

Sodium hydrate . . 1 fluid ounce (20 % solution).

Water. ... 3 ,, ounces.

A brass tray immersed in this solution assumed a dark
brown colour with a patchy appearance. This result was
only obtained with difficulty, and a second attempt to
procure a similar result completely failed. From this
it may be inferred that a bronzing effect can only be
produced on brass by this method when the solution is
just freshly prepared, and even then the colour is very
poor.

Results equally as good can be obtained by omitting
the mercury sulphide and sodium hydrate, as the potassium
sulphide is the only effective reagent.

§ 161. The following solution of potassium sulph-
antimoniate is recommended by Bottger for copper : —

A solution for testing this method was made by dis-
solving 100 grains of potassium sulphide in 12 fluid
ounces of water and then saturating the hot solution
with freshly precipitated antimony sulphide. The follow-
ing results were obtained : —

(a) Brass tray . . rich shellac-brown.

(b) „ ,, ,, but paler.

This solution acts on brass very slowly, (a) was produced
after fifteen to twenty minutes' immersion ; (b) occupied
about half an hour, (a) was done in the freshly pre-

in COLOURING OF BRASS 211

pared solution ; (6) was done after several other articles
had been bronzed. Both were scratch-brushed at inter-
vals. Copper takes a brown to black colour very quickly
in this solution, the shade of colour depending on the
length of the immersion.

§ 162. The following solution acts much more quickly
on brass than the preceding one, producing a dark gray
deposit of metallic arsenic, which becomes lighter on
scratch-brushing : — x

White arsenic oxide ... 1 ounce.

Hydrochloric acid ... 2 fluid ounces.

Sulphuric acid . . . . J „ ounce.

Water 12 ,, ounces.

§ 163. A very good black colour can be obtained on
brass, as previously explained, by a solution of

Copper nitrate 50 parts.

Water 100 „

If the work is too large for immersion, it is heated and
the solution is applied by means of a paint-brush, when
the heating is continued until the surface is dry. It is
then gently rubbed with a linen pad, and brushed with or
immersed in a solution of

Potassium sulphide . . . 10 parts.

Water 100 ,,

Hydrochloric acid . . . . 5 ,,

Immersion of the work in the liquid produces much
better results, and, after draining off the superfluous
liquid, it is heated on a hot plate or over a clear Are till
dry. We have obtained more uniform results by using
a solution about three times more dilute than the pre-
ceding solution of copper nitrate, viz. —

1 See Appendix.

212 METAL-COLOURING AND BRONZING part

■ i. ^^^^^^^^^^^^^— ■ I I ■ ■ ■ — ^ 1^— ^^— ^^^^^^^^^— — ■ ■ ■ ■ ■!■■■■ — I

Copper nitrate 100 parts.

Water 600 „

The heating process must not be continued longer than
is necessary to convert the whole of the green salt which
forms on drying into the black copper oxide.

A good black can be thus produced on brass in this
way without recourse to the second pickling in potassium
sulphide, but this second pickling is probably advan-
tageous in fixing the colour.

§ 164. The following bronzing liquid is due to
Bottger : —

Mercury chloride .... 100 grains.

Antimony chloride . . . 100 ,,

Water 20 fluid ounces.

The above salts are dissolved in the water and a current
of sulphuretted hydrogen passed through until the
whole of the mercury and antimony are precipitated as
sulphides. The precipitates are allowed to settle, when
the supernatant liquid is poured off and the precipitates
washed several times with water; they are then boiled
in a 20 per cent solution of sodium hydrate until they
are completely dissolved.

One hundred grains of a solution of sodium thio-
sulphate are then added to the dissolved sulphides and
the whole allowed to stand till the liquid is clear. The
clear solution is then decanted off, heated, and the work
immersed in it. Or the solution may be painted on to
the hot work in the usual way —

(a) Brass tray slaty-black.

(ft) Coppered tray „

in COLOURING OF BRASS 213

There is apparently no difference in the reaction of this
solution on brass and to that on copper.

The colour produced is probably due to a deposit
(by simple immersion) of antimony, the deposit being
rendered darker by the presence of a sulphide in the
solution. If the work is smooth and bright, the black
deposit will be correspondingly smooth and bright, but
if the work has a dead or dull appearance then the
deposit will be correspondingly dull.

The appearance of the articles after bronzing may be
improved by lightly scratch -brushing, which operation,
when judiciously done, gives a better finish to the work
without sensibly diminishing the intensity of the colour.
The same remarks also apply to the final polishing with a
calico mop and rouge. The work may be rendered grayer
by heating it after bronzing, but the colour obtained
is not so rich as before.

With regard to this solution it should be stated that
the mercury compound is quite unnecessary and useless, as
the bronzing is quite as effective without it. Moreover,,
mercury sulphide is insoluble in sodium hydrate, so that
in reality the mercury compound is thrown away and only
the antimony compound used.

This was proved by the following experiment : —

Antimony sulphide . . . 100 grains.

Sodium hydrate .... 350 ,,

Water 4 fluid ounces.

The mixture was boiled for some minutes, the undissolved

portion allowed to settle, and the liquid used for bronzing.

The results were precisely similar to those recorded above.

One hundred grains of sodium thiosulphate were added,

214 METAL-COLOURING AND BRONZING pabt

and an article immersed in it, with precisely the same result;
therefore the only effective bronzing agent is the solution
of antimony sulphide in the caustic soda.

When the work which has been bronzed in either of
these solutions is scratch-brushed, the deposit is that of
slightly violet-tinted antimony.

§ 165. Another method of producing a black colour
on small brass articles is recommended by Buchner, as
follows —

Copper carbonate .... 10 parts.

Liquid ammonia 70 „

Water 150 „

The article is to be rubbed with the solution until it
blackens, when it is heated over a hot coal-fire (without
putting it in the fire). After cooling it should be rubbed
with olive oil.

We tested the method in the following way : —

Copper sulphate .... 250 grains.

Water 6 ounces.

Sodium carbonate .... 300 grains.

The action of the sodium carbonate is to precipitate the
copper as carbonate, which is allowed to settle, and the
supernatant liquid filtered off. The residue is well washed,
just sufficient liquid ammonia added to redissolve it, and
the liquid diluted to one pint.

The articles were then immersed in the solution and
heated over a clear flame. After several immersions and
subsequent heatings the brass assumed a reddish-black
colour, and a copper article similarly treated became dead
black.

ill COLOURING OF BRASS 216

All these ammoniacal copper solutions, when heated,
deposit a black substance on the bottom of the vessel
containing the bronzing solution, ammonia being expelled.
This black substance is soluble both in ammonia and in
nitric acid. We have used this solution largely for parcel
bronzing, and it is probable that this deposit is the cause
of the black colour we obtained on brass.

The filtrate obtained from the precipitated copper car-
bonate was also tested as to its power of bronzing brass,
which received a light yellowish-drab colour on immersing
it in the hot solution.

The colour on the copper article which was made black
in the present bronzing liquid is anomalous, as we failed
to reproduce it. It was thought that the solution was too
impoverished to give the effect as it had been in use for
some time, so a new solution was made, and the experi-
ments repeated with a coppered tray, but the result was
not a permanent black as before. Although we succeeded
in getting the tray black all over, the coating would not
withstand brushing with a soft brush without exposing
the copper. This method, as described above, is of little
use for bronzing copper. 1

§ 166. The following method, which is a modification
of the former one, and which taught us the cause of the
varied results previously obtained, is one of the best
with which we are acquainted for giving a rich, firm,
durable coating to brass. The only precaution required
is to avoid excess of ammonia which practically dissolves
all bronzes produced by means of copper solutions.

Copper nitrate . M . . . 10 ounces.

Water 20 ounces.

Ammonia solution

1 See Appendix.

216 METAL-COLOURING AND BRONZING part

The copper nitrate is dissolved in the water, and the
ammonia cautiously added, until the precipitate which
forms at first is just redissolved. It is probable that the
black deposit is due to copper oxide. The following
results were obtained : —

(a) Brass ash-tray . . . black (lustrous).

(6) „ ,, . . . steely-black.

(d) Part coppered, part brass . black and crimson.

(a) was swilled after bronzing and dried out in sawdust ;

(b) was Tieated after drying out; (c) and (d) were not
heated ; (d) became black on the brass parts, and a crimson-
coloured bronze on the coppered parts.

The solution becomes very turbid after a time, and in
this condition appears to yield the best results. It reacts
very quickly on brass, producing a magnificent dark blue-
black colour, which is firmly adherent, and is capable of
taking a good polish.

On somewhat strongly heating the articles after bronz-
ing the dark blue changes to a steely-black, which is not
so rich in colour as before heating.

The bronzing reaction does not take place till the
solution is warm, and goes on rapidly when the solution
is worked hot. The bottom of the dish in which the
bronzing was done was covered with a deposit, which was
partly black and partly reddish-brown.

The only effect produced on copper by the solution was
a transparent crimson coloration. It was thought that,
as the presence of zinc in the brass enabled the solution to
be decomposed, with the formation of the black coating
before mentioned, the copper might be also blackened by

in COLOURING OF BRASS 217

immersing it in the hot solution in contact with zinc, but
no such change could be effected in- this way. The
crimson colour on copper makes a very effective contrast
with the blue-black on brass for parcel bronzing.

A black colour may be obtained by dipping the work in
the clear solution (which for this purpose may contain an
excess of ammonia) and heating on a hot plate, or over a
clear fire until the black colour appears. This plan, how-
ever, is much more tedious, involves more labour, and
yields an inferior result.

§ 167. It may be desired, previous to the operation of
bronzing, to impart a brass surface to copper articles so as
to enable them to take a black colour by one of the
methods referred to in the preceding paragraphs. Or,
on the other hand, to give a copper surface to a portion of
a brass article, with the object of getting a contrast in the
colours and so produce a most artistic effect.

For the purposes of coppering, a solution of copper sul-
phate, with the aid of an electric current, is most generally
employed. This salt is low in price and easily obtainable.
It is also very valuable on account of its electrical rela-
tions with regard to different metals, by which copper is
deposited in a practically pure state, even from impure
solutions.

Copper may also be deposited on brass by simple
immersion. If the copper article is surrounded by iron
wire and suspended in a dilute solution of copper sulphate
or copper nitrate, it soon becomes coated with a firmly
adherent, but very thin, coating of copper.

A surface coating of bronze may be given to copper
goods by immersing them in a boiling solution of cream
of tartar containing tin. A half-hour's boiling will be

218 METAL-COLOURING AND BRONZING pari

sufficient for tinning, especially if a few drops of tin
chloride are added to the solution. When the coating is
sufficiently thick, the objects are rinsed in hot water and
dried ; they are then heated moderately until they have
taken a bronze colour.

For brassing copper, the preceding solution is replaced
by a saturated solution of ammonium chloride, containing
some granulated zinc, in which the copper articles are
boiled as before. When the zincing is completed the
article is rinsed in hot water, dried, and heated as with
bronze until the brass colour appears. To accelerate the
zincing, a little zinc chloride may be added to the bath.

If, after heating, the articles appear iridescent or
patchy in places, these irregularities may be removed by
polishing with tripoli and scratch-brushing.

The change which occurs in this method may be easily
understood. By the boiling with tin or zinc respectively
a coating of one of these metals is obtained on the surface
of the copper, and on heating unites with the copper, form-
ing a surface alloy, even by the aid of a very moderate
temperature.

§ 168. In a previous paragraph it was explained that
a very beautiful black could be obtained by means of a
solution of copper nitrate neutralised by ammonia. The
same result may be obtained by the following : —

Copper sulphate 400 grains.

Water £ pint.

Liquid ammonia

The copper sulphate is dissolved in the water, and suffi-
cient ammonia added to just redissolve the precipitate
which first forms. Brass pickled in the cold solution

in COLOURING OF BRASS 219

acquires an asphaltum brown colour which easily rubs of.
Brass pickled in the hot solution gives an adherent
deposit of the same colour as above.

A stronger solution than the above-mentioned one was
then tried, viz.—

Copper sulphate . . 2 ounces.

Water .... just sufficient to dissolve

the copper sulphate.
Ammonia ... to neutralise and make

slightly alkaline.

A brass tray immersed in this solution, made hot, was
almost immediately turned a beautiful and permanent
blue-black colour. As copper assumes a beautiful Floren-
tine tint in this solution, brass articles, which have been
parcel coppered and then bronzed in the above solution,
present a very beautiful contrast in the copper and brass
parts respectively.

§ 169. The following is one of those complicated mixtures
which are often met with in books dealing with workshop
receipts, and used by bronzers who often treasure them as
profound secrets known only to themselves. It need only
be stated here that a simpler mixture is not only cheaper
but yields better results : —

Hydrochloric acid ... 20 fluid ounces.

Copper sulphate .
White arsenic
Copper acetate
Iron sulphate
Ammonium chloride

1£ ounces.

14 „
160 grains.

160

»»
80 „

A series of experiments with the above solution gave a
steely-gray colour to brass, but the coating withstood
scratch-brushing very imperfectly.

One hundred grains of sodium thiosulphate was then

220 METAL-COLOURING AND BRONZING part

added to the solution, and brass articles immersed in it
received a darker shade of colour than before ; the coloured
film was more firmly adherent to the metal, and better with-
stood the action of the scratch-brush. Warming the work
previous to the immersion favoured the formation of the
colour, but two or three immersions and subsequent
scratch-brushings were necessary to get a good effect.
Warming the solution does not appear to make much
difference.

BROWN COLOURS ON BRASS

§ 170. For some years, writes the Bavarian, Industrial,
and Trade Paper in 1886, " beautifully coloured bronze
figures, which rapidly won the public favour, have been
brought into the market by the French. The models
for them were chiefly executed by French sculptors, and
then cast in brass or bronze. The colouring produced on
these figures is of great variety and taste, according to
their character, which greatly adds to their beauty and
popularity. The bronzing varies from all shades of faint
or clay-yellow to reddish-brown, and red to dark brown
and black. It has a good bronze appearance and adheres
firmly to the metal, that is — it chemically unites with it.

"A firm in Philadelphia, which was engaged in the
fabrication of art-metal wares, but not of such figures as
above described, received from one of its customers two
uncoloured figures, with the request to colour them similar
to the French bronzing. After trying different recipes,
the idea was suggested of making use of sulphur com-
binations of arsenic and antimony, which was carried out
as follows —

"The articles are thoroughly cleaned and rinsed in

in COLOURING OF BRASS 221

water until every trace of acid is removed, for if any
acid is left in the pores or crevices it comes out after
bronzing, and produces ugly black spots and streaks,
which it is impossible to properly remove. The solution
is applied to the article by means of a pad of cotton-wool
or a thick soft brush. It is best to commence by putting
on a dilute solution of ammonium sulphide as thinly as
possible, and quickly go over certain limited parts of the
, surface. The quicker and the more equally this is done,
the better will be the final effect. It is then allowed to
dry, and any separated sulphur, etc., brushed off. A dilute
solution of arsenic sulphide, dissolved in ammonia, is now
applied, which produces a colour similar to that of Mosaic
gold. Another coating of the same solution is given when
the former one is dry, and so on until the desired tint is
obtained. The more of this liquid there is used the
darker will be the colour. And it is possible to get a very
dark brown.

" A reddish-brown colour is obtained by means of a
solution of antimony sulphide in ammonium sulphide, and
one is able to produce the most delicate pink and the
deepest dark red colour. By rubbing some parts more
than others a high metallic lustre is obtained.

" It should be remembered that ammonium sulphide and
ammonia dissolve the colouring matter again, so that if
any badly-coloured parts appear they may be dissolved off
and the colouring repeated. In such a case it is generally
preferable to remove the whole colour from the figure and
commence again. Just as the solutions of antimony and
arsenic can be dissolved in ammonia or ammonium sulphide,
so can the same be dissolved in potassium or sodium hydrate
or sulphide, and sometimes the latter are the more useful.

222 METAL-COLOURING AND BRONZING part

" If the figure is pickled dead instead of bright, then the
tone of the bronzing will be different. When the metal is
left too long in the acid it assumes a greenish-gray surface,
which should be rubbed with a cloth until it shines and a
good lustre appears on the metal This surface takes a
dead yellow colour by treatment with the above-mentioned
metallic sulphides. The article should not be warmed for
bronzing."

§ 171. A beautiful brown colour, known as Bronze
Barb6dienne is produced, according to Langbein, in the
following way : —

" Freshly precipitated arsenic sulphide and antimony
sulphide are digested with ammonium sulphide solution
until they are nearly dissolved and just a faint muddiness
remains. The solution is raised to a temperature of 35° C.
and the brass-ware immersed in it. The colour is at first a
golden-yellow, then brown, when it should be removed and
well scratch-brushed in order to bring out the colour, then
on re-immersion in the bath the articles take the desired
tone. The operations may require repeating several times
in some cases. If, after using several times, the solution
fails to work satisfactorily some more antimony sulphide
solution is added. As this solution decomposes on stand-
ing it is made fresh each time it is required for use.

" By this method only solid brass articles can be coloured
brown; to brassed zinc and iron articles the solution
imparts brownish-black tones, which however are very
beautiful. Upon brass, as well as upon zinc and iron
coated with brass, Bronze Barb&lienne may be produced
as follows : — Mix 3 parts of red antimony sulphide with
1 part of finely powdered red iron oxide, and triturate the
mixture with ammonium sulphide so as to form a thin

in COLOURING OF BRASS 225

paste. Apply the mixture to the object by means of a
brush, and after allowing it to dry in a drying chamber,
remove the powder by brushing with a soft brush."

§ 172. Various shades of colour may be produced on
brass by means of a solution of barium sulphide, but the
action is slow unless the solution is strong and used hot.
With a hot solution containing 100 grains of barium
sulphide to 1 pint of water an umber-brown colour, with a
rich blue sheen, can be obtained after a few minutes' immer-
sion. The first effect produced upon brass is a gold
colour, this is succeeded by an iridescent crimson, passing
to the brown colour mentioned above.

Brass, which has been parcel coppered and then
bronzed in this solution, looks very well provided that the
copper predominates.

§ 173. Smoke-bronze. — Bronzing with smoke is some-
times resorted to in order to give the metal an ancient
appearance. This is effected by exposing the work to
the smoke of a fire for some days, when it receives a firm
coating of a dark colour. The articles are generally
suspended over the smoky fire of a furnace by means of
brass wire. When the furnace is sufficiently heated the
smoke is maintained by burning hay and other substances
which produce copious smoke with the coal. When the
right tint is attained they are removed from the furnace
and allowed to cool without touching them with the
hands. The hotter the articles have been made the darker
will be the colour. If the articles which have been
smoked have been previously coated with a green bronze,
then it is well to finish with a waxed brush.

§ 174. A useful solution for colouring brass may be
made by dissolving copper sulphate in water —

224 METAL-COLOURING AND BRONZING part

Copper sulphate .... 8 ounces.
Water 1 pint.

(a) Brass ash-tray . . pale umber-brown.

(b) „ ,, . . . light coinage bronze.

The brass trays bronzed in this solution lost weight by
the process, probably due to the solution of the zinc and
oxidation of the copper. A copper tray bronzed in the
same solution gained in weight.

While copper bronzes very slowly in this solution, brass
readily takes a colour, especially if it is scratch-brushed
after a short immersion in the solution, and then re-
immersed. The tray (a) was heated on the lacquering
stove after bronzing, when it lost its umber-brown colour
and assumed the appearance of a light coinage bronze
similar to (6), which was bronzed after the solution had
been in use for some time.

In another series of experiments with common copper
sulphate employed in the same proportions as above, the
brass assumed a slaty-brown colour. As the same quality
of brass was employed in both cases, it is presumed that
the difference was due to impurities in the solution. The tray
first turned red, and on scratch-brushing assumed a nickel-
yellow colour, then on re-immersion became slaty-brown.

§ 175. The above experiments were varied by using
basic copper sulphate mixed with sufficient water to form
a paste.

Preparation of the mixture —

(a) Precipitate copper oxide from 1000 grains of copper sulphate

by caustic potash.

(b) Mix the precipitate with 1000 grains of copper sulphate.
(g) Mix (b) with sufficient water to form a thin paste.

in COLOURING OF BRASS 225

The work was then covered with the paste, and
heated over a charcoal fire. The process was repeated
once. The residue left on the work was difficult to remove
and the surface was not uniformly coloured.

'A brass ash-tray was pickled in the above mixture after
diluting with water to form a solution, when the tray
assumed a light slate-green colour.

§ 176. Another variation of the copper sulphate solu-
tion was tried in the following way : —

Copper sulphate solution . . . 600 grains.
Water 2 pints.

The above solution was boiled and rendered neutral by
the addition of a solution of sodium hydrate, then 1500
grains of red iron oxide were added.

The work was immersed in the above liquid, taken out,
and heated. The operations were repeated until a brown
tone was secured. This solution gave very good results
on copper and brass by immersion only, and the colour
was darkened by the subsequent heating. The effects by
heating the work over a charcoal fire were better than by
heating over a gas-flame.

The experiments were repeated with a solution as
above, but omitting the iron oxide. The colours were
similar, but paler in tone. Brass articles received a light
brown, with a bluish-green sheen.

§ 177. The following mixture is recommended by one
bronzer, but we failed to obtain any good results either on
copper or brass, even when the work was left in the solu-
tion for a long period of time : —

Copper sulphate solution £ pint (1 of salt to 4

of water).

226 METAL-COLOURING AND BRONZING part

Tartaric acid solution . 10 fluid ounces (100

grains of acid per
ounce).

Sodium hydrate solution . 12 fluid ounces (1 of

alkali to 5 of water).

(a) Coppered ash-tray . . . brassy colour.

(b) Brass ash-tray .... nickel-yellow colour.

The above was then tried with a slight excess of ammonia
added, when the following results were obtained : —

(a) Brass ash-tray .... pale greenish-brown.
(6) „ „ .... very dark brown.

The solution worked very slowly and cannot be recom-
mended. Doubtless the discordant results were due to
the action of ammonia.

§ 178. The following solution containing copper acetate
in addition to copper sulphate yields much better and
quicker results on copper than on brass : —

Copper sulphate .... 1 ounce.

,, acetate 2 ounces.

Water 1 gallon.

A brass ash-tray received after a short time a light
umber-brown colour, with bluish sheen, while a cast brass
article assumed a coinage bronze colour only after a consider-
able time. The great disadvantage of solutions containing
acetic acid is the iridescence which it gives to the bronzed
surface. The first change observed in this as in many
other solutions is a slight tarnishing. If the work is then
rinsed in water, scratch-brushed, and re-immersed in
the solution the tarnishing goes on much more rapidly
and uniformly than at first, and this second tarnishing
is quickly succeeded by the colour which the solution is

ill COLOURING OF BRASS 227

intended to impart to the metal. Contact with zinc
accelerates the preliminary tarnishing.

The above solution may be varied by the addition of
potassium nitrate, when brass receives a paler bronze
colour than in the preceding solution.

§ 179. Another solution similar to the preceding one is
valuable for copper and gives a light colour to brass.

Copper acetate

112 grains.

,, sulphate .

540 „

Vinegar

160 fluid grains.

Water.

(a) Brass ash-tray .

light umber-brown, with green sheen.

(5) Cast brass article

greenish-yellow on the raised parts,

and the same, but redder on the

groundwork.

A brass ash-tray heated on the lacquering stove after
bronzing became paler in colour, passed through a nickel-
yellow tint, then darkened, and became somewhat like
antimony, but inclining to purple.

§ 180. The effect of copper nitrate, neutralised by am-
monia, in producing a black bronze, has been referred to in
a previous part ; but brown colours can also be obtained by
its agency, with the aid of suitable mixtures and care-
ful manipulation. Several shades, varying from nickel-
yellow to jet-black, are capable of production by its means.
The following are some of the results after numerous ex-
periments : —

A saturated solution of copper nitrate was used —

(a) Brass ash-tray .... slaty -brown.

{b) „ ,, .... black.

(a) was immersed in the hot solution and received the colour

228 METAL-COLOURING AND BRONZING part

mentioned above, which became darker with a longer im-
mersion ; (6) was bronzed by immersion the same as (a),
and then heated on a hot iron plate. The process was re-
peated two or three times with alternate scratch-brushings.

Dilute solutions of copper nitrate produce merely a stain
on brass and copper work immersed in them. A black
bronze cannot be produced by pickling in copper nitrate
alone, it is necessary to heat the work afterwards without
rinsing in water. If the work is placed on a hot plate
and covered with an inverted glass beaker, the following
changes will be observed : — The work dries of a light
green colour, and brown fumes are given off. After a
little time the green colour changes to black by the de-
composition of the nitrate and the formation of black
copper oxide, which assumes a brighter and more uniform
appearance by brushing with a waxed brush. The copper
casting (c) treated in a similar way assumed a dark Indian
red colour on the deep parts and groundwork, while the
more prominent parts became black.

§ 181. The following mixture containing copper nitrate
was tested as a bronzing agent : —

Copper nitrate .
Tartaric acid
Sodium hydrate
Water

1 ounce.

»»

8 fluid ounces.

No useful effect could be obtained by immersing brass
or copper in this solution. Four fluid drachms of ammonia
were then added, with the result that a brass tray was
bronzed a very dark brown. From this it may be inferred
that the addition of tartaric acid and sodium hydrate is
unnecessary, as the same effect may be produced by copper
nitrate and ammonia alone.

in COLOURING OF BRASS 229

§ 182. A solution of copper nitrate of various strengths,
with sodium hydrate added until a slight precipitate just
begins to form, gave the following results on stamped
brass : —

1. A 20 per cent copper nitrate solution . . greenish-brown.

2. A 10 ,, ,, ,, „

o. A lo ,, , , , , . . „

It was impossible to say from the observed action that
one was better than the other.

§ 183. The following mixture may be useful in certain
cases, as a fairly good bronze was obtained on brass : —

1. Copper nitrate, 20 per cent solution, 3 fluid ounces.
Copper acetate, saturated ,, ,,

A brass tray received a neutral greenish-brown colour.

2. Copper nitrate, 20 per cent solution, 8 fluid ounces.
Copper acetate, saturated ,, 6 ,,

A bras* tray received a coppery yellowish-green.

3. Copper nitrate, 20 per cent solution, 1 fluid ounce.
Copper acetate, saturated ,, 4 ,, ounces.

A brass tray was bronzed a similar colour to that done
in the first solution.

§ 184. A method which is somewhat widely used for
producing on brass different shades of colour, varying
from blue to brown, and from a blue-black to dead-black,
is conducted in the following way : — To a solution of
a soluble copper salt, such as the sulphate, is added
a solution of sodium carbonate, which precipitates the
copper as copper carbonate. This is allowed to settle, and
the supernatant liquid filtered off. The precipitate is well
washed, and dissolved in ammonia solution to form the
bronzing solution.

230 METAL-OOLOTJRING AND BRONZING part

Copper sulphate 600 grains.

Water 1 pint.

The sodium carbonate solution consists of two parts
water and one part of the crystallised salt. The colour
obtained will, to a large extent, depend on the condition
of the solution, i.e. on the quantity of ammonia added.
The best results are obtained when a slight trace of the
precipitate remains undissolved, because then the operator
knows that the ammonia is not in excess. The following
results were obtained with a warm solution : —

(a) Brass tray, just immersed, taken

out, and swilled . . . brownish-black.

(b) Brass tray, immersed longer than (a), deep brownish-black.

(d) „ „ (c), light umber.

(e) „ immersed a considerable time, dead brown.

This is one of the most valuable solutions for brass
that we have worked with, every shade of colour is
beautiful. The tray (d) showed an exceedingly beautiful
lustrous brown colour.

§ 185. An extraordinary series of different colours from
the same solution may be obtained on brass by using a
solution of lead acetate containing some sodium thiosul-
phate. The following table shows the results obtained by
the author on a series of brass plates : — 1

Sodium thiosulphate .... 200 grains.

Lead acetate 200 „

Water 1 pint.

1 See Appendix.

Ill

COLOURING OF BRASS

231

Metal.

Time of
Immersion.

Temperature

Solution.

Remarks.

Colour.

Brass

5 sec.

87° C.

• • •

Pale gold

10 „

■ • •

Deeper gold

15 „

• • •

Brown gold

20 „

Not uniform

25 ,,

...

Crimson

30 „

• • •

Purple

35 „

Blue on edge
Blue extended

40 „

45 „

Iridescent

• • •

Bluish - crimson -
green

50 „

Pale blue edge

Mottled bronze
centre

55 „

Do., and irides-
cent

Mottled bronze
centre

1 min.

• • •

Pale blue

65 sec.

89*' C.

• • •

Mottled purple
Less mottled than

70 „

• • •

above

75 „

Pale blue on edge

Pinkish-bronze

80 „

Bluish sheen

Nickel colour

85 „

Mottled

Blue and pink

90 „

Pink centre

Very pale blue

100 „

Pink diminished

»t

110 „

Mottled

Purple and yellow

120 „

>»

Bluish-yellowand

pink
Pale purple

2£min.

m • •

3 ,,

Yellowish on edge

Bronze centre

4 „

87"c.

Mottled

Pink and yellow

5 „

»>

Pink and gray
Pink and pale

10 „

• • •

blue

§ 186. A series of experiments were made with a solution
in which sulphuric acid was substituted for lead acetate.

232 METAL-COLOURING AND BRONZING part

Sodium thiosulphate .... 200 grains.

Sulphuric acid J ounce.

Water 20 ounces.

A variety of colours was obtained on brass but of very
inferior quality as compared with the former solution.

BRONZING AND COLOURING OF ZINC

§ 187. The metal zinc is known in commerce under two
different names — zinc and spelter, the latter appellation
being applied to the metal when in the cast state. It is
then highly crystalline, with a bluish shade. On rolling
the metal into sheets this crystalline structure is largely
destroyed. The metal, when exposed to moist air, takes
up oxygen, forming a gray film of sub-oxide, and after-
wards forms basic zinc carbonate by the action of car-
bonic acid. This grayish-white deposit is of little artistic
value, and is not worth imitating by chemical means, as is
the case with copper and its alloys, so that the colouring
of zinc for ornamental purposes is entirely artificial, no
natural patina being used as a model or as a standard for
imitation. Zinc compounds are chiefly white in colour,
such as the oxide, sulphide, carbonate, and chloride.
These white compounds turn to a dirty gray colour when
exposed to ordinary atmospheric influences.

One of the most valuable properties of zinc is its
power of taking the finest impressions of a mould into
which it is poured, and it is one of the very best metals for
fine castings, and, as already stated, the cast metal can be
coated with copper, brass, and other metals with great
facility, and these deposits can be made to receive many

in COLOURING OF ZINC 233

of the various bronze shades of solid copper, brass, etc.
The use of zinc for cast and wrought work has spread
very rapidly, and a new branch of industry has become
firmly established, almost rivalling in bulk and import-
ance that of brass. With regard to the rolled form of the
metal, the modern discovery that it could be rendered
malleable by working it at a temperature of about 100° to
150° C. has greatly extended its usefulness and stimu-
lated its application for many purposes. It is true that
zinc coated with brass or copper is not equal in all cases
as a base for bronzing to that of the solid metals them-
selves, but in many others it is equally good, especially
when a good firm deposit has been secured.

§ 188. Black Colour on Zinc. — Zinc articles often re-
quire to be coloured black so as to imitate the beautiful
dark shades which lend such a charm to brass and copper
goods. There are many solutions in which a black deposit
is formed on zinc by simple immersion, but several of
these deposits are unfortunately non-adherent, and, in
nearly all cases, it is necessary to repeat the operation
several times, with alternate heating, to get a firmly
adherent coating. In this category may be classed certain
salts of copper, to which an excess of ammonia has been
added; or the insoluble salts of copper, such as the
hydrate and carbonate which have been dissolved in an
excess of potassium tartrate.

There are many difficulties in the way of producing a
good durable black coating on zinc, equal to that formed
on brass for philosophical instruments, such as mountings
for miscroscopes, and other optical apparatus, so as to
absorb the light and prevent reflection. The black for
such a purpose must be a true bronze and not merely a

284 METAL-COLOURING AND BRONZING part

coating of paint or varnish, which would not attain the
object in view. The bronze must be uniformly dis-
tributed over the surface and perfectly adherent. It
must also be quite opaque and as free as possible from
lustre.

In order to obtain a uniform result the manner of con-
ducting the operation, the temperature, the purity, the
strength, and composition of the solution must not vary.
Many failures result from inattention to these details, and
many different recipes have been given from time to time
in consequence of this. Many operatives recommend the
use of salts of silver added to the bronzing bath to yield a
more intense black colour, and some recommend the
addition of gold. In all cases it must be borne in mind
that zinc exerts a considerable reducing action on solutions
which are unaffected by copper and brass, and this leads
to the necessity of exercising great caution in operating
with it. If, for example, zinc which has been bronzed in
a solution containing copper nitrate is heated over a fire,
as in the analogous process for brass, the copper will be
reduced to the metallic state, and gives to the surface an
appearance far from that of a good bronze colour. 1

§ 189. Dullo recommends a solution of antimony chloride
in alcohol for producing a black colour.

Antimony chloride . . 200 grains.

Alcohol ..... 4 fluid ounces.

Hydrochloric acid . . . 2 ,, drachma

Zinc blackens in this solution, and the work, after allowing
to dry by exposure to air, or by igniting the alcohol,
requires to have oil or varnish applied to fix the colour, or
it will rub off. The deposit of antimony in this case being

1 See Appendix.

in COLOURING OF ZINC 235

in a powdery state makes it impossible to apply this
method for producing a firmly adherent patina.

Brass, when immersed in this solution in contact with
zinc, becomes coated with antimony.

Tin is darkened in colour by immersion, but the deposit
easily rubs off. The deposit does not dry after varnishing.

Iron is unaltered by dipping in the solution.

Lead and iron are blackened by immersion in contact
with zinc, but only at the points of contact.

All the above effects produced by zinc contact are im-
practicable on a commercial scale.

§ 190. Knaffl states that a firmly adherent black coating
may be given to articles of rolled and cast zinc by means
of a solution of the double sulphate of nickel and ammonia.
The method was tested by means of the ordinary nickel-
plating solution diluted with water. No deposit was
obtained upon zinc, brass, tin, iron, or lead.

Sulphuric acid in small quantity was then added to the
above solution and the liquid gently heated, when zinc on
immersion received an adherent dark purplish-gray colour.
Tin was stained by immersion in contact with zinc Iron
appeared to be unaffected. The deposit was rendered
much paler by scratch-brushing.

§ 191. Eletzinski states that a solution of molybdic acid,
or ammonium molybdate in nitric acid, made very dilute,
furnishes a good liquid for producing a brown patina on
cast zinc. The object assumes iridescent colours on immer-
sion whichheconsiders to be due to molybdenum oxide. The
following proportions were tried with the following results :-*-

Ammonium molybdate . . 100 grammes.
Ammonia .... 150 cubic centimetres.
Water J litre.

236 METAL-COLOURING AND BRONZING paut

In English measures this equals —

Ammonium molybdate . . 1550 grains.

Ammonia 2325 ,,

Water 1 pint.

Zinc acquired a beautiful iridescent appearance after a
few minutes' immersion in the solution. On continuing
the process the iridescent colours were succeeded by a light
yellowish-brown colour, and this, on warming the solution,
was followed by a slaty-black which was more opaque than
any of the preceding colours.

Brass and tin are unaffected when immersed alone, but
tin when placed in contact with zinc assumes a beautiful
dark violet colour which is firmly adherent to the metal.
Iron in contact with tin is simply stained.

§ 192. Puscher states that the following method gives a
good black colour on zinc : —

Copper sulphate .... 30 parts.

Potassium chloride . . . 30 ,,

Water 400 „

If any iron oxide is present it will remain undissolved and
should be filtered off. The perfectly cleaned zinc goods
are then immersed in the clear solution at the ordinary tem-
perature till a deep black deposit is formed, when they are
removed, washed, and dried. The coating is firm and will
stand rubbing with a cloth. In the case of large articles
they should be painted with the solution or moistened
with a sponge. Should the articles show copper-brown
spots, then the parts should be again moistened with the
solution till the proper black colour is attained. By
rubbing, the surface assumes an indigo colour, with bright
lustre. By brushing with linseed oil or wax the colour
becomes a deep lustrous black. The dry blackened zinc

ill COLOURING OF ZINC 287

• »

article may be immersed in a solution of Syrian asphalt
dissolved in benzol, and then rubbed with a woollen cloth.
The following solution was tried : —

Copper sulphate, saturated solution . 1 part.

Potassium chloride „ . 1 ,,
Water 2 parts.

The work was dipped momentarily in the solution, for if
it is left in only a few seconds, the black deposit
which forms on the surface will swill off when rinsed in
water. It is better not to swill, but to drain off the solu-
tion and put the work on a hot plate until just dry. The
process must then be repeated until a satisfactory colour
is obtained. The colour we obtained on both rolled and cast
zinc was a grayish-black. Zinc is coppered in a dilute
solution.

§ 193. Sainte-Claire Deville has shown that the trans-
formation of manganese nitrate into peroxide is effected at
a temperature of 200° to 250° C. PL Neumann has taken
advantage of this reaction to produce a black colour on
zinc. Manganese nitrate may be put in contact with zinc
without any notable change taking place, but when heated
the salt undergoes the change mentioned above. Now
zinc requires a temperature of 415° C. to melt it, so that
the temperature at which the nitrate is decomposed and
the metal blackened does not affect the zinc as regards
fusion.

He states that " the method is applicable to small and
large goods either by immersion or by painting with a
brush. After coating them with the solution they should
be left to dry very slowly over a charcoal fire, or, in the
case of small articles, over the flame of a spirit-lamp. In

288 METAL COLOURING AND BRONZING pakt

this way a more uniform and a more intense black colour
is obtained. After each application of the solution and
drying, the work must be scratch -brushed. When the
proper colour is obtained (which may be only after seven
or eight repetitions) it should be rubbed with the smallest
quantity of linseed oil to give it a good finish.

" Should the nitrate not be available, the black oxide
may be dissolved in hydrochloric acid, then precipitated
as carbonate with sodium carbonate, and the latter just
dissolved in nitric acid. The solution is then evaporated
to a syrupy consistence and allowed to stand for the
nitrate to crystallise out.

" The black colour adheres firmly to the zinc, which
can be bent or even hammered without it chipping off.
It is also able to resist the action of the atmosphere, in
consequence of its chemical nature, even better than the
black copper oxide formed on brass."

In consequence of the above method being so strongly
recommended we spent a considerable amount of time
in testing it with the following solutions : —

T j Manganese nitrate, saturated solution J fluid ounce.

1 Water 4 ,, ounces.

TT I Manganese nitrate, saturated solution \ fluid ounce.

( Water 4 ,, ounces.

For the first immersion the stronger solution was used,
and for the repetitions the weaker solution was employed.
The zinc articles were taken out of the solution and dried
gradually on a hot iron plate to the required temperature.
After drying they assumed a reddish-brown colour, and on
heating to a still higher temperature they became gray and
finally a blackish-gray. The black colour was not dense

in COLOURING OF ZINC 280

enough for a useful bronze. The solution is better suited
to cast than to rolled zinc.

Sheet -steel takes a fairly good colour in this solution.
From the experiments recorded above we consider the
method to be but an inferior one, as other solutions have
yielded much better results with much less time and
trouble. We cannot endorse the opinion expressed by
Neumann as to its valuable properties. We have tried
solutions varying in strength from 1 in 20 up to a com-
pletely saturated solution, and heated the zinc articles
from about 200° C. up to nearly the melting-point of zinc,
without obtaining thoroughly satisfactory results.

§ 194. A good black having a brownish tint can be
formed on zinc goods by immersing them in a solution of
copper nitrate of the following strength : —

Copper nitrate 1 ounce.

Water 5 fluid ounces.

A good black colour is produced almost immediately by
immersing zinc in this solution. The brownish tint
referred to above is probably due to the deposit containing
finely precipitated copper. Immediately after pickling
the work may be rinsed in clean water, and dried out in
sawdust.

§ 195. A grayish-black colour can be obtained on zinc
by immersion in a solution of

Copper acetate .... 250 grains.

Ammonium chloride . . 250 ,,

Water 5 fluid ounces.

On immersing the article in the solution a black deposit
forms almost immediately on the surface, which looks
very rich and lustrous after swilling in water, but it is

240 METAL-COLOURING AND BRONZING part

not adherent and easily rubs off, leaving a brown colora-
tion on the surface of the metal If the article is previ-
ously dipped in liquid gum and then immersed in the
solution, swilled, and dried gently on a hot plate, the
coating becomes much darker, and may afterwards be
vigorously brushed without removing the grayish - black
coating. By repeating the process several times a fairly
good black deposit may be obtained.

§ 196. BOttger recommends the following solution for
imparting a black colour to zinc : —

Platinum chloride .

40 grains.

Gum Arabic .

. 40 „

Water ....

We have tested the above solution in various ways,
both of the strength given and in different degrees of
dilution, but the result is always a black patchy colora-
tion, which on scratch-brushing reveals a deposit of regu-
line platinum. With very dilute solutions the colour is
more gray than black, and inclining to purple in places,
which may be largely removed by brushing. The colour
produced by platinum chloride is likely to be more per-
manent than those obtained by copper salts, but it is
generally less dense in appearance.

§ 197. Brown Colour on Zinc. — A reddish-brown colour
is produced on zinc articles by means of the following
solution : —

Copper chloride ..... 1 part.

Water 4 parts.

Ammonium chloride, saturated solution . 5 ,,

On pickling zinc in this solution a black deposit is formed,
which is easily rubbed off.

ni COLOURING OF ZINC 241

If the pickled zinc is dried on a hot stove, the black
coating is decomposed with the formation of a reddish-
brown coating, which is improved by brushing. The
heating process must not be continued longer than is
necessary to produce the brown colour, for if heated much
beyond that stage the coating will also rub off completely.

§ 198. A firmly adherent dark brown colour can be ob-
tained on zinc by immersion in the following solution : —

Copper sulphate .... 50 grains.

Sugar 75 „

Sodium carbonate . . . 500 ,,

Water 4 ounces.

§ 199. Gray Coating on Zinc. — An excellent coating
of arsenic on zinc, giving its surface a gray colour, may
be obtained by means of the following solution : —

Arsenious oxide .... 100 grains.

Sodium phosphate . . . 36 „

Potassium cyanide . . . 100 ,,

Water 5 fluid ounces.

The work must be first rendered perfectly clean and then
immersed in the cold solution. If too large for immersion
the liquid may be painted on to the article by means of a
brush.

§ 200. Puscher gives the following solutions for pro-
ducing a green patina on zinc goods : —

(a) Copper nitrate solution . , 5 fluid ounces (1 of salt to

100 of water).
/, >. j Ammonium carbonate . . 50 grains.
I Water 5 fluid ounces.

The work is recommended to be dipped first in (a), then
in (b) and set aside to dry.

242 METAL-COLOURING AND BRONZING part

In testing the above method a black deposit was ob-
tained after immersion in solution (a), which was unaltered
by dipping in solution (6). After allowing it to stand
twenty-four hours no change was observed, except the
formation of a grayish-white film, probably of ammonium
carbonate.

The solution (a) was tried of various strengths from
1 to 20 per cent, but with the formation only of a black
deposit in each case. There was no indication of a green
patina.

§ 201. Different Colours on Zinc. — Bdttger has obtained
a variety of colours on zinc, which should contain as
little as possible of lead, by means of the following
solution : —

Copper tartrate . . . 300 grains.

Sodium hydrate . . . 400 ,,

Water 10 fluid ounces.

It is quite necessary for success in this process that the
zinc articles should be scrupulously clean.

When immersed in the solution the metal first becomes
coated with a film of copper, and then assumes after a
minute or two colour films in the following order : —
Yellow^ light brown, brown, crimson, blue, purple, very
pale purple, light sea-green, then light crimson again.

All these colours are iridescent, and are evidently due
to thin films. It is difficult to get any one of them uni-
formly distributed over the surface of the article. There
are generally two or more colours present on a small area
of surface, and they are always very beautiful.

A hot solution works very rapidly, and is more difficult
to be controlled than a cold solution.

Continued immersion in the hot solution for five to ten

ill COLOURING OF ZINC 243

minutes produces a dense gray deposit, with a slightly
reddish tint.

§ 202. Similar results to those mentioned in the pre-
ceding paragraph may be obtained by the following
solution, which is due to Ludersdorff : —

Copper sulphate

100 grains.

Cream of tartar

100 „

Sodium carbonate .

750 „

Water ....

8 fluid ounces.

This solution behaves much in the same way as the pre-
ceding one; the colours, however, are more dense, but
less brilliant, and distribute themselves over the surface
after the same manner as the mottled designs commonly
produced on the edges of books.

The colours are produced in both hot and cold solu-
tions, and are preceded by the deposition of a film of
copper. All the colours of the spectrum appear to be
present at any one time, so that it is impossible to produce
a single colour alone on the surface.

PARCEL COPPERING AND BRONZING AS APPLIED TO

FINE ZINC CASTINGS

§ 203. After thoroughly cleaning the zinc work in the
usual way, a stout coating of brass should be deposited in
the hot cyanide brassing solution by means of an electric
current, after which it should be well swilled in water and
dried out in hot sawdust.

After removing the loose sawdust, all those parts
which are not to be coppered should be carefully painted
over with copal varnish of good quality by means of a
camel-hair pencil. A little asphaltum varnish may be

244 METAL-COLOURING AND BRONZING pabt

added to the copal varnish in order to render it darker in
colour than the brass, so that the operator may see every
stroke he makes with the pencil he uses in the process.
The work must now be allowed to remain for a few hours
in a warm place to dry.

When the varnish is dry, the work is suspended in an
electro-depositing bath of copper cyanide until a sufficient
coating of copper is obtained.

It should then be well rinsed in water, dried, and
soaked in turpentine and brushed until the varnish is re-
moved and the underlying brass is exposed. It is then
well rinsed in hot potash solution, and finally in plenty of
water, when it is ready for bronzing.

A good liquid to employ in this case is a hot solution of
copper nitrate. The article is immersed for a little while,
then removed, swilled in water and well scratch-brushed to
give a good and uniform surface, then re-immersed in the
bronzing bath, and the operation repeated if necessary.

The colour obtained upon the brass by this method is
a very dark green (nearly black), and upon the copper a
light nut-brown or bronzed flesh tint.

If the casting is a figure subject, it is best after brassing
all over to stop off all the draped parts and deposit copper
on the face and all the naked parts. The hair, sandals,
etc., may also be stopped off with advantage, but the
taste and judgment of the operator will generally dictate
the most effective treatment in this respect

Of course several of the other solutions mentioned in
the section on bronzing of copper and brass may be em-
ployed, but the one recommended above is very effective,
and the two colours blend together very harmoniously.

Brass, copper, or other metals may be treated in a

in COLOURING OF IRON AND STEEL 245

similar way to the above. In the case of a brass article,
the electro-brassing would of course be omitted. It is
very important in the case of zinc, or any metal that has
to be both brassed and coppered, that the brassing should
be done first, and not vice versa, as the free ammonia in
the brassing solution dissolves the copal varnish.

BRONZING AND COLOURING OF IRON AND STEEL

§ 204. Iron in the pure state is not a commercial
article, and therefore the effect of various agents in pro-
ducing colours on its surface need not be considered here ;
but iron in the condition of wrought or malleable iron,
steel, and cast iron is of very great importance, and will
therefore claim our attention from their industrial value.

Wrought iron is almost silver -white in colour, but
rapidly tarnishes when exposed to moist air at the
ordinary temperature, and still more rapidly oxidises
when heated under the same influence. Steel is bluish-
white when soft, but becomes much whiter when hardened.
When it contains much carbon it is close-grained and
lustrous. Many elements when alloyed with it, such as
chromium, make it still whiter and more brilliant. Cast
iron is grayish-white in colour, varying with the condition
in which the carbon exists, as well as with the amount
and nature of the impurities present.

Iron and steel become coated with two distinct grades
of colour by oxidation, according to the kind of oxide
which is produced. The red oxide is generally formed by
a slow process of oxidation, and varies in tone from brown
to a bright red colour, containing iron and oxygen in the
proportion of two atoms of iron to three atoms of oxygen.

246 METAL-COLOURING AND BRONZING part

The black oxide contains three atoms of iron to four
atoms of oxygen, and is generally produced when iron is
strongly heated in contact with air or oxygen. Various
shades of colour may therefore be produced by a com-
bination of these two chief oxides.

Iron forms a brassy -looking compound with sulphur
when the proportion of atoms of iron to atoms of sulphur
is as one is to two, or as three is to four. When the pro-
portion is as one is to one, then the colour varies from a
bronze to black, depending upon the physical condition of
the compound.

These various shades may be produced by means of vari-
ous solutions, or by heating in contact with gases or vapours.
A still greater variety of shades may be induced by taking
advantage of electro-deposition, as explained under the
heading of Electro-Chemical Metal-Colouring. In addition
to the colours obtained by the processes referred to above,
there is also the method of obtaining rainbow tints by the
deposition of exceedingly thin transparent films of oxides
or other compounds which produce an optical effect, as in
the case of the well-known process of tempering steel.
The surface of the iron or steel is made bright, and the
article gradually heated. When the temperature reaches
220° C, a faint yellow colour appears ; this is succeeded
by other colours as the process proceeds. The following
table shows the temper used for various articles, the lowest
temperature indicating the hardest temper : —

Temperature.

Colour.

Article.

220° 0.

Faint yellow.

Surgical instruments,

230° „

Straw yellow.

Razors.

255° „

Brownish yellow.

Scissors, chisels, etc.

265° „

Purplish brown.

Axes, planes, etc.

Ill

COLOURING OF IRON AND STEEL

247

Temperature.

Colour.

Article.

277° C.

Purple.

Table-knives.

288° „

Light blue.

Swords, springs.

293° „

Dark blue.

Fine saws.

316° „

Blackish-blue.

Hand-saws.

In order to obtain the above colours with certainty,
various alloys, whose melting-points are known, are used.
Messrs. Parkes and Martin have proposed the following as
suitable for this purpose : —

Lead.

Tin.

Melting-point.

214° C.

221° „

228° „

240° „

257° „

262° „

276° „

284° „

100

289° „

§ 205. Gray Colour on Iron. — A good and uniform
dark gray colour can be imparted to iron and steel by
first depositing a coating of copper on the articles and
then immersing in the following solution : —

Ammonium sulphide solution
Water ....

1 fluid ounce.
6 fluid ounces.

It will be well to repeat the immersion once or twice to
get a good colour. The deposit will withstand scratch-
brushing. Potassium or sodium sulphide may be used in
the place of ammonium sulphide.

§ 206. A light gray colour may be imparted to iron

248 METAL-COLOURING AND BRONZING part

and steel by means of a solution of antimony chloride.
The articles are first coppered as in the previous method,
then immersed in the following solution : —

Antimony chloride . 80 grains.

Water 1 J fluid ounces.

Hydrochloric acid to make the solution clear.

A deposit of antimony may be obtained on plain steel
without the previous coppering, but it has a very poor
appearance as compared with the method recommended
above.

§ 207. A good gray colour can be obtained on iron and
steel by immersion in a hot solution of

Arsenious oxide . 50 grains.

Hydrochloric acid sufficient to dissolve the above.

Water .... 1 fluid ounce.

No effect is produced upon either plain or coppered steel
in a cold solution.

§ 208. Blue Colour on Iron. — A blue colour is said to
be obtained on iron and steel by the following method : —

Glean the articles free from all dirt and grease, then
immerse in vinegar, well wipe off and dry. Now moisten
them with hydrochloric acid by means of a cloth, allow to
dry, and place in sand in a sand-bath until the desired
blue colour is obtained. We could not succeed in getting
a uniform colour by this method. Probably if the metal
had been polished very bright a better result might have
been obtained.

§ 209. The following method of producing a blue
colour on iron and steel is due to Bottger : —

Potassium ferrocyanide ... 25 grains.

Ferric chloride 25 „

Water 20 fluid ounces.

in COLOURING OF IRON AND STEEL 249

•
We were unable to get any coloration on steel in this
solution even after eighteen hours' immersion. Immersion
in the solution, and drying on hot plate without rinsing
in water, was tried without any useful effect.

§ 210. Puscher has produced a variety of colours on
iron and steel by means of a mixture of lead acetate and
sodium thiosulphate. The following proportions were
tried, with the results enumerated below, the colours
altering with the time of immersion : —

Lead acetate 50 grains.

Sodium thiosulphate . . . . 50 ,,
Water 5 fluid ounces.

The solution must be used hot

1. Light brown colour.

2. Darker ,,

3. Purple and blue mixed.

4. ,, ■ „ but paler tone.

5. Beautiful uniform light blue colour.

6. Steel-gray.

7. Black colour, after half an hour's immersion.

In order to produce a uniform colour all over, the article
must be completely immersed, so as to be equally heated.

§ 211. Black Colour on Iron. — A bluish-black colour
may be obtained on iron and steel by means of a solution
of sodium thiosulphate.

Sodium thiosulphate ... 50 grains.
"Water 5 fluid ounces.

The bluish-black colour produced by this solution has a
somewhat warm coppery tint. The effect was produced
by pickling in the hot solution during about ten minutes,
with scratch-brushing at intervals. Two scratch-brushings
were found to be sufficient. No advantage was obtained

250

METAL-COLOURING AND BRONZING

PART

from using a stronger solution, viz. 100 grains of the thio-
sulphate to 5 fluid ounces of water.

§ 212. A solution for producing a dead-black colour
on iron and steel, which is said to give good results, is
as follows : —

Bismuth chloride

1 part

Mercury chloride

2 parts.

Copper chloride .

1 part.

Hydrochloric acid

6 parts.

Water ....

50 „

The solution is recommended to be applied to the work by
means of a brush, the liquid allowed to dry on, and the
work boiled for half an hour in water. The operation is
repeated until the proper colour is obtained. The colour
is said to be fixed and improved by immersing during a
few minutes in a bath of boiling oil, then removing and
heating until the oil is driven off.

We could only obtain a zinc-gray colour by this method
after three repetitions, and even this colour was unsatis-
factory.

§ 213. Meriteus states that a bright black colour can
be obtained on iron by making it the anode in distilled
water, kept at 70° C, and using an iron plate as a cathode.
The method was tested as follows —

A piece of bright sheet pen steel was placed in distilled
water and made the anode by connecting with the positive
pole of a plating dynamo, and a similar sheet was con-
nected with the negative pole to form the cathode. An
E.M.F. of about 8 volts was employed. After some time
a dark stain was produced, but wanting in uniformity.

The experiment was repeated with larger plates, when
a good blue-black colour was obtained on the anode in

in COLOURING OF IRON AND STEEL 251

half an hour. On drying out in sawdust the colour
appeared less dense, and inclined to a dark straw tint. The
back of the plate was also coloured, but not regularly.
The face of the cathode was discoloured with a grayish
stain on the side opposite to the anode, but on the other
side the appearance was almost identical with the back of
the anode. The water became of a yellowish colour.

Fresh distilled water was then boiled for a long time so as
to expel all trace of the oxygen absorbed from the atmo-
sphere, and the experiment repeated as in the former cases.
No perceptible change took place after the connection had
been made with the dynamo for a quarter of an hour.
After the interval of one hour a slight darkening occurred,
but the effect was much less than that produced in five
minutes in aerated water.

The action of the liquid in colouring the steel is* evi-
dently one of oxidation, due to the dissolved oxygen, which
becomes more chemically active under the influence of the
electric condition, and gradually unites with the iron.

§ 214. The following solution for producing a dark

colour on iron and steel has been recommended : —

Ferric chloride 10 parts.

Water 200 „

Potassium sulphide .... 1 part.

The articles may be immersed in the solution or painted
with it, then allowed to dry by exposure to the air, and
finally brushed with a waxed brush. A fairly good black
bronze may be obtained in this way.

The above solution, with the omission of the potassium
sulphide, may be used for bronzing iron or steel. The
colour is brown after two immersions with subsequent
air-drying, and uniformly distributed over the surface.

252 METAL-COLOURING AND BRONZING pabt

A dense and fairly uniform grayish-black colour can be
produced on iron by digesting flowers of sulphur with
turpentine, painting fjie work with the liquid, allowing to
dry, and then heating over a clear fire, or on a hot stove.

BRONZE COLOUR ON IRON

§ 215. A bronze-like colour is produced on iron and
steel goods by exposing them to the vapours of heated
aqua regia, then dipping them in melted vaseline, and
heating until the vaseline begins to decompose.

This process yields a nice warm brown bronze colour on
iron and steel.

The steel articles, in our experiments, were heated by
holding them over a smokeless flame without bringing
them in contact with it. The excess of vaseline was
shaken off, and the process completed by holding the
article for a very short time in the flame itself. The work
was afterwards wiped with a soft cloth, but if it is rubbed
too hard some of the coating is removed.

§ 216. Iron and steel goods, such as gun-barrels, for
example, may be bronzed a brown colour by means of a
paste of antimony chloride. It is used for bronzing
mixed to the consistence of cream with olive oil,
and is known in the trade as "bronzing salt." The
iron is slightly heated, covered evenly over its surface
with this mixture, then left until the requisite colour is
obtained. The paste may be made more active by adding
a little nitric acid.

§ 217. Another method of producing a brown colour on
iron and steel may be employed by using a mixture of —

in BRONZE COLOUR ON IRON 253

Nitric acid J ounce.

Spirits of nitre J „

Alcohol 1 „

Copper sulphate 2 ounces.

Ferric chloride J ounce.

Water 2 pints.

Dissolve the copper sulphate in water and then add the
other ingredients. In the case of gun-barrels the burnish-
ing and marking is effected by means of the burnisher and
scratch-brush. The polishing is effected by rubbing with
a piece of smooth hard wood, called polishing wood. They
are then varnished with shellac varnish, or with lacquer,
and finally polished with hard wood as before.

Instead of the above mixture, the following may be
used : —

Copper sulphate 1 ounce.

Spirits of nitre 1 „

Water 20 ounces

In any case the iron must be previously well cleaned and
rendered quite bright; it is then freed from grease by
rubbing with whiting and water, or with quicklime and
water. The bronzing mixture is then put on and allowed
to remain twenty-four hours, when it is brushed off with a
stiff brush. If the article is not sufficiently deep in colour
the operation is repeated once or twice.

§ 218. The following method is said by Buchner to be
suitable for bronzing gun-barrels : —

Ferric chloride .... 400 grains

Antimony chloride ... 10 „

Gallic acid 10 ,,

Water 5 fluid ounces.

254 METAL-COLOURING AND BRONZING part

We found that iron and steel acquired in this solution,
when used hot, first a very pale blue tint, then on longer
immersion a darker colour, passing into a purple. After a
still longer immersion the purple was succeeded by a plum-
bago-gray colour. The deposit is evidently due to antimony.

No immediate effect is obtained in a cold solution.

§ 219. Iron and steel articles may be bronzed by
decomposing certain organic compounds on their surface
by means of heat, with the formation of a deposit of a
mixture of ferric and magnetic oxides. If, for example,
the surface of an iron article is coated with a layer of
linseed oil, and the whole is heated over a fire, hydro-
carbons are eliminated which assist in colouring the metal.
With regard to bodies which cannot be conveniently
heated in this way, a uniform coating of oxide may be
imparted by plunging them in water at a temperature of
80° to 100° C.

According to Hess, iron articles may also be bronzed by
immersing them in an acid solution of ferric chloride, and
allowing them to remain in it for some time. The water
of the solution is decomposed, and the liberated oxygen
unites with the iron, forming a dark layer of oxide. The
work is then well swilled in hot water and allowed to dry.
It may be finally brushed with a waxed brush. The
advantage of this method lies in its rapidity.

COLOURING OF TIN

§ 220. Although tin forms several coloured compounds,
such as stannous oxide and sulphide and stannic sulphide,
they are scarcely ever employed for bronzing the metal, as
the colours obtained are not sufficiently satisfactory. The

in COLOURING OF TIN 255

best method is to first coat the tin with copper or brass
by means of electro - deposition, and then to bronze the
electro-deposited surface.

§ 221. We have tested a few solutions for the direct
colouring of tin with the results recorded below —

I. Copper sulphate .... 120 grains.
Iron sulphate . . . 120 ,,

Water 5 fluid ounces.

This solution is best worked hot. The first effect is a
deposition of copper, which imparts a brassy appearance
to the tin. This colour subsequently becomes deeper and
assumes the well-known colour of copper, after which a
dark red colour appears on the edges of the work (a
stamped tin plate) which gradually extends over the
surface as the operation proceeds. On drying out in saw-
dust and brushing with a waxed brush a good bronze
colour remains.

II. A solution of copper sulphate containing 20 per
cent of the salt gave a similar result to the former, but a
shade darker colour. The result was better with a hot
than with a cold solution.

III. A saturated solution of copper acetate acted much
slower than the copper sulphate and gave a brassy colour
to the tin.

IV. An acid solution of copper sulphate used cold
gave a deposit of copper, which being non- adherent
washed off when swilled in water, exposing a crystalline
tin surface.

V. A solution of copper chloride gave a black deposit
on the tin at first, then a red deposit of copper, which
afterwards washed off completely by means of water,

256 METAL-COLOURING AND BRONZING pabt

leaving a crystallised surface. The same result was
obtained in the hot and cold solution.

VI. A solution of equal parts of copper chloride and
ammonium chloride gave results identical, with those of
No. V.

VII. Copper nitrate solution gave a very thin film of
copper on the tin, when a cold solution was used ; but
with a hot solution a white incrustation, probably of
m etas tannic acid, was formed.

VIII. Solutions of ferric chloride, lead acetate, and
sodium thiosulphate appear to have no action on tin
unless the latter is immersed for a very considerable
time, and even then the reaction is but slight. A solution
of copper sulphate and sodium thiosulphate gave no
result when cold, but a dark deposit was formed when the
liquid was heated — this coating, however, easily rubbed
off.

CRYSTALLINE SURFACE ON TIN

§ 222. Moire* M^tallique. — The very beautiful crystal-
line appearance of tin, to which the above name is given,
may very readily be produced by first heating the tin
plate to a point where the tin begins to show signs of
melting on the edges, and then quenching by immersing
the whole in certain cold acid solutions.

The following solution yields excellent results : —

Nitric acid I measure.

Sulphuric acid 10 measures.

Water 89 „

in COLOURING OF GOLD 257

This gives a surface of small and very numerous
crystals.

Much larger crystals and, therefore, fewer in number
upon a surface of given area may be produced by pick-
ling the article during a few seconds in a warm acid
mixture as above. The metal is immersed when cold.
After the immersion it is dried out in sawdust, and
varnished with a transparent varnish of any desired
colour.

COLOURING OF GOLD

§ 223. Ormolu Colour. — To obtain this fine colour
on gilt work it is first lightly scratch-brushed, strongly
heated, and then allowed to cool a little. The ormolu
colouring matter is a mixture of haematite, alum, and sea-
salt made into a thin paste with vinegar, and applied with
a brush until the surface of the article is covered, except
such parts as are required to be burnished. The article is
heated until it begins to blacken, then removed from the
fire, plunged in cold water, washed, and rubbed with a
brush which has been dipped in vinegar or dilute nitric
acid.

French clocks and other ornamental work are coloured
in this way. The gilt articles are lightly scratch-brushed,
then coated with a paste of potassium nitrate, alum, and
red iron oxide reduced to a fine powder, and worked into
a paste with a solution of saffron, atmatto, or other
colouring matter, according to the tint required. When
the gilding is strong, the article is heated until the
coating of the above mixture curls over by being touched
with the wet finger. When the gilding is only slight,

258

METAL-COLOURING AND BRONZING

PAET

the mixture is only allowed to remain upon the article
for a few minutes. The article is then well washed with
water to remove the paste. Such parts as have acquired
too deep a colour are afterwards struck with a brush
made with long bristles. By a series of vertical strokes
with the brush the uniformity of surface is produced. If
the first operation has not been successful the colouring is
removed by dipping in dilute sulphuric acid, and after
well rinsing the operation is repeated.

Bed Ormolu is produced by means of a mixture of —

Alum ....

30 parts.

Potassium nitrate

. . 30 „

Zinc sulphate

- • 8 „

Common salt

• • 3 „

Red ochre ....

. . 28 „

Iron sulphate

. . 1 „

To these may be added annatto or madder ground in
water.

Yellow Ormolu is produced by the following : —

Potash alum 50 parts.

Red ochre 17 „

Zinc sulphate 10 n

Common salt . . . . 3 „

Potassium nitrate .... 20 n

Dead Ormolu, for clocks, is composed of —

Potassium nitrate .... 37 parts.

Alum 42 „

Common salt 12 M

Powdered glass and calcium sulphate . 4 ,,

Water 5 „

in COLOURING OF GOLD 250

The whole of these substances are well ground and mixed
with water.

§ 224. Colouring Gold. — This operation consists of
imparting a colour to gold articles after every other
process has been completed. Its object is to give to
alloyed gold all the appearance of fine gold itself by
dissolving out the base metal from the surface of the
articles and leaving a facing of gold of deep rich colour.
Two distinct modes of colouring are adopted by jewellers,
termed respectively dry colouring and wet colouring. The
latter is most frequently practised, as the former cannot
be effectively applied to gold inferior to 18 carat.

Wet Colouring. — The ingredients of the mixture
employed in this process have a powerfully solvent action
on the base metal with which the gold is alloyed,
and a weaker action on the gold itself, so that the
article loses weight in direct ratio to the length of time
it is submitted to the colouring process, and this
loss is greater as the gold is lower in quality. Gee
states that the colouring is hastened and the loss in
weight reduced to a minimum by using old colouring
liquid, and he assumes that the dissolved gold is, to some
extent, deposited again on the article, because the loss in
weight of some common qualities of gold was found to be
very little, and the amount of gold recovered from the
spent colouring liquid very small indeed. This statement
is in accord with the well-known fact that in any liquid
in which a metal, say copper, is electro-positive to the
metal in solution, say gold, the latter is deposited on the
former.

Many different mixtures are used for colouring gold,
some of which will be afterwards given in tabular form.

260 METAL-COLOURING AND BRONZING part

The following, which he has found to be effective, has
been supplied to the author by an experienced Birmingham
jeweller :-

Potassium nitrate . . . . 12 ounces.

Common salt 6 „

Hydrochloric acid .... 3 „

The nitrate and salt are pounded to a fine powder and
placed in a previously warmed plumbago crucible about 8
inches by 7 inches, then stirred with a wooden spoon for a
minute or two. The acid is then added with about 1 ounce
of boiling water, and the mass constantly stirred until it
boils up to the top of the pot The work, which has been
previously cleansed in hot potash or soda solution, is then
suspended in the colouring liquid by means of a silver or
platinum wire for about one minute, then well swilled in
boiling water. A little more water is added to the
colour-pot, and when the liquid boils up the work is
again immersed for another minute and swilled in boiling
water as before. This operation of dipping and swilling
is repeated several times, the colouring liquid being
weakened by adding water before each immersion, until
the desired appearance is attained. The work is finally
well washed in hot water and dried in boxwood sawdust.
The whole process takes five to seven minutes.

The coloured work is next scratch-brushed, on a lathe,
with a revolving brush made of very fine brass wire and
having stale beer dropping on it. If the colouring has
been properly conducted, a beautiful rich and dead colour
will be produced.

Dry Colouring. — This term is applied to the colouring
process when no liquids are used as constituents of the
mixture. The ingredients used are —

in COLOURING OF GOLD 261

Potassium nitrate .... 8 ounces.
Common salt . . . . . 4 „
Alum . . . . . 4 ,,

These substances are ground to a fine powder, well mixed
and placed in a previously heated blacklead " colour "-pot,
of the same dimensions as that described for use in wet
colouring, but the same pot must not be employed for dry
colouring as has been used for the wet process. It is
well to get the pot nearly red-hot before placing the
" colour " in it. The mixture must then be constantly
stirred with an iron rod. It will first boil up as a
greenish liquid, then solidify, and afterwards boil up a
second time and become thoroughly fused, having a
brownish-yellow colour. At this stage the work, which
has been previously annealed and dipped in dilute aqua-
fortis, is dipped in the "colour," being suspended on a
silver or platinum wire, the latter being preferred, and
then kept in motion for about a minute and a half, then
immersed in boiling water containing a little aquafortis.
The immersion and swilling are again repeated, when
the articles possess a beautiful colour. They are then
washed in hot water containing a little potash, and finally
dried in warm boxwood sawdust.

In dry colouring the work should be as highly polished
as possible previous to the colouring, for the brighter it is
the better will be the final colour. The time given above
is only intended as a general guide, as some work will
colour much quicker than others, and the time can only
be arrived at by experience. The following mixtures
have been recommended for colouring : —

262

METAL-COLOURING AND BRONZING

PART

Dry Process

Wet Process.

Potassium nitrate 8 oz.
Common salt . . 4 , ,
Alum . . . . 4 ,,

Potassium nitrate
Common salt . .
Alum ....
Hydrochloric acid
Water in each
case in sufficient
quantity . . .

8
4
4

• • •

14
7
7
2

• • •

15
7
7
1

• ■ •

• * •

• • •

Sal-ammoniac . 4oz.
Potassium nitrate 4 ,,
Borax . . . . 4 ,,

The following is a useful mixture for removing tarnish
from coloured gold which has been kept in stock for
some time : —

Sodium bicarbonate
Calcium chloride
Common salt
Water

2 ounces.
1 ounce.

1 „
16 ounces.

Well mix the ingredients and apply with a brush.

§ 225. The colour of electro-deposited gold is not the
same in every case, but varies in tint according to the
nature of the anode and the purity of the solution. If
copper goods are gilded in the solution, it becomes charged
with some dissolved copper ; and the same remarks apply
to silver articles, which impart silver to the solution. In
the former case the deposited gold becomes redder, and in
the latter case paler, yielding what is known as green gold,
from the greenish cast of the metal. Red gold may there-
fore be obtained by adding some copper cyanide to the
bath. Green gold is produced by adding some silver
cyanide to the gilding bath. By a suitable addition of
both silver and copper cyanides a rose-coloured gold is
obtained.

ill COLOURING OF SILVER 263

Colouring of Gilt Work. — The most simple method is to
vary the strength of the current The old method, which
is still practised, is to brush with certain mixtures, such
as gilder's wax. For a redder colour the wax is prepared
with a greater content of copper, while for a greener
colour more zinc salt is used. Langbein gives the fol-
lowing : —

I. For red colour. Wax 12 parts, copper acetate 4
parts, zinc sulphate 4 parts, copper scale 4 parts, borax
1 part, red iron oxide 6 parts, copperas 2 parts.

II. For green colour. Wax 12 parts, copper acetate
4 parts, zinc sulphate 8 parts, copper scale 2 parts, borax
1 part, red iron oxide 6 parts, copperas 2 parts.

The wax is melted in an iron pot, and the other in-
gredients are added, after finely pulverising, in small por-
tions at a time and intimately mixed by starring till the
whole is cold, so that the powder cannot settle on the
bottom in lumps. Finally, the soft mass is moulded into
sticks one-third of an inch in diameter.

COLOURING OF SILVER

§ 226. The metal silver does not unite directly with
the oxygen of the air, so that if no other substance than
oxygen were present silver goods would remain unaltered
when freely exposed to the atmosphere ; but if sulphuretted
hydrogen is present, it attacks the silver in moist air,
converting the surface into a layer of silver sulphide, which
may vary considerably from a light golden colour to a
brownish-black, according to the thickness. If, therefore,
a silver article is subjected to the air of a town or other

264 METAL-COLOURING AND BRONZING part

place where coal-gas is burnt, it gradually changes in
colour ; and if this change is to be avoided, the article must
be coated with a layer of transparent varnish.

The term oxidising then, as applied to the colouring
of silver goods, is a misnomer, as oxygen plays but an in-
significant part in the colouring ; and as the natural tar-
nishing of silver is chiefly due to sulphur, it would be more
correctly termed sulphurising. But the term oxidising
has now been accepted in trade and commerce as signifying
the colouring of silver by any means ; and if we divest it
of its strictly chemical meaning, it will suffice to indicate
the effect produced, and the term will be used in this sense
in the present section.

Various agents are employed in the oxidising of silver,
but most of them contain sulphur in some form or other,
chiefly as metallic sulphides. In some cases advantage is
taken of the fact that some metals when deposited by
simple immersion are precipitated on the article in such a
fine state of division as to be black, or at least dark
coloured. Such is the case with platinum, when deposited
from a solution of its chloride, and thus forms one of the
most valuable colouring agents for silver.

In addition to the processes which come under the
geueral signification of oxidising, ordinary electro-deposition
of other metals on silver is also employed to impart a
variety to the pure white surface of the metal. Thus a
very minute coating of copper, and then a very thin layer
of silver upon that, imparts a dead appearance like frosted
silver. Gold of different qualities is also deposited on
portions of the surfaces of silver articles to give a variety
to their appearance.

In the case of silver alloys, which are chiefly formed of

in COLOURING OF SILVER 265

silver and copper, and consequently have a less white colour
than pure silver, their surfaces may be whitened by dis-
solving out the particles of copper from the alloy, and thus
leaving a surface of pure silver. During the manufacture
of such alloys into articles, the metal becomes hardened
and requires annealing, which consists of making it red-
hot and allowing to cool. But as copper is an oxidisable
metal and silver is not, the former becomes oxidised by
the action of the oxygen of the air, while the silver remains
unaltered ; and if the whole alloy is pickled in a suitable
acid the copper oxide is dissolved out, leaving the surface
silver-white in colour. A dilute solution of sulphuric acid
is generally employed as a pickle in the above case. A
very strong pickle may be formed by dissolving one part
of potassium bisulphate in ten parts of water used cold.
Should the surface of the article not be clear, it should be
well scoured with fine white sand or scratch-brushed.

OXIDISING PROCESSES

§ 227. Some workers recommend the use of a paste
similar to that used for bronzing copper, on the assumption
that such a mixture is oxidising. But silver is an unoxid-
isable metal, therefore it would not be affected by the con-
ditions mentioned abova The following mixture was
tested : —

Plumbago 60 grains.

Red iron oxide 10 ,,

Turpentine . . . . • to form a paste.

The paste was painted on to the work, allowed to dry,

266 METAL-COLOURING AND BRONZING part

then , rubbed with a clean brush, and lastly with a pad
moistened with alcohol. Little, if any, change was pro-
duced on the silver, the very slight coloration being simply
due to dirt, which was readily removed. 1

§ 228. Silver goods may be readily coloured by means
of a solution of platinum chloride. The platinum chloride
may be dissolved in alcohol, ether, or water, according to
the effect it is desired to produce. The solution is applied
by means of a camel-hair pencil or by means of a linen
pad. The process is hastened by making the work warm
and vigorously rubbing on the liquid with the pad. The
aqueous solution is best applied hot.

The strength of the solution will vary with the depth
of colour desired, the more platinum chloride there is
present the darker will be the colour and the quicker will
be its action. From 1 to 10 per cent of platinum chloride
in solution is recommended.

§ 229. The following is a most valuable solution foi
colouring silver goods : —

Barium sulphide .... 5 grains.
Water 5 fluid ounces.

This solution imparts a beautiful golden tint to silver,
and when worked cold almost any desired shade, from
pale to deep gold, may be obtained, according to the time
of immersion. If the immersion is continued the gold
colour passes through crimson-purple to brown, the former
being more or less iridescent. The same colours are ob-
tained by using the solution hot^but they succeed each
other more rapidly.

To produce the gold colours it is best to work with a
cold solution, because the changes succeed each other more
1 This paste may be used for work with detail in relief.

ill COLOURING OF SILVER 267

slowly, and the process can be stopped when the required
shade of colour is obtained.

The browns have a rich umber tone, and are very beau-
tiful, passing from very light to very dark shades. It is
preferable to work with a warm solution for the lighter
shades, and to increase the temperature when the darker
tones are required. Scratch-brushing improves the work
but renders the colour somewhat paler.

A bluish-black colour may be produced almost immedi-
ately by working with a hot solution of twice the strength
mentioned above, viz. —

Barium sulphide .... 10 grains.
Water ...... 5 fluid ounces.

In this latter solution nickel and nickel -plated goods
slowly acquire a very beautiful golden-bronze tint, quite
different from that produced on silver. In a cold dilute
solution a colour something between that of gold and of
the English bronze coinage is also produced on nickel. 1

§ 230. The following is a solution commonly recom-
mended for oxidising silver goods : —

Potassium sulphide . 5 grains.

Water 10 fluid ounces.

The variety known as "liver of sulphur" is generally
used and imparts a reddish -brown colour to silver, the
colour being darker the stronger the solution. This solu-
tion should be worked at 60° or 70° C. With a stronger
solution than the one given above, the colour produced is
bluish -black almost immediately after immersion in the
hot solution.

Instead of using potassium* sulphide alone some oxid-

1 See Appendix.

268 METAIrCOLOURING AND BRONZING pari

isers prefer to add a quantity of ammonium carbonate,
thus —

Potassium sulphide .

10 grains.

Ammonium carbonate

20 „

This solution worked hot gives a nearly black colour to
silver, which assumes a plumbago tint on scratch-brushing.

If the above solution is diluted to double its bulk with
water, various shades of brown may be obtained. The
colours, however, are not so rich in tone as those produced
in a solution of barium sulphide (§ 229).

§ 231. Another solution which produces identical results
with the foregoing is composed of —

Potassium sulphide . ... 12 grains.

Ammonium chloride . . 40 „

Water 10 fluid ounces.

It has been recommended to deposit a thin film of mer-
cury on silver goods before oxidising, but there is no
advantage gained by this method, except that the action
is slower and a bluish-brown colour finally results. With
a strong solution the action is quicker and the final colour
darker in tone.

§ 232. Ammonium sulphide may be substituted for
potassium sulphide in the above recipes. The following
results were obtained by its use. The solution was
worked at a boiling temperature : —

Ammonium sulphide solution 20 fluid grains.

Water 10 ,, ounces.

The first result of the immersion of a silver-plated article
in the above liquid was to produce various shades of gold,

in COLOURING OF SILVER 269

the colour deepening with the length of time of immersion.
These colours were followed by iridescent crimson, umber-
brown, and finally a steely-brown.

To obtain a black colour the following proportions
should be employed : —

Ammonium sulphide . . . 100 fluid grains.
Water 10 ,, ounces.

Nickel-plated goods simply acquire a dark golden tint in
each of the above ammoniacal solutions.

The following modification of the above solution works
quicker on a silvered surface but slower on a nickeled
surface : —

Ammonium sulphide ... 20 fluid grains.
Ammonium chloride . . . 40 ,, ,,
Water . . .- . 10 ,, ounces.

§ 233. The use of sulphuretted hydrogen gas for
oxidising silver articles in place of metallic sulphide
solutions has been recommended. The articles require to
be moistened with water and then fully exposed to the
action of the gas. As this gas is very objectionable in
consequence of its suffocating odour, the operation should
be performed in the open air, or in a fume chamber with
a good draught The method was tried on a silver-plated
tray which had been rendered perfectly clean and
moistened with water. A stream of the gas was allowed
to impinge upon the tray for some time. It would
probably be better to enclose the work in a box and to
pass the gas first through water and then into the box by
means of a suitable orifice. The result obtained in our
case was not so satisfactory as with the use of either of

270 METAL-COLOURING AND BRONZING part hi

the sulphide solutions before described. The coloration
was not uniform.

§234. Silver articles may be coloured by means of
chlorine, applied either in the form of gas, or as a soluble
chloride. When silver is immersed in chlorine water
a thin deposit of silver chloride is formed, which darkens
on exposure to light.

The following solution has been recommended for
producing a warm brown tone on silver : —

Copper sulphate .... 50 grains.
Ammonium chloride . . 26 ,,

Vinegar 1 fluid ounce.

A brown colour, with iridescence, was obtained on testing
this method, but the film blackened on exposure to light,
so that no advantage is gained by its use, and the final
result is less effective than with the sulphide solution,
especially the barium sulphide method.

§ 235. Fart Gilding and Oxidising.— Very beautiful
effects are produced on metal work by first gilding the
articles all over in the usual way, then stopping off
certain parts, preferably the most prominent ones, with
copal varnish, allowing to dry, then plating the uncovered
parts with silver. After rinsing in water the articles may
be immersed or painted with one of the before-mentioned
oxidising solutions. The varnish is then removed with
turpentine.

§ 236. A pink tint may be obtained on silver or silver-
plated goods by immersing them for a few minutes in a
hot solution of copper chloride, then well rinsing in water,
and drying out in sawdust ; or instead of drying, dipping
in methylated spirits and igniting the spirit.

PAKT IV

ELECTROCHEMICAL METAL-COLOURING

§ 237. Electricity is often a very important aid in
colouring metals, not only because a coloured metal or
alloy, such as copper, brass, etc., can be deposited on a
white metal like zinc or iron, but because the metal
deposited, if sufficiently thick, may be used to take the
same chromatic effects as though the whole mass were of
the same material as that on the surface. An electric
current often hastens a chemical change, so that a metal
which only takes a bronze colour very slowly in a certain
bath may have the operation considerably accelerated by
its use.

A metal or alloy may be deposited on another metal or
alloy by simple immersion in a solution of a soluble salt
of the metal to be deposited, but the coating thus obtained
is always very thin, and thick deposits cannot be obtained
in this way. The same remarks apply to the case when
the immersed metal is placed in contact with another
metal.

Some metals are said to be electro-positive and are
represented by the sign + , while others are termed electro-

272 METAL-COLOURING AND BRONZING part

negative and are represented by the sign - , if then an
electro-positive metal is immersed in a solution of an
electro-negative metal the latter will be deposited on the
former. If, for example, a piece of iron is immersed in a
copper solution it will receive a coating of copper. If a
piece of a negative metal, such as silver or platinum, is
made to touch the immersed iron the deposition will be
hastened. In these cases the iron is dissolved and passes into
the solution, while the copper passes out of the solution and
is deposited on the iron. A good illustration is afforded in
the production of the red (Florentine) bronze of Lafleur.
A brass article is wrapped round with iron wire and
immersed in a very dilute solution of copper sulphate or
copper nitrate (spent pickle is generally used), when it
soon becomes coated with a layer of copper in a simple
and inexpensive way.

The table on p. 273, according to Fischer, represents the
metals according to their electrical character : —

In the horizontal lines are the precipitants, and in the
vertical lines the metals precipitated. The sign +
opposite to zinc, for example, indicates that zinc
precipitates gold, osmium, etc., but not nickel as that
metal is negative.

It will be seen from this table that some metals are
deposited by mere immersion in the solution of another
metal, while others are not, because they are of the same
electrical order, or more positive than the immersed metal.
It seldom happens that a metal is coated by mere immersion
in a solution of the same metal. The base metals, zinc,
cadmium, tin, lead, and iron become more readily coated
than the noble metals, which are very easily deposited by
most of the common metals. The solutions of base metals

ELECTRICAL CHARACTER OF METALS

273

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+ + I I I I I

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T^PItf

I I + I I I I

I I

•UU

T**I

+ + + I I + I

+ + + + I I I

I I

'itynoung

+ + + + + + I

I I

uoddoo

+ + + + + + l

I +

•Xuouifluv

++++++++

I I

•QJU9SJV

+ + + + + + + +

I I

'JUnoxdjn

++++++++++++

I +

•umjjnnaji

++++++++++++++

I I

M^ES

+++++++++++++++

I +

•nitijpoqa

+++++++++++++++ I I I

•uinipvnvj

+ + + + + + + + + + + + + + + + I I

•umupttfcj;

++++++++++++++++ I l

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+ + + + + + + + + + + + + + + + 1 I

•ramraso

+++++++++++++++++ I

++++++++++++++++++

274 METAL-COLOURING AND BRONZING part

do not readily yiejpl up their metal. Of all ordinary metals
zinc deposits metal from the greatest number of solutions,
and appears to have the greatest depositing power.

In reading the foregoing paragraphs it must not be
supposed that the deposited metal, whatever its kind may
be, always possesses the metallic lustre, as in many cases
it has the appearance of a black or dark -coloured film
on the surface, and when deposited very rapidly it is
thrown down in the form of a dark-coloured or black
powder. By rubbing, however, the loose powder may
often be removed, and the metal assumes its ordinary
colour and lustra

By the simple immersion process a thin coating only of
metal is obtained, and even that is usually imperfect,
because the surface to be coated, and the deposited metal,
act electrically as two different substances, the former
being electro-positive and the other electro-negative. In
consequence of this electrical difference there is set up a
voltaic action at minute points all over the surface ; this
action is not perceptible at first because it is of microscopic
minuteness, but it gradually spreads from those points all
over the surface and causes the metal beneath the coat-
ing to dissolve and the deposit to become loose and full of
spots.

It is possible, however, to coat a metal perfectly with
another metal by means of simple immersion by adopting
the following rule, first proposed by Dr. Gore : — Take an
electro-positive metal, such as copper, dip it into a solution
of a less positive metal, say mercury, when it will receive a
coating of mercury. Now immerse it in a third and still
less positive metal, say gold ; the film of mercury and any
parts of the copper which have not been perfectly quicked

it COPPERING BY A SEPARATE CURRENT 275

dissolve, and a film of gold is deposited in their stead.
Now re-dip the copper in the mercury solution, and any
still non-coated particles of it are dissolved, and deposit
mercury in their place; then dip again into the gold
solution and a similar effect takes place as before.

COPPERING BY A SEPARATE CURRENT

§ 238. Zinc, iron, tin, etc., are for the most part coppered
by a separate current in alkaline baths, which may be
classified into two groups, viz. those containing potassium
cyanide, and those without it.

A very good coppering bath may be obtained thus — dis-
solve 3^ ounces of copper cyanide in a solution consisting
of 12 ounces of potassium cyanide and 2 quarts of water,
boil, filter off any residue, and then dilute with 6 quarts of
water. This bath works very well when heated to 60° or
70° C. Cyanide of copper is not very soluble in cyanide
of potassium, the liquid formed does not readily dissolve
the anode, nor does it conduct well ; it also has a strong
tendency to evolve hydrogen at the cathode, but this
may be lessened, or wholly prevented, by avoiding the use
of any free cyanide of potassium, employing a weaker
current, and adding some aqueous ammonia and copper
oxide.

§ 239. Watt recommends a solution composed of 1
gallon of water, 6 ounces of potassium cyanide, 4
ounces of potassium carbonate, 2 ounces of liquid am-
monia, and 2 ounces of copper sulphate. Dissolve the
copper sulphate in distilled water, and when cold add
the potassium carbonate and the ammonia. When the

276 METAL-COLOURING AND BRONZING pari

precipitate is redissolved, add the potassium cyanide.
Filter off any insoluble matter and employ the liquid for
coppering.

§ 240. The following coppering bath is recommended
to be used at the ordinary temperature : — Water 10 quarts,
crystallised sodium bisulphite 7 ounces, sodium carbonate
14 ounces, neutral copper acetate 7 ounces, pure potassium
cyanide 5 ounces, liquid ammonia 4£ ounces. The excess
of sodium carbonate serves no 'useful purpose and had
better be omitted, neither is the addition of ammonia
necessary.

§ 241. Eoseleur recommends the following : — Reduce
20 parts of crystallised copper acetate to powder and rub
into a paste with a little water, add to it 200 parts of an
aqueous solution containing 20 parts of washing soda, and
stir the mixture ; a light green precipitate is formed ; 20
parts of sodium bisulphite are now dissolved in 200 parts
of water, and the solution mixed with the former one;
the precipitate becomes a dirty yellow. Lastly, dissolve
20 parts of pure potassium cyanide in 600 parts of water,
and add it to the previous mixture. If the solution is not
colourless, add more potassium cyanide until it is so.
This liquid may be used either hot or cold and requires a
current of moderate strength. A current of '4 Ampere at
a tension of 3 to 3^ volts is calculated for 15 square
inches of surface of objects.

§ 242. According to Gore another good bath for copper-
ing, which may be employed either hot or cold, consists of
2500 parts of water, 35 parts of pure potassium cyanide, 30
parts of sodium bisulphite, 20 parts of aqueous ammonia,
and 35 parts of copper acetate. The cyanide and bisulphite
are to be dissolved in one part of the water, and the ammonia

iv SOLUTIONS FOR COPPERING 277

and copper acetate in the other, and the two solutions mixed
together. If the blue solution of copper acetate in ammonia
does not then become quite colourless, a little more cyanide
must be added. If these liquids are used hot the deposi-
tion is more rapid. If they become green or blue after
working, it is from an excess of copper dissolved, and
either the anode should be reduced in size or some
potassium cyanide added. If the anode acquires a brown
or white insoluble coating, the liquid is deficient in copper,
and some more of the copper acetate in ammonia solution
must be added. Too large an excess of potassium cyanide
causes a strong evolution of hydrogen bubbles on the
objects; but no deposition of copper, or only a slight
amount takes place, and this has a tendency to peel off.
The excess of cyanide can be neutralised by the addition
of more copper salt. In any bath containing cyanide,
each addition, and especially that of a metallic salt, causes
a certain irritation, and causes irregularity of working for
some time. Boiling tends to prevent this irregularity.

§ 243. Eoseleur specially recommends the following
solution for zinc articles : — Water 10 quarts, pure cream of
tartar 6^ ounces, crystallised sodium carbonate 15 ounces,
copper sulphate 6£ ounces, caustic soda solution of 16° £.
fib.

To prepare this bath, dissolve the tartar and sodium
carbonate in two-thirds of the water, the copper sulphate
in the remaining third, and mix the two solutions. Filter
off the precipitate and dissolve it in the caustic soda
solution, then add this solution to the former one.

For small zinc objects which require to be coppered in a
sieve Koseleur recommends a solution composed of —
Water 10 quarts, crystallised sodium bisulphite If ounces,

278 METAL-COLOURING AND BRONZING part

neutral copper acetate 8 ounces, pure potassium cyanide
8£ ounces, and ammonia \ ounce.

In preparing copper solutions, the copper acetate pre-
scribed in the preceding formulae may be replaced by
copper carbonate.

§ 244. Dr. Eisner coats base metals with copper by
employing a solution composed of — 1 part of potassium
bitartrate in powder dissolved in 10 parts of boiling
water, and as much freshly prepared and wet hydrated
copper carbonate added as the liquid will dissolve. The
dark blue liquid is filtered and rendered still more
alkaline by the addition of a small quantity of potassium
carbonate. This liquid may be used to coat articles of
tin, cast-iron, and zinc.

§ 245. Gauduin's coppering solution consists of a solu-
tion of copper oxalate with ammonium oxalate and free
oxalic acid. It is said to work well when heated to about
70° C.

§ 246. In preparing cast-iron work for electro-copper-
ing, after the pieces have been pickled and scoured, they
should be carefully examined for sand-holes, and if any
such cavities appear on the work, they must be well
cleared from dirt by means of a steel point. After this,
well brush again with sand and water. It is a good plan
to give the article a slight coating in the coppering bath,
and then examine to see if any places are uncovered.
The body should have a final brushing with moist sand,
then well rinsed in clear water, and put into the coppering
bath. The alkaline bath should only be used to give a
complete covering of copper and the article should then
be transferred to a copper sulphate bath to finish.

If the current is too great in relation to the surface of

iv BRASSING BY A SEPARATE CURRENT 279

the work, the copper is deposited as a brown or brownish-
black powder. In the sulphate solution, if the liquid is
too dense, streaks are apt to be formed on the receiving
surface, and the article (especially if a tall one) will
receive a thick deposit at its lower part, and a thin one at
the upper portion, or even have the deposit on the upper
end redissolved. If there is too little water, crystals of
copper salt form on the anode and at the bottom of the
vessel. The copper obtained from the usual double
cyanide of copper and potassium solution by a weak
current is of a dull aspect, but with a strong current it is
bright

BRASSING BY A SEPARATE CURRENT

§ 247. A brassing solution may be made of different
proportions of copper cyanide and zinc cyanide, or other
copper and zinc salts dissolved in a suitable solvent ; and
since a solution of copper cyanide requires a different current
strength from one of zinc salt, it will be seen that, accord-
ing to the greater or smaller current strength, now more
of one, now more of the other metal is deposited, which
influences the colour of the deposit. Hence the proper
regulation of the current is the chief condition for obtain-
ing good deposits. For solutions containing two or more
metals the less positive one is generally deposited first if
the current is weak. In a brassing solution, with a weak
current, the copper is largely deposited, while a more
powerful current deposits both copper and zinc; if too
strong, zinc will be chiefly deposited. A solution contain-
ing copper and zinc salts in about equal proportions is
considered to be the most suitable and the least subject to
fluctuations.

280 METAIrCOLOURING AND BRONZING pakt

In making a solution for the deposition of the alloys,
brass, bronze, German silver, etc., Watt recommends that,
instead of forming a solution from the salts and their
solvents, the alloy should be dissolved in nitric acid, and
from the acid solution thus obtained the depositing bath
should be formed. Metals of the best quality should be
used and the anode should be formed of the same
quality of alloy as that used in making the solution.

§ 248. One gallon of a good brassing solution can be
made according to the method referred to in the last
paragraph by taking

Good brass 1 ounce.

Nitric acid 4 ounces.

Water 4 „

Cut up the brass into small pieces, place in a glass beaker,
add the water and then the acid. The beaker should be
gently heated on a sand-bath in a fume chamber so that
the fumes may pass into the flue of the chimney. When
the acid is completely exhausted, the red fumes will cease
to come off. If the whole of the brass has been dissolved
before the acid is exhausted some more of the metal must
be added, and the heat continued until the acid fumes
cease, while some undissolved metal remains in the flask,
so as to prevent excess of acid.

The acid solution must be poured into a larger vessel,
diluted with three or four times its bulk of water, and
liquid ammonia added, until the precipitate which first
forms is redissolved (stirring the liquid in the meantime
with a glass rod) and a clear blue solution is obtained.
A moderately strong solution of potassium cyanide must

SOLUTIONS FOR BRASSING

281

now be added to the solution with constant stirring until
the blue colour entirely disappears, leaving a yellow-tinted
solution. A moderate excess of free cyanide is then added,
and the solution made up to a gallon with water. The
solution must be allowed to rest for a few hours and the
clear liquid poured from the sediment into the plating
vessel. If the solution is to be used hot it may be further
diluted with water.

§ 249. Roseleur recommends the following : —

Copper sulphate .

Zinc ,, . . .

Sodium carbonate (crystallised)

bisulphite

carbonate (crystallised)
Potassium cyanide (pure) .
Arsenious acid .
Water

»t

»»

5} ounces.

5J
15|

7
7

802 grains.
10 quarts.

The bath is prepared by dissolving the copper and zinc
sulphates in one portion of hot water, and the 15 J ounces
of sodium carbonate in another portion ; mix the two
solutions and well stir. The copper and zinc are pre-
cipitated as carbonates, and the vessel with its contents is
allowed to rest until the precipitate subsides, when the
clear liquid is poured off and thrown away. Now add,
without washing the precipitate, about 6 quarts of water,
and dissolve in it, with constant stirring, the 7 ounces
each of sodium carbonate and bisulphite, adding these
salts a little at a time. Dissolve the potassium cyanide in
4 quarts of cold water, and add this solution, with the ex-
ception of about half a pint, in which the arsenious acid is
dissolved, to the first solution, and finally add the solution

282 METAL-COLOURING AND BRONZING part

containing the arsenious acid. If the solution is not clear
after well stirring, more potassium cyanide must be added.

Brassing solutions, as a rule, work more irregularly
than other cyanide baths, the deposit being too red or too
gray, while occasionally one side of an object is coated well
and the other not at all. To force the solution to work
well it should be thoroughly boiled, and the water lost by
evaporation replaced. If boiling is not admissible, then
the solution must be worked through for some hours with
the current, until it gives the right colour. The addition
of the arsenious acid is said to make the brassing brighter,
but if the above-mentioned proportion is exceeded, the
deposit will be too light and show a gray tone.

§ 250. Another brassing solution is composed of

Sodium carbonate
Crystallised sodium bisulphite
Copper cyanide .
Zinc „

Water .
Potassium cyanide

The sodium carbonate and bisulphite are dissolved in two to
three quarts of water. The cyanides of copper and zinc are
rubbed to a thin paste with water in a porcelain mortar ;
the paste is then added to the solution of sodium salts,
and the potassium cyanide added with brisk stirring, until
the metallic cyanides are dissolved. Dilute to ten quarts.
§ 251. A simple brassing solution may be rapidly made
by using

Copper sulphate . 1 part.

10J

ounces.

>»

quarts.

till solution is

clear.

Zinc ,,

Water

Potassium cyanide
Ammonia .

1 ,,

sufficient to dissolve the salts.

iv SOLUTIONS FOR BRASSING 283

The copper and zinc sulphates are dissolved in water and
potassium cyanide added, with brisk stirring, until the
precipitate first formed redissolves ; to the resulting
solution one-tenth to one-fifth liquid ammonia is added
and the solution diluted moderately with water. For a
light-coloured brass two parts zinc sulphate to one part
of copper sulphate are used.

§ 252. For coating iron, steel, and tin with brass the
following solution of Roseleur's may be used : —

Sodium bisulphite .... 2 parts.

Potassium cyanide (75 per cent) . . 5 ,,

Sodium carbonate 10 „

Distilled water 80 „

Then add to the above solution

Fused zinc chloride .... 1 part.

Copper acetate 1J parts.

Water 20 „

§ 253. All work which has to be electro-brassed must
be scrupulously clean. Cast-iron work may be pickled in
a solution of

Sulphuric acid J pound.

Water 1 gallon.

Scoured with sand and swilled with water. If the iron
articles are coated with rust, it may be removed by
brushing with sand and strong hydrochloric acid, after
which they should be immersed in the sulphuric acid
pickle.

Zinc articles are prepared for brassing by pickling
in

Sulphuric acid 1 ounce.

Hydrochloric acid 2 ounces.

Water 1 gallon.

284 METAL-COLOURING AND BRONZING pabt

After pickling the work is well rinsed, scoured with sand
and water, then swilled and placed in the brassing bath.
Iron and zinc articles should not be suspended in the
bath at the same time if it can be avoided, and if this is
not convenient, the iron articles should enter the bath first,
and when they have been coated all over, the zinc work
may be introduced.

In brassing, the distance of the objects to be coated
from the anodes is of considerable importanca If objects
with deep depressions or high reliefs are humg in a brassing
bath, it is sometimes found that the relief parts are paler
in colour than the groundwork, which will show a redder
deposit ; in such a case the distance from the anode must
be increased. After an electro-brassing bath has been in
use for some time, its whole character may be altered,
because (1) the anode is composed of two metals of un-
equal solubility in potassium cyanide, and does not there-
fore maintain the solution constant; (2) as ammonia is
often a constituent of the plating bath, this, being a
volatile body, evaporates during the working and therefore
alters the composition of the solution ; (3) the zinc oxide
liberated at the brass anode becomes less soluble than the
copper oxide set free at the same electrode, and the free
cyanide in the solution is largely taken up by the latter
to the exclusion of the less soluble zinc oxide ; in this way
the solution becomes more highly charged with copper
than with zinc ; (4) when a current of low electro-motive
force is used in depositing brass, the copper is more readily
deposited than the zinc, and vice verm ; (5) the amount
of anode surface immersed in the solution in proportion to
that of the cathode sensibly affects the deposition of brass ;
if, for example, the anode surface is very large compared

iv DEPOSITION OP BRONZE 285

with the surface to receive the deposit, zinc will be princi-
pally deposited, but if the opposite condition prevail, then
copper is deposited. 1

DEPOSITION OF BRONZE BY A SEPARATE CURRENT

AND BY CONTACT

§ 254. The electro-deposition of an alloy of copper and
tin, known under the general name of bronze, may be
performed in a similar manner to that of brass, by sub-
stituting a salt of tin in place of a salt of zinc.

For coating wrought and cast iron with bronze, Gountier

recommends the following solution : —

Potassium ferrocyanide . . 10£ ounces.

Cuprous chloride

Tin chloride

Sodium thiosulphate .

Water.

Eisner prepared a bronzing bath by dissolving 21 ounces
of copper sulphate in 10 quarts of water, and adding a
solution of 2£ ounces of tin chloride in caustic potash.

The above methods are far from satisfactory, and as
the various bronze shades may be closely imitated by
depositing different qualities of brass, by altering the pro-
portions of copper and zinc, varying the current, etc., the
deposition of bronze is rarely required.

§ 255. Langbein states that the following bath is reli-
able : — Prepare, each by itself, solutions of copper phosphate
and stannous chloride in sodium pyrophosphate. From a
copper sulphate solution, precipitate copper phosphate by
means of sodium phosphate, allow the precipitate to settle,
and after pouring off the clear supernatant liquid, bring it

1 Watt's Electro-Deposition, p. 882.

H ,, *
14 „

10 quarts.

286 METAL-COLOURING AND BRONZING pakt

into solution by means of a concentrated solution of sodium

pyrophosphate. On the other hand, add to a saturated

solution of sodium pyrophosphate the solution of the tin salt

as long as the milky precipitate at first formed redissolves.

To these two metallic solutions add a solution of sodium

pyrophosphate, which contains about If ounces of the salt

to the quart, until the precipitate appears quickly and of the

desired colour. Some sodium phosphate requires to be added

to the bath from time to time, and if the colour becomes too

light, solution of copper, and if too dark, solution of tin.

§ 256. Mr. Fred Wiels deposits bronze on iron, steel,

and other metals by means of a solution of

Sodium-potassium tartrate 150 parts.

Water 1000

Soda lime (containing 50 to 60 per cent of free soda

and 35 per cent of copper sulphate) . . 80

Tin chloride in sodium hydrate .... 80

The iron or steel articles are cleaned in dilute sulphuric
acid, washed with water, and finally with water made
alkaline with soda, then scratch-brushed, again swilled and
immersed in the plating bath in contact with a piece of
lead or zinc, or suspended by zinc wires.

M. Weiss Kopp imparts a bronze colour to electro-
coppered articles of cast-iron, by rubbing them with a
mixture of 4 parts of sal-ammoniac, 1 of oxalic acid, and
1 of acetic acid, dissolved in 30 of water.

DEPOSITION OF NICKEL AND ITS ALLOYS

§ 257. A solution of nickel for plating by means of a
separate current may be made by dissolving the double
sulphate of nickel and ammonia in water, in the proportion

>»

iv DEPOSITION OF NICKEL 287

of 12 ounces of salt to the gallon. The solution should
be tested with blue litmus paper, and if the paper is turned
red, it shows an excess of free acid which must be
neutralised by cautiously adding ammonia until the liquid
is slightly alkaline. Or a nickel-plating solution may be
made by suspending a large plate of nickel in dilute
sulphuric acid, and placing it in contact with the positive
pole of the battery. The negative pole is joined to a
small plate of nickel which is also suspended in the
solution. A current is then passed until sufficient nickel
has been dissolved from the anode, which is determined by
weighing it before and after passing the current; the
solution is then neutralised with ammonia. To each
ounce of nickel sulphate in solution add one ounce of
ammonium sulphate, and test with litmus paper as described
above. All articles to be nickel-plated are required to be
scrupulously clean and previously polished bright.

As nickel solutions are used in the concentrated state
the nickel salt begins to crystallise after a time, especially
in a warm place, in which case water must be added. It
should also be borne in mind that the anode does not
maintain the strength of the solution, so that it is necessary
to add crystals of the double salt from time to time to
replenish the weakened solution.

In nickeling iron, zinc, etc., the articles should be first
covered with a strong current, and then a weaker current
may be employed to finish with.

§ 258. An alloy of nickel and cobalt possesses greater
hardness than either of the metals separately, and Langbien
states that the greatest degree of hardness is attained with
nickel 70 to 75 parts, and cobalt 30 to 25 parts. The
most suitable plating baths are as follows : —

288 METAL-COLOURING AND BRONZING pari

•

Nickel-ammonium sulphate . . 21 ounces.

Cobalt-ammonium sulphate . 5 J „

Ammonium sulphate ... 9 „

Water 15 quarts.

Instead of ammonium sulphate, 10£ ounces of boric acid
may be used.

For the first bath the salts are dissolved, in water, and
in case the liquid is too acid, ammonia is added until blue
litmus paper is only slightly reddened. It is best to use
rolled and cast anodes in equal proportions ; and when the
bath becomes alkaline to restore its original slightly acid
reaction by the addition of a little citric acid

The second bath must not be neutralised but worked
in its strongly acid reaction. If after dissolving the
double nickel and cobalt salts the solution is acid, the
free sulphuric acid should be neutralised by one or two
ounces of nickel carbonate, then the solution acidified by
the boric acid.

§ 259. Stolba recommends the following solution for
nickeling by zinc contact without the aid of a battery : —

Heat a boiling concentrated solution of zinc chloride in
a copper kettle, with double its volume of soft water, then
add, drop by drop, pure hydrochloric acid until the
precipitate formed by diluting the zinc chloride solution
with water disappears. Then add as much zinc powder as
will lie on the point of a knife, the effect being that the
copper of the kettle, as far as it comes in contact with the
solution, is in a few minutes zinced. Now bring into the
kettle sufficient nickel sulphate to colour the liquid
perceptibly green, then introduce the articles to be nickeled,
together with small pieces of zinc or zinc wire so as to
present many points of contact, and continue boiling.

iv DEPOSITION OF GERMAN SILVER 289

With a proper attention to the details it is claimed that the
articles will be uniformly nickeled in fifteen minutes ; if
such is not the case the boiling must be continued, fresh
pieces of zinc added, or, if the solution does not look
sufficiently green, fresh nickel salt introduced.

For the success of the process the articles must be
perfectly clean. The solution must not become turbid on
boiling by the separation of basic zinc salt, nor acid by free
hydrochloric acid, otherwise the nickeling will be dull and
blackish. The turbidity is removed by cautiously adding
drop by drop some hydrochloric acid, and any acidity by
means of sodium carbonate. The articles after plating are
well swilled, dried, and polished with whiting.

The above method of nickeling may also be carried out
for iron and steel articles with the zinc contact. To a 10
per cent zinc chloride solution add enough nickel sulphate
to impart a deep green colour, and heat to the boiling
point. Then, without troubling about the turbidity caused
by the separation of basic zinc salt, immerse the articles
in it so as not to touch each other, and keep the whole
boiling for thirty to sixty minutes, replacing the water
lost by evaporation in the meantime. The deposition of
metal is of course very thin.

§ 260. By boiling a mixture of 8£ ounces of nickel-
ammonium sulphate and 8£ ounces of ammonium chloride
in 1 quart of water, together with clean iron filings, and
introducing into the liquid the copper or brass articles, the
latter become coated with a thin layer of nickel. The
nickel solution requires to be frequently renewed.

§ 261. German silver, an alloy of copper, zinc, and
nickel, may be deposited on other metals, according to
Watt, by dissolving a portion of the alloy (say one ounce) in

290 METAL-COLOURING AND BRONZING part

nitric acid, taking care to leave a little undissolved metal
at the finish so as to ensure that the free acid has been
completely taken up, then pouring off the clear solution.
Now dissolve 4 ounces of potassium carbonate in a pint
of water, and add this solution gradually to the former
solution with careful stirring until no further precipitation
takes place. The precipitate must be washed several
times with hot water and then redissolved by adding a
strong solution of ammonia in about the right quantity to
just redissolve it, and sufficient potassium cyanide to de-
colorise it. A little excess of potassium cyanide is then
added to form free cyanide, and the bath worked with a
German silver anode. The battery power required would
be the same as that used for working a brassing solution.

DEPOSITION OF SILVER BY A SEPARATE CURRENT

§ 262. The preparation of a silver-plating solution com-
prises three distinct operations, viz. —

(a) The conversion of silver into silver nitrate.

(6) The precipitation of the whole of the silver from
the above solution, as silver cyanide or chloride.

Note. — Distilled water or filtered rain water should always be
used.

It is assumed that the potassium cyanide used in
making the following solution contains 75 per cent of

KCy:—

To prepare one quart of solution, proceed as follows : —
Put 240 grains of fine silver in an evaporating dish,

iv DEPOSITION OF SILVER 291

cover with nitric acid diluted with an equal bulk of water,
and heat gently on a sand bath until dissolved. If the
action becomes violent, add a little cold water, and remove
the source of the heat. Any excess of acid should be
avoided. Evaporate cautiously to dryness to expel the
free acid, and dissolve the residue (which is silver nitrate)
in water, and make up the solution to a quart with
distilled water.

Make a solution of potassium cyanide by dissolving 200
grains in about 4 ounces of water, and add it in successive
small portions to the silver nitrate solution, with brisk
stirring, until a precipitate ceases to form in the clear
liquid. If too much cyanide is added, some of the pre-
cipitated silver cyanide will be dissolved. Allow to stand
till clear, then decant the clear liquid, and well wash the
precipitate several times with water.

Dissolve 400 grains of potassium cyanide in 4 ounces
of water, and transfer the solution to a tall glass jar.
Make a paper scale, divided into inches and eighths, paste
it upon the jar, and note the level of the solution. Add
this solution to the precipitated silver cyanide, with
brisk stirring, until a very small portion remains undis-
solved. The solution is the double cyanide of silver and
potassium. Observe by reference to the paper scale the
proportion of potassium cyanide solution used, then add
an equal quantity to supply the necessary amount of
"free cyanide"; add sufficient water to make the solu-
tion up to one quart, and filter. It is then ready for use.
This forms one of the best silver-plating solutions.

§ 263. When deposited silver is required to have a
dead pearly - white appearance, a solution prepared as
follows should be used —

292 METAL-COLOURING AND BRONZING tart

Prepare a solution of silver nitrate as before, and
precipitate the silver with hydrochloric acid in excess.
Well wash with hot water until the wash water ceases
to redden blue litmus paper. Dissolve 600 grains of
potassium cyanide in 6 ounces of water, and transfer to
the glass jar containing the paper scale mentioned in the
previous experiment. Add the solution gradually to the
silver chloride precipitate, stirring the whole time until
it is almost completely dissolved.

Bead off the proportion of cyanide used, and add one-
half more for " free cyanide." Make up to one quart with
water, and filter for use.

When the percentage of potassium cyanide in the
commercial article is known, much time may be saved
by adding the exact quantity required to precipitate the
silver as cyanide. This quantity is found by multiplying
the weight of silver by 0*6 and by the reciprocal of the
percentage.

§ 264. Battery Method of making a Silver-Plating
Solution, — Dissolve 2 ounces of potassium cyanide in 1
quart of water; place in this liquid a silver plate to
form the anode, and a much smaller silver plate for the
cathode. Pass a current through the solution until a
piece of bright German silver the same size as the anode
receives a good deposit when it is made the cathode in
place of the small silver plate. Or pass the current until
\ ounce of silver is dissolved from the anode.

Copper, brass, and German silver work should be
"quicked" immediately before plating. The quicking
solution may be prepared as follows —

Dissolve \ ounce of mercuric oxide in a strong solution
of potassium cyanide. Note how much of ' the potassium

IV DEPOSITION OF GOLD 293

cyanide solution was used to dissolve the red powder,
and then add one-half of that quantity for free potassium
cyanide, and make up the bulk of the solution with water
to one quart.

The work should be dipped in this solution until it
becomes uniformly white. It should then be rinsed in
water and immersed in the plating solution to receive a
deposit of silver.

Work that has been quicked before plating is less
liable to strip in undergoing the subsequent processes of
scratch-brushing and burnishing.

Iron, steel, and zinc should be coppered in the cyanide
bath before silvering.

Pewter, Britannia metal, etc., are usually steeped in a
hot potash solution and transferred without rinsing to a
" plating solution," containing a large excess of potassium
cyanide, in which it is "struck," with a current dense
enough to give evidence of incipient burning of the deposit.
When struck the work is scratch-brushed, and transferred
to an ordinary plating solution to receive its final deposit.
A beginner would no doubt find it easier to treat these
metals as he would iron and zinc by first giving them a
coat of copper in the cyanide bath.

•

DEPOSITION OF GOLD ON OTHER METALS

§ 265. Gold is dissolved in aqua regia in the same
way that silver is dissolved in nitric acid. The solution
of gold chloride is evaporated over a water bath nearly
to dryness to expel excess of acid, and the residue is
dissolved in a small quantity of water. To the concen-
trated solution add a solution of good potassium cyanide

294 METAL-COLOURING AND BRONZING part

as long as a precipitate of gold cyanide is formed. Well
wash the precipitate, and then add just sufficient potassium
cyanide solution to dissolve it. Afterwards add, say one-
fourth of the quantity of potassium cyanide used in dis-
solving the precipitate, to give the requisite amount of
" free cyanide."

Another method of preparing a gilding solution is to
precipitate the gold as fulminate by adding ammonium
hydrate to a dilute solution of chloride of gold until the
precipitate ceases to form. Thoroughly wash the pre-
cipitate, and whilst it is still wet add a solution of
potassium cyanide until it is exactly redissolved. Add free
potassium cyanide as in the former method.

Notes. — Fulminating gold is very explosive when dry. The
precipitate must therefore be dissolved in potassium cyanide im-
mediately after washing.

This solution is more easily prepared than the preceding one,
and works well

Gold- plating solutions may be also prepared by the
battery method in a similar manner to that described for
silver, but it is necessary to use 2 ounces of potassium
cyanide per pint of water, and to substitute a gold anode
and cathode for the silver ones. This is the best method
for non-chemical students.

All these solutions should be worked at about 150° F.,
with a gold anode, and with a current from two Bunsen
cells arranged in series.

A good gilding solution should contain one ounce of
gold per gallon, but a solution containing less than half
that amount will work fairly well.

§ 266. Recovery of Gold from Old Solutions. — 1st
Method. Add hydrochloric acid in excess to the solution,

IV WATER GILDING 296

in the open air or in a fume chamber, so as to avoid in-
haling the fumes of hydrocyanic acid, syphon off the
clear liquid and collect the cyanide of gold. Dissolve it
in aqua regia, evaporate nearly to dryness, add water,
and precipitate the gold with ferrous sulphate.

2nd Method. Evaporate the solution to dryness, ignite
the residue, dissolve the saline mass in water, and finely-
divided gold remains. Dissolve this gold in aqua regia,
and recover it, as in the first method, by precipitating it
with ferrous sulphate.

§ 267. Recovery of Silver from Old Solutions.— Add
hydrochloric acid in excess, syphon off the clear liquid,
collect the precipitated silver chloride, dry and fuse in a
clay crucible with twice its amount of sodium carbonate
and a little nitre, pour into mould, allow to cool, and
detach the slag by striking it with a hammer. Or the
solution may be evaporated to dryness, the residue fused
at a red heat, the soluble matter dissolved in water, and
the finely-divided silver fused with carbonate of soda as
before.

§ 268. Water Gilding. — This is the name given to the
method of gilding by simple immersion. The following
was patented by G. R. Elkington of Birmingham and
carried on for some time in that city : —

Hydrogen potassium carbonate . . 60 parts.

Gold chloride l£

Water 200

»>

The mixture is boiled for two hours, during which period
the solution, at first yellow, assumes a green colour,
when it is completed. Brass and copper articles are gilt
by dipping them for about half a minute in the hot solu-

296

METAL-COLOURING AND BRONZING

PARI

tion. German silver goods require to be placed in contact
with a zinc or copper wire.

§ 269. According to Langbein the two following
methods have stood the test of experience : —

{Crystallised sodium pyrophosphate
12 per cent prussic acid .
Crystallised gold chloride .
Water

2| ounces.
44 drachms.

li „

1 quart.

Heat the mixture to the boiling point, and immerse the
objects of copper and its alloys in the liquid with constant
moving until they are gilded. Iron, steel, tin, and zinc
articles should be previously coppered. Baths containing
sodium pyrophosphate give good results when fresh, but
they quickly decompose and can seldom be completely
exhausted. In this respect No. II. is better.

II.

r Crystallised sodium phosphate

Pure caustic potash .

Gold chloride .

Potassium cyanide .
^ Water ....

2{ drachms.

1 quart

»*
»

Dissolve the sodium phosphate and the potash in part of
the water, and the gold chloride and the potassium
cyanide in the other part. Mix the two solutions together,
and boil for use as before. It is best to dip the articles
in the partly used solution and finish in a freshly prepared
one. The deposit of gold by simple immersion is, in all
cases, very thin.

ELECTRO-CHROME

§ 270. Nobili, in the year 1826, discovered that when
a solution of lead acetate is electrolysed, by means of a
strong current, using a large platinum anode and a

iv ELECTRO-CHROME 297

platinum wire cathode, a deposit is formed on the anode ;
and that if a polished steel plate be employed as the
anode, using a current from 6 Groves cells, the deposit is
in the fonn of a thin film, and exhibits all the colours of
the spectrum. By placing the anode horizontally beneath
the vertical cathode wire the colours are in the form of
rings concentrically arranged round the wire which forms
the centre and in the order of the chromatic scale.
These are known as Nobili's rings. The experiments may
be conducted in the following way : —

Make a concentrated solution of lead acetate by
boiling lead acetate with water, filter off any insoluble
residue, and pour the clear liquid into a shallow dish.
A plate of polished steel is then immersed in the solution
and allowed to rest on the bottom of the dish; it is
connected with the positive pole of the battery or dynamo.
A small disk of copper is joined with the negative pole of
the battery by means of a wire, li now the copper plate be
brought as near as possible to the steel plate without
touching it, in a few moments a series of beautiful pris-
matic colours will appear upon the steel surface, when the
plate should be removed and well swilled in water.

The effect is due to the deposition of thin films of
lead peroxide, the varied hues being occasioned by the
different thicknesses of this film, the light being reflected
through them from the polished metallic surface beneath.
By reflected light every prismatic colour is visible, and
by transmitted light a series of prismatic colours com-
plementary to the first series will appear, occupying the
place of the former series. The colours are seen to the
greatest advantage by placing the plate before a window
with its back to the light, and holding a piece of white

296 METAL-COLOURING AND BRONZING pam

paper at such an angle as to reflect the light upon its sur-
face. The colours appear very durable and will bear a
considerable amount of friction without being removed.
If the deposition continue too long the coloration will be
less marked and become more or less red, green, or brown.
If well rubbed, when dry, with the finger a rich blue-
coloured film will be laid bare, by the removal of the
delicate film above it

Becquerel, Gassiot, and others have, by varying the
strength of the battery and of the solutions employed, and
interposing non-conducting patterns between the anode
and cathode, and by using cathodes of different shapes,
obtained effects of great delicacy and beauty.

The following formula is due to Becquerel : — Dissolve
| lb. of caustic potash in 2 quarts of distilled water, add
5£ ounces of litharge, boil the mixture for half an hour
and allow the residue to settle. Pour off the clear liquid,
dilute with an equal bulk of water, and use the solution
when cold for the deposition. The solution is rapidly
deprived of its metal, because lead is deposited on the
cathode at the same time.

Mr. Gassiot's method to obtain " metallo-chromes " is
to place over the steel plate a piece of card, cut into
some regular device, and over this a rim of wood, the
copper disk being placed above this as before. One of
the simplest plans is to turn up a piece of copper wire in
the form of a ring, cross, or star, and connect it with the
positive pole of the battery.

Metallo-chromy is used to ornament metallic toys, for
colouring bells, and the hands and dials of watches, by
means of Becquerel's solution.

Salts of other metals, such as manganese, bismuth,

iv COPPERING BY IMMERSION 299

cobalt, nickel, etc., which yield deposits of peroxide at
the anode, may be employed instead of those of lead.
Watt has produced excellent effects on nickel-plated goods
in this way. By this means may be imparted to different
polished surfaces all the richest colours of the rainbow.
They commence with silver blonde, and progress onwards
to fawn colour, and thence through various shades of
violet to the indigo and blues; then through pale blue
to yellow and orange ; thence through lake and bluish-lake
to green and greenish -orange and rose -orange; thence
through greenish-violet and green to reddish-yellow and
rose-lake, which is the highest colour on the chromatic scale.
The colours occur sometimes on the anodes, and some-
times on the cathodes, according to the liquid employed,
and with a variety of metals in a number of different
liquids. At other times they arise wholly from deposits
from the liquid, as with peroxides on anodes of platinum,
or films of metal upon the cathodes ; and sometimes they
consist of insoluble substances formed by the union of
the anode with an element of the liquid.

COPPERING ZINC, IRON, STEEL, TIN, LEAD, BRASS,
ETC., BY SIMPLE IMMERSION AND BY CONTACT
WITH ANOTHER METAL

§ 271. A simple solution for coppering zinc by simple
immersion may be employed as follows —

Copper sulphate . . 250 grains.

Water .... 8 fluid ounces.

Ammonia ... to precipitate and redissolve.

The copper sulphate is dissolved in the water, and
liquid ammonia added until the green precipitate which

300 METAL-COLOURING AND BRONZING part

forms at first is redissolved and a deep blue coloration
results. The solution imparts a bright deposit of copper
on zinc when there is a slight excess of ammonia present.
If the solution is just neutral or only slightly alkaline the
deposit of copper is more or less dull.

The deposit obtained in the hot solution appears to be
thinner and paler than that obtained in the same solution
when cold. Doubtless this is due to the greater solubility
of copper in hot ammonia solution. If the deposit is
gently scratch-brushed, the reduction of thickness produced
by this means causes the coating to appear brassy, and if
the scratch-brushing is vigorously applied the whole of the
deposit is removed. The deposit is therefore very thin
but reguline, and firmly adherent to the zinc.

Evidence of deposition of copper upon iron was
obtained, but the coating was not reguline in character,
and only obtained with difficulty. By immersing iron in
contact with zinc no better result was obtained. Lead
and tin were uncoppered even after a considerable time of
immersion with or without contact with zinc. No reaction
was obtained on brass by immersion in this solution with-
out it was in contact with zinc or iron.

A modification of this method is recommended by
Grager for coppering large articles of zinc, steel, and iron.
He employs two solutions, (a) and (6). The following pro-
portions were used in our experiments : —

t Hydrochloric acid

(a) \ Water .

I Zinc chloride .

( Copper sulphate

(b) \ Water .

I Ammonia

1 fluid ounce.

i ounce.

4 ounce.
8 fluid ounces,
to precipitate and redissolve.

tv COPPERING OF CAST-IRON 301

The clean article is first painted all over with the solution
(a), and while it is still moist a coating of the solution (b)
is imparted.

Zinc is coppered by solution (6) alone without the
assistance of solution (a). Iron is well coppered by using
both solutions as directed above. Tin is not coppered
unless it is in contact with zinc, when it becomes coated
with copper at the point of contact only. The coating is
very thin and is easily removed with the scratch-brush.
Brass was not coppered even in contact with zinc The
solution (b) becomes turbid on working and requires the
addition of ammonia occasionally.

§ 272. Eisner recommends the following solution for
coppering cast-iron goods. The proportions given below
are those used in our experiments : —

Copper sulphate .... 1 ounce.

Water 5 fluid ounces.

Potassium hydrate .... in excess.

The amount of potassium hydrate employed was determined
by the quantity required to just precipitate the whole of
the copper as black copper hydrate. The precipitate was
then well washed and dissolved in a concentrated solution
of sodium bisulphite. 1200 grains of the bisulphite were
used, but a slight brown residue was left undissolved.

It is an excellent solution for coppering sheet iron and
wire as well as for cast iron. Zinc receives a black deposit,
so that a solution of the above strength is not suitable for
coppering that metal, although the deposit may be greatly
improved by subsequent rubbing with whiting. Tin and
brass do not become coated by simple immersion in this
solution, but the latter when in contact with zinc receives

302 METAL-COLOURING AND BRONZING part

a good reguline deposit, which is rendered paler in colour
by scratch-brushing.

§ 273. According to Bacco a solution of the double
cyanide of copper and potassium is used for coppering zinc
by simple immersion, and for coppering iron by immersion
in contact with zinc. The following results were obtained
by us : —

, v / Copper sulphate ... 1 ounce.

I Water 5 fluid ounces.

(h\ I Pot* 88 * 11111 cyanide . . 1 J ounces.
I Water 5 fluid ounces.

The solution (a) was added to (b) and the precipitate of
copper cyanide which was formed at first was redissolved,
yielding a clear coffee-coloured solution containing some
free potassium cyanide.

No deposit of copper was obtained upon either zinc,
iron, or tin by simple immersion. The solution was then
divided into two equal portions, A and B.

Copper sulphate was added to A until a precipitate was
just produced. In this solution zinc was imperfectly
coppered. To the solution B 250 grains of pure potassium
cyanide were added. No effect was produced by the
immersion of zinc, iron, or tin.

In adding potassium cyanide to copper sulphate a
copious evolution of hydrocyanic acid takes place, and
care must be taken not to inhale this gas, as it is a
powerful poison.

§ 274. A solution of copper tartrate in potassium
tartrate has been recommended for coppering zinc. The
following proportions were tried : —

iv COPPERING OF ZINC 303

Cream of tartar . . 100 grains.

Copper tartrate . . . 10 ,,

Water 8 fluid ounces.

The solution was rather turbid from excess of copper
tartrate. The cold solution gave a fairly bright deposit
on iron immediately after immersion, but on swilling the
same and drying out in sawdust the deposit assumed a
brassy colour, which, on scratch-brushing, completely dis-
appeared, exposing the bright iron surface. No deposit of
copper was obtained in the cold solution upon either zinc,
tin, or brass. When the solution was made hot it failed
to copper either iron, zinc, tin, or brass.

Ammonia was added to the above solution in slight
excess, and the same was tried both hot and cold without
yielding a deposit of copper to either of the above-named
metals.

§ 275. The following modification of the above method
is due to Ludersdorff.

Copper sulphate . . . 250 grains.
Water 3 fluid ounces.

The solution was then treated with a solution of

Sodium carbonate (crystallised) . 500 grains.
Water 5 fluid ounces.

The precipitate of copper carbonate was allowed to settle,
filtered, and washed. Then a solution was made of

Cream of tartar .... 400 grains.
Water 8 fluid ounces.

The solution thus formed was used to dissolve the preci-
pitate of copper carbonate, and when the solution was clear
it gave an acid reaction,

304

METAL-COLOURING AND BRONZING

PART

Iron and tin were coppered in this liquid when cold by
placing in contact with zinc. Both iron and zinc received
a good bright deposit of copper in the hot solution- In
the cold solution the deposit was darker and less brilliant
than that obtained by means of the hot solution.

§ 276. Hess recommends the following method of pre-
paring a copper tartrate solution : — First dissolve copper
sulphate in water and add to it a solution of sodium
hydrate and cream of tartar. The following proportions
were tried : —

/ » / Copper sulphate
{ ; I Water .

r Cream of tartar
(&)-! Sodium hydrate
I Water .

240 grains.
4 fluid ounces.

440 grains.
440 „

4 fluid ounces.

The solution (a) was added to solution (6), when a blue and
alkaline liquid resulted.

The cold solution gave a bright deposit of copper upon
zinc, but no effect was obtained upon brass, iron, or tin.
A good deposit of copper was formed on iron and brass by
the aid of a zinc contact but not upon tin. The same
solution when hot does not appear to be suitable as it
deposits copper in a more or less dark or powdery con-
dition.

Instead of the above method of forming a solution the
following may be used : —

Copper tartrate
Sodium hydrate
Water .

120 grains
160 „
4 fluid ounces.

This solution acts precisely as the former one.

§ 277. Stolzel coppers various metals by using a paste

iv COPPERING OF VARIOUS METALS 305

consisting of cream of tartar, copper sulphate, and water.
The following proportions were tried : —

Copper sulphate .... 1 part.

Water 4 parts.

Cream of tartar .... to form a thin paste.

The paste was then brushed on to the work with a hard
brush. Sheet iron, cast iron, and tin were well coppered
by this means. Zinc received a black deposit. Brass was
unaffected. Upon tin the best result appears to be
obtained by smearing the paste upon the surface, allowing
it to remain a few minutes, then washing it off with
water, and scratch-brushing. In all cases scratch-brushing
renders the deposit paler.

§ 278. An excellent method of coppering zinc, which
has the great advantage of cheapness, is adopted by the
author, who uses the blue deposit which accumulates at
the bottom of vessels containing aquafortis, in which brass
and copper goods are dipped or pickled, and which is
known as blue-stone. The proportions recommended
are —

Blue-stone . . . 500 grains.
Water .... 20 fluid ounces.

Ammonia ... to neutralise the free acid.

The precipitate formed by the addition of ammonia is
readily dissolved when the alkali is added in excess.

Zinc quickly receives a good bright deposit of copper.
Iron, tin, and brass are quite unaffected when immersed
alone in the solution; if placed in contact with zinc a
coating of copper is obtained, but only on the parts which
actually touch the zinc, so that the method is only adapted
for zinc.

306 METAL-COLOUBING AND BRONZING part

In preparing this solution for coppering zinc care must
be taken to avoid having a large excess of ammonia, as
when too much of this latter reagent is present the deposit
is black instead of reguline. If too much ammonia should
have been added the fault may easily be corrected by
adding more of the copper salt and water in proportion.

Brass may be coppered by using the above solution
without the addition of ammonia. It is necessary, how-
ever, to have the article in contact with a piece of iron, or
better still, to well wrap iron wire round it. When the
coppering is complete the brass should be thoroughly swilled
in clean water and dried out in sawdust.

Small steel and iron articles may be coppered by simple
immersion in a solution of

Blue-stone .... 1 part.

Water 20 to 30 parts.

Sulphuric acid .... 1 part.

Or they may be buried in sawdust which is kept well
moistened with the coppering solution.

Large iron articles, after thoroughly cleaning, are best
painted with a saturated solution of sodium hydrate, then
with a saturated solution of the blue-stone. The same
precaution must be taken as before described to ensure
uniformity.

BRASSING ARTICLES BY SIMPLE IMMERSION

§ 279. According to Bacco, a solution of equal parts of
copper sulphate and zinc sulphate, to which a solution of
potassium cyanide has been added until the precipitate
which first forms has been redissolved, and then a little

rv BRASSING BY IMMERSION 307

ammonia poured in, gives a deposit of brass on zinc
articles by immersing them during the space of twenty-
four hours.

Take the following proportions : —

Copper sulphate .... 4 ounces.

Zinc sulphate 4 ,,

Water 1 gallon.

Dissolve the two salts- in a portion of the water, then add
potassium cyanide as directed above, and one -tenth its
volume of ammonia. Dilute to one gallon. If a lighter
shade of brass is required, take

Zinc sulphate 8 ounces.

Copper sulphate . . . . 4 ,,
Water 1 gallon.

In our experiments, using the above solution, a very good
coating of brass was obtained in about two hours ; if left
in longer the deposit became too red in colour, resembling
copper. The solution was found to work much better
cold than hot, as the coating which is first obtained is
dissolved again in the hot solution, especially if there is
much free potassium cyanide and ammonia present.

§ 280. A simple immersion brassing solution is recom-
mended by Ludersdorff as follows —

Copper sulphate .... 250 grains.
Water 3 fluid ounces.

The copper sulphate is dissolved in the water, and a
solution of washing soda added to precipitate the copper*
as carbonate. This is filtered off, well washed with water,
and dissolved in a solution of cream of tartar. Three
grains of ammonium chloride is then added to each fluid
ounce of the solution, when the liquid is ready for use.

308 METAL-COLOURING AND BRONZING part

In working with such a solution we obtained a dark
reddish-coloured deposit on zinc immediately it was im-
mersed. The colour at the beginning was about the
colour of an old bronze penny, but not uniformly distri-
buted. On taking the article out of the solution it
became nearly black in parts. On scratch -brushing the
deposit was removed and the zinc surface exposed.

§ 281. A good deposit of the colour of brass may be

rapidly obtained on zinc by immersing it in a solution

Copper tartrate .... 50 grains.

Sodium hydrate .... 380 ,,

Water 1 fluid ounce.

Cream of tartar solution . . . 6 „ ounces.

The coating is improved by scratch-brushing. If clean
iron articles are immersed in the above solution they are
soon coated with a thin film of copper.

If less cream of tartar is added than that given above,
so as to form a thin paste instead of a solution, and the
paste is applied to the zinc articles by means of a vigorous
rubbing, they become rapidly coated with copper. The
same remarks apply to iron goods. If the coppered
brass articles are heated in an oil-bath to about 150° C,
the copper and zinc near the surface seem to unite to form
a layer of brass.

COATING METALS WITH TIN

§ 282. Many metals are coated with tin, because that
metal prevents the base which it covers from being
oxidised. It is often applied direct in the metallic state,
but it may also be deposited very readily from its solu-

iv COATING METALS WITH TIN 309

tions by simple immersion, or by contact with another
metal. The salt of tin most commonly used, in conse-
quence of its solubility, is the chloride. Stannous chloride
is the most useful salt, and should be made as required,
by dissolving the metal in hydrochloric acid, as the salt
becomes less soluble after standing for a long time.

Sodium and potassium stannate solutions may be also
made by dissolving these salts in water, or by boiling tin
oxide in a strong solution of sodium or potassium hydrate.
The same liquid may be produced by passing a current of
electricity through a solution of sodium or potassium
hydrate, using a large tin anode. This solution decom-
poses by exposure to the atmosphere, and deposits tin
oxide at the bottom of the vessel.

A very interesting example of the deposition of tin
may be seen by immersing a rod of zinc in a dilute
solution of stannous chloride, to which a few drops of
nitric acid have been added, and allowing the whole to
remain undisturbed for some time.

§ 283. C. Paul uses the following solution for tinning
zinc, iron, brass, copper, etc. : —

Water 10 parts.

Sulphuric acid 1 part

Copper sulphate 1 part.

A dilute solution of copper sulphate is added to the
acidified water with stirring after the articles have been
immersed. When they have become coated with a thin
layer of copper, they are removed, washed, and then
wetted with a solution of

Stannous chloride 1 part.

Water 2 parts.

Hydrochloric acid 2 „

310 METAL-COLOURING AND BRONZING part

They are next shaken with a mixture of finely-powdered
chalk, copper sulphate, and ammonium sulphate, which is
prepared by dissolving one part of copper sulphate in
sixteen parts of water, and adding ammonia until a clear
dark blue liquid results. The articles are now tinned by
immersion in a solution of

Stannous chloride .... 1 part.

Cream of tartar .... 3 parts.

Water to form a solution.

§ 284. The following method is simpler than the
above : —

Stannous chloride .... 1 ounce.
Cream of tartar .... 2 ounces.
Water 5 fluid ounces.

Zinc and lead are tinned by simple immersion, but there
is also deposited a pasty mass of tin, which rubs off,
leaving a thin reguline and firmly adherent deposit of tin
on the surface.

Iron, brass, bronze, and copper are tinned in this
solution when in contact with zinc; the deposit extends
uniformly over the surface, and withstands scratch-
brushing.

It would appear, from the rapid precipitation of tin by
zinc, that zinc should only have a very short immersion in
this solution, and be worked at the ordinary temperature.

§ 285. Gore recommends the following solution for
coating articles of iron or zinc with tin : —

Stannous chloride 1 part.

Ammonium alum 30 parts.

Water 1000 „

Heat the solution to boiling and immerse the previously

iv COATING METALS WITH TIN 311

cleaned articles in it until they attain a fine white colour,
and add fresh stannous chloride as the solution becomes
weaker.

Zinc and lead are well tinned in the above solution.
Brass, iron, bronze, and copper may be tinned if placed in
contact with zinc, but the deposit only occurs on the side
adjacent to the zinc, so that the articles must be com-
pletely surrounded by that metal if they are to be tinned
all over. Zin« and lead should not be allowed to remain
in the solution longer than is absolutely necessary to obtain
a good reguline deposit, as beyond this point the tin is
deposited in a pasty mass.

§ 286. According to Roseleur, articles of zinc may be
tinned by simple immersion in a solution composed of

Fused stannous chloride .... 1 part.

Sodium pyrophosphate .... 5 parts.

Water 300 „

We found this solution to work very slowly indeed ;
after several hours' immersion a black deposit was formed,
and on scratch-brushing a coating of tin was revealed. Iron
and steel were well and rapidly coated in this solution by
placing in contact with zinc.

§ 287. Roseleur also recommends the two following
liquids for coating iron articles by zinc contact : —

(Cream of tartar 1 part.

Stannous chloride .... 1

Water 1

Stannous chloride .... 6 parts.

II. ^ Sodium pyrophosphate ... 60

Water 3000

>»

312 METAL-COLOURING AND BRONZING part

Each liquid must be used hot and kept in constant motion.
The articles are immersed in contact with fragments of
zinc, the entire surface of which should be equal to about
one-thirtieth that of the articles to be tinned. The opera-
tion lasts from one to three hours. Roseleur gives the
preference to No. II. solution. He also prefers to use
coils of zinc instead of fragments of the metal, as being
less liable to cause markings on the articles than the
latter. For tinning small articles they are placed in
layers on perforated zinc plates which must be quite clean
before immersion.

§ 288. A solution for tinning may be made with

Stannous chloride (concentrated solution) . \ fluid ounce.

Water 6 „ ounces.

Sodium hydrate in slight excess.

Zinc is tinned in this solution by simple immersion, but
not so well as in the previous methods. Brass and iron
are tinned by zinc contact.

COATING METALS WITH ZINO

§ 289. Articles to receive a deposit of zinc must be
thoroughly clean, especially iron castings, and frequently
moved about in the bath while the deposition is proceeding.
They should also be well scratch-brushed during the process
with a steel brush. Polishing is effected by means of the
usual calico bobs, using lime and oil.

I. For zincing iron by contact a concentrated solution
of zinc chloride and ammonium chloride is suitable. The

iv COATING METALS WITH ZINC 313

liquid is boiled and the objects immersed in contact with
large surfaces of zinc.

II. Boil commercial zinc powder with a concentrated
solution of sodium hydrate, then immerse the articles to
be zinced in the boiling , solution, when they will soon
become covered with a bright layer of zinc. Copper
articles are best put in contact with a piece of zinc.

PART V

MECHANICAL METAL-COLOURING

§ 290. Colours may be imparted to various objects by
which an imitation of bronze may be obtained, by adding
to the surface a mere mechanical deposit -which is not an
integral part of its substance, and therefore not a true
bronze, but simply a second substance imposed on the
surface.

This may be well illustrated by the method of bronzing
with Dutch -metal powder. (The scraps obtained from
beating the metal to form the thin sheets known as leaf
are rubbed with honey or gum to form the powder.) The
body to be bronzed is first varnished with japanner's gold-
size, and the metal powder applied to the surface by
means of a pad of chamois leather. ~*

§ 291. A coating of gold may be also imparted to
articles in a similar way. Leaf -gold is ground with virgin
honey on a stone until the leaves are broken up and
minutely divided. The mixture is removed by means of
a spatula and stirred up in a basin of water, whereby the
honey is dissolved and the gold set free. The basin is
then left undisturbed until the gold subsides, the water
is poured off, and fresh quantities added until the honey

part v MECHANICAL METAL-COLOURING 315

is entirely washed away, after which the gold is collected
on a filtering paper, and dried for use.

The gold-size is applied to the perfectly clean and dry
article by means of a brush, carefully going over the whole
again to remove any excess of size that may have lodged
in any of the crevices. Now allow it to remain in a clean
and warm place for several hours until it is just sticky but
not liquid, then apply the gold powder or bronze powder
as the case may be.

§ 292. Instead of Dutch-metal leaf, tin sulphide may
be used as a bronzing powder. One part of tin sulphide is
rubbed with six parts of bone-ash, and the mixture applied
to the objects by means of a moist linen pad.

Copper which has been precipitated by means of iron
may be also employed, as well as various alloys of copper
and zinc, and copper and tin, after being reduced to a very
fine state of division.

White bronze powder may be obtained from an alloy
of equal parts of bismuth, tin, and mercury, known as
Mosaic silver, which, being of a very brittle nature, can be
readily crushed to powder. Instead of this alloy metallic
tin may be used. It is reduced to powder by pouring
it into a box, the inside of which has been well rubbed
with whiting, and when the metal is just on the point of
solidifying, vigorously shaking, when the metal breaks up
into an exceedingly fine powder. The coarser fragments
are removed by means of a fine sieve. The finely-sifted
portion is made into a thin paste with liquid glue and
applied to the object by means of a brush. This pro-
duces a dead colour which may be made bright by means
of a burnisher.

A gray tint may be imparted to objects by first giving

316 METAL-COLOURING AND BRONZING part

them a coating of blacklead and then applying the pre-
paration of white bronze.

But it is quite unnecessary to deal further in this work
with the manufacture of bronze powders, as they are
manufactured on the large scale, and can be so cheaply
purchased in all shades of colour that no bronzer would
dream of making them for himself.

§ 293. Bottger recommends the following method for
bronzing wood, porcelain, glass, etc. : —

A concentrated solution of potassium silicate is first
prepared, and the articles are coated with it in as thin and
as uniform layers as possible by means of a brush, or if the
articles are small, they may be immersed in the solution.
While the articles are still moist, bronze powder is
applied, and then they are left to dry in the air at the
ordinary temperature, when any superfluous powder is
removed by a soft brush. The layer of bronze powder
adheres very firmly to the object, and may be polished or
burnished.

Bottger states that this method is very well adapted
for articles of iron or porcelain which have to be subjected
to heat, as the bronze is not altered by a moderate
increase of temperature. The frames of mirrors, or tables,
gilt either with real or imitation gold, which have been
damaged by use and had their gilding removed, may be
repaired by means of this process, using gold or bronze
powder as desired. Glass, wood, and many other bodies
may be easily decorated by this process.

§ 294. Materials for Bronzing as used by Japan-
ners. — Metallic Bronze Powders. — These may be purchased
in almost any required colour at prices varying from about
three shillings to thirty-two shillings per pound. The

v MECHANICAL METAL-COLOURING 317

cheapest quality is used for bronzing common cast-iron
work, and the better qualities for a superior class of
goods. For general purposes that sold at seven shillings
per pound and retailed at sixpence per ounce is usually
preferred.

The colours commonly kept in stock by retailers are
white (imitation silver), various shades of imitation gold,
such as pale, deep, and green gold respectively; various
shades of crimson or copper bronzes, as well as all shades
between crimson and green gold, including orange, citron,
etc. Besides these well-known bronze powders others have
been more recently introduced possessing very brilliant
tints, probably produced by dyeing the metallic powders
with aniline dyes.

Varnishes. — The varnish used for fixing the bronze
powders is called gold-size, of which two kinds are in
general use, viz. quick-size and slow-size. The former
dries or becomes tacky (in which condition it is suitable
to receive the bronze powders) in about fifteen minutes
after the work has been varnished with it, whilst the
latter will require several hours to arrive at the same
condition.

The varnish used to protect the work from oxidation
after bronzing is that known as copal varnish. Several
qualities are used by japanners. A nearly colourless
varnish must be used for light or richly tinted bronze
work, but a darker and cheaper variety may be employed
for the darker-coloured work. The price varies from six
to sixteen shillings per gallon.

Turpentine. — This is used for thinning the different
varnishes used by japanners.

§ 295. Bronzing Operation. — All work, except that of

318 METAL-COLOURING AND BRONZING pabt

the commonest kind, is japanned and dried in a stove
before being bronzed. The method adopted depends on
the kind of work to be done.

Iron furniture castings, of the common kind, are
varnished all over with gold-size and set aside until they
are nearly dry or "tacky," when the bronze powder is
rubbed on the relief parts by means of a leather pad.
The articles are then put into a stove or oven, where they
are allowed to remain until the gold-size dries perfectly
hard. They are then coated with copal varnish and
returned to the stove to again dry. In some cases the
metallic bronze powder is mixed with gold-size to the
consistency of paint, and this is brushed on to the relief
parts of the work, which is dried in the stove, and not
subsequently varnished; this method is not nearly so
effective as the one previously described.

Castings for superior kinds of work are treated as
follows : — Instead of painting the gold-size all over the
casting, the operator pours a little out upon his stone,
and grinds just sufficient chrome-yellow into it to impart
a yellow tint. He then applies the tinted vanish to the
work by means of a camel-hair pencil, carefully following
the outlines of the part to be bronzed. The object of
tinting the varnish is to enable the workman to see the
parts that have been outlined. This operation is termed
"picking out." As in the first method, the bronze
powder is rubbed in after the size has acquired the
proper degree of stickiness, and the work is then stoved
and varnished.

Bronze powders are much used in ornamenting japanned
and polished work, such as cash-boxes, teja-trays, etc. In
this case the artist, or japanner as he is called, draws the

v MECHANICAL METAL-COLOURING 319

design upon the surface of the work with a camel-hair
pencil by means of the tinted varnish, and when this is
sufficiently dried the bronze powder is rubbed in, using
one or several colours, when the operation is finished as
described above.

§ 296. Bronze filleting is done as follows : — The work,
say a cash-box, has a band or fillet, about one inch wide,
drawn around the lid at a distance of half an inch from
the edge, and when this is sufficiently dry, pale imitation
gold-bronze powder is rubbed in and dried in a stove as
before. The black japanned box lid now presents the ap-
pearance of having a brass band inlaid upon it. The box
is now returned to the japanner, who binds the edges of
the bronze band with a fine sharply drawn line also in tinted
varnish, he also draws other fine lines around the body of
the box, and when all these lines have acquired the right
degree of stickiness by drying, he rubs in bronze powder,
but of a different colour to that used for the fillet
Copper-bronze powder is generally used for this purpose.
The work is then returned to the stove and varnished as
usual. The fillet now appears like a brass band bound
with narrow copper edges, whilst the copper lines upon the
body of the box give a finished appearance to the work.

§ 297. For most purposes the slow japan gold-size is
preferred, because a large quantity of work can be sized
one evening and be ready for the application of the bronze
powder on the following morning. If quick-size were
used instead of slow-size the bronze powder would have
to be rubbed in soon after the sizing, as it would be
too dry after standing all night. The bronze powder
fixed with slow-size is also less liable to become " shady "
than that fixed with quick-size.

320 METAL-COLOURING AND BRONZING part

Gold-leaf and Dutch-metal leaf is also used in con-
junction with bronze powders for decorative purposes.
The method of fixing the metal upon the work is precisely
similar to that described for bronze powders, except that
the gold or metal leaf is laid carefully on the sticky
pencilled design, gently pressed down with a pad, and, as
the leaf adheres only where the design has been drawn,
the excess is wiped off with a silk duster.

The gold scrap is carefully collected and sold to the
refiners. The metal scrap is sold to the bronze-powder
makers.

Gold-leaf is sold in books, about Is. 3d. each, each
book containing twenty -five leaves. Dutch -metal leaf
costs from Id. to 3d. per book of twenty leaves.

§ 298. When the gold-leaf upon the work is required
to have a beautifully bright surface, or to be burnished as
it is called, the process is conducted as follows : —

The japanned surface of the work to be gilded bright
is previously highly polished. The japanner first wets
the surface of the work with a solution of isinglass, and
then cuts the gold-leaf into strips of suitable width, which
he places carefully upon the prepared surface. The
excess of islinglass solution is then drained away, and
the gold adheres firmly to the work.

The design is next drawn upon the surface of the gold
with a camel-hair pencil, using black asphaltum varnish
for this purpose. When the design is finished and dry,
the gold not covered by the varnish is removed by gently
rubbing with cotton wool moistened with water. The
varnish being insoluble in water protects the gold, which
is to constitute the decoration, from being injured by the
process.

▼ MECHANICAL METAL-COLOURING 321

The next step is to remove this varnish so that the
gold beneath shall be exposed to view. To do this the
varnish is washed off with turpentine, in which liquid it
is easily dissolved, whilst the isinglass which binds the
gold to the work is unaffected because it is insoluble in
turpentine.

The design now appears in gold and only requires to
be gently rubbed with chamois leather and whiting to
give it a brilliant lustre. In this condition it could easily
be washed off with water, because isinglass is soluble in
water. To prevent this, the entire gilt surface is coated
with one or two layers of colourless copal varnish, which
is dried hard in a japanner's stove, after which it affords
a permanent protection to the metal

Gold-leaf is beaten exceedingly thin, hence when a
burnished appearance is desired it must be laid upon a
bright surface. If the gilding is done upon a dead
surface the gold will be correspondingly dead.

As in the case of bronze powders gold-leaf can be
obtained in several shades of colour, such as pale gold,
deep gold, and green gold ; the two first being formed by
alloying gold with different proportions of copper, and the
other by alloying gold with silver.

§ 299. Bronzing of Plaster of Paris Articles.— The
bronzing of articles made of plaster, terra-cotta, wood, and
other non-metallic substances may be effected in various
ways ; and although it is with the colouring of metals that
this work is chiefly concerned, it has been thought advis-
able to give some information with regard to the colouring
of other bodies. In many cases metals are electro-deposited
on non-metallic surfaces, and then the subsequent bronzing
is a colouring of the deposited metal.

322 METAL-COLOURING AND BRONZING part

§ 300. According to Thenard and D'Arcet, plaster
figures may be bronzed with a green patina in the follow-
ing way : —

1st. Boil some linseed oil with caustic soda, so as to
form a neutral soap ; then add a concentrated solution of
common salt, and boil until grains of the soap swim on the
surface. Filter and well press the precipitate until all the
lighter particles are removed ; then dissolve the soap in
distilled water and filter off any insoluble residue.

2d. Prepare a clear solution of 4 parts copper sulphate
and 1 part iron sulphate, boil, and add a little at a time
a portion to the above soap solution, and well boil. Wash
the precipitate and add it to the remaining solution of
sulphates. Well wash the precipitate by decantation with
hot water, then with cold water, and finally filter it off.

Take 3 parts of boiled oil, 1£ parts of the above prepared
soap of copper and iron, and 1 part of pure white wax,
and melt the whole together. Heat the plaster figure to
about 80° C, and apply the molten mixture. If the article
is small, it may be immersed in the composition, then re-
moved and dried at 80° to 90° C. The operation is
repeated until the plaster will absorb no more of the liquid ;
it is then left to stand exposed to the air until all odour
disappears, and finally rubbed with a pad of^fine linen.

§ 301. Another method of coating plaster objects with
a green bronze is to paint them first with a solution of
copper acetate containing some plumbago in suspension
(4 of copper acetate to 1 of plumbago), and when this is dry
to give them a coating of gum, and lastly a coating of gold-
size. Bronze powder is then rubbed on the prominent
parts and the whole brushed with a waxed brush when dry.
Six parts of copper acetate are now dissolved in vinegar,

v MECHANICAL METAL-COLOURING 823

1 part of yellow chrome added, and 1 part of plumbago.
This mixture is painted on with a brush so as to leave the
deep parts green. After standing for five or six hours, the
article is coated with a transparent varnish, and the pro-
minent parts relieved if necessary with bronze powder.

It will readily be understood from the preceding that a
great variety of tones may be imparted by means of dif-
ferent bronze powders and mixtures, which are coloured
when dry.

§ 302. An iron colour may be imparted to plaster
articles as f ollows —

Digest in the essence of terebinth 2 parts of red
iron oxide and 1 part of soot, heat on a sand bath and
add a little yellow wax. This mixture is painted on the
plaster article with a brush, and the object dried in a stove.
When the object has absorbed the fat part of the coating,
a little brown umber is sprinkled on to imitate rust in the
deeper parts, and white bronze on the prominent parts, the
powders being fixed with gold-size in the usual way. Par-
ticular parts may be relieved by rubbing with the finger.
Other coloured powders may be added if desired.

§ 303. A red bronze may be produced on plaster figures
by the method described in § 300, by increasing the pro-
portion of soap of iron or omitting the copper. Or in a
solution of strong glue to digest some Prussian blue, soot,
and yellow ochre. The article is coated several times
with this mixture, with alternate drying. Lastly, any
special parts may be touched up with a fine brush which
has been dipped in tin sulphide and varnish.

§ 304. Many colours on so-called bronze objects are
simply produced by different pigments, such as those on the
draperies of figures, wings of birds, skins of animals, etc.

324 METAL-COLOURING AND BRONZING part v

A common method is to paint with two or three coats,
allow to dry, then give a coating of varnish or simply of
the white of an egg.

A golden -yellow is produced from a decoction of
the young root of the hawthorn mixed with a little
saffron.

Green is obtained by boiling some nightshade in 1
part vinegar and 1 part water.

Red is produced by boiling some Brazil wood with alum,
and the same boiled with caustic soda is used to produce a
brown. The colours are applied by means of a varnish.

Plaster figures are made to imitate silver by means of
an amalgam of mercury, tin, and bismuth, then coated
with a transparent varnish.

§ 305. The best means of imitating bronze on plaster
articles is to deposit a coating of copper or brass on their sur-
faces by means of the electric current. The whole surface
must first be saturated with boiled linseed oil and allowed
about two days to dry. The surface is then brushed over
with a concentrated alcoholic solution of silver nitrate,
allowed to dry, and submitted to a current of sulphuretted
hydrogen, which converts the surface into a coating of
silver sulphide, and this being a good conductor of elec-
tricity, enables the metal to be deposited when the article
is placed in the depositing bath.

REMEDIES FOR ACCIDENTS, ETC., IN
PROCESSES OF BRONZING

§ 306. As some of the substances referred to in the
text are of a highly poisonous nature, and others have an
injurious influence on the flesh, when the skin happens to
be broken, it is desirable to know what to do in case of an
accident.

Workmen exclusively engaged in pickling objects are
advised to neutralise the action of acid vapours upon the
enamel of the teeth and the mucous membranes of the
mouth and throat by frequently rinsing the mouth with a
dilute solution of sodium carbonate.

In cleansing articles from grease by means of caustic
potash or soda, the skin may be irritated and made very
sore, especially if any part is cut or broken. The operator
should frequently swill his hands in cledh water, and, pre-
vious to each repetition of the immersion, dip them in a
very dilute solution of sulphuric acid, dry them, and well
rub with equal parts of glycerine and water. It is advis-
able, in the case of a cut finger, to cover the wound with
plaster, and well wrap it round with some linen rag.

Great care should be exercised with the drinking-vessels
used by the workmen in a plating or bronzing shop, as
many cases of poisoning have occurred from want of

326 METAL-COLOURING AND BRONZING part

thought in this direction. There is a great temptation in
cases of emergency to use the same jug or cup for ladling
out a solution as is used for ordinary drinking purposes,
and if such a vessel is not properly swilled out serious con-
sequences may follow.

Many individuals are very sensitive to nickel solutions,
eruptions, which are very painful and heal slowly, breaking
out upon the arms and hands. In case such a person
has to touch nickel salts, or to put his hand into a nickel
solution to remove an object which has fallen to the bottom
of the bath, he should immediately well wash his flesh
with clean running water.

In cases of internal poisoning the following plans should
be pursued, as everything depends on promptitude in
applying the remedy : —

Poisoning by Nitric, Hydrochloric, or Sulphuric Acids.
— Administer abundance of tepid water to act as an emetic,
or swallow milk, the whites of eggs, some calcined mag-
nesia, or a mixture of chalk and water. If those acids, in
a concentrated state, have been spilled on the skin, apply
a mixture of whiting and olive oil. If the quantity is
very small, simple swilling with plenty of cold water will
suffice. A useful mixture, in case of burning with strong
sulphuric acid, is formed with one ounce of quicklime
slaked with a quarter of an ounce of water, then adding
it to a quart of water. After standing some time pour off
the clear liquid and mix it with olive oil to form a thin
paste.

Poisoning by Potassium Cyanide, Hydrocyanic Acid,
etc. — If cyanides, such as a drop of an ordinary plating
solution, has been accidentally swallowed, water, as cold
as possible, should be run on the head and spine of the

v REMEDIES FOR ACCIDENTS 327

sufferer, and a dilute solution of iron acetate, citrate, or
tartrate administered. If hydrocyanic acid vapours have
been inhaled, cold water should be applied as above, and
the patient be caused to inhale atmospheric air containing
a little chlorine gas. It is a dangerous practice to dip the
arms into a plating solution to recover any work that has
fallen off the wires, because the skin often absorbs cyanide
liquids, causing painful sores. In such a case well wash
with water, and apply the olive oil and lime water mixture.

Poisoning by Alkalies. — These bodies are the opposite
of acids in character, so that acids may be used as anti-
dotes. It is preferable to employ weak acids, such as
vinegar or lemonade ; but if these are not at hand, then
use exceedingly dilute sulphuric acid, or even nitric acid
diluted, so that it just possesses a decidedly sour taste.
After about ten minutes take a few teaspoonfuls of
olive oil.

Poisoning by Mercury Salts. — The white of an egg is
the best antidote in this case. Sulphur and sulphuretted
hydrogen are also serviceable for the purpose.

Poisoning by Copper Salts. — The stomach should be
quickly emptied by means of an emetic, or in want of this,
the patient should thrust his finger to the back of his
throat so as to tickle the uvula, and thus induce vomiting
After vomiting drink milk, white of an egg, or gum water.

Poisoning by Lead Salts. — Proceed as in the case of
copper salts. Lemonade, soda water, and sodium carbon-
ate are also serviceable.

Poisoning by Acid Vapours. — Admit immediately an
abundance of fresh air, and inhale the vapours of ammonia,
or a few drops of ammonia may be put into a glass of water
and the solution drunk. Take plenty of hot drinks and

328 METAL-COLOURING AND BRONZING pabt v

excite warmth by friction. Employ hot foot-baths to re-
move the blood from the lungs. Keep the throat moist by
sipping milk.

§ 307. Removal of Stains, etc. — To remove stains of
copper sulphate, or salts of mercury, gold, silver, etc., from
the hands, wash them with a very dilute solution of am-
monia, and then with plenty of water ; if the stains are
old ones, they should be rubbed with the strongest acetic
acid, and then treated as above.

Grease, oil, tar, etc., may be removed from the hands
or clothes by rubbing with a rag saturated with benzine,
turpentine, or carbon bisulphide.

APPENDIX

§ 59 (d). Although this is not suitable for plain work, it may be
satisfactorily employed for light and shade effects in repousse*
work. Take :—

Bees'-wax 8 parts

Vaseline lpart

Melt the above together and saturate a rubbing cloth with the
molten mixture and allow to cool. Heat the work and grease the
surface uniformly by rubbing with the cloth. Then a brown or
black colour may be obtained by heating the article and finishing
by rubbing with a clean cloth or leather.

§ 61. The following is a superior method to the one given in
§ 61, p. 103. Excellent Florentine bronzing mixtures are now
specially prepared by drysalters, and it is better to buy the mixtures
than to prepare them oneself.

Mix the powder to a paste and paint all over the surface of the
work. Heat upon a hot plate until dry, but not to a temperature
higher than can be just handled without burning one's fingers.
Finish by brushing with a soft brass wire scratch-brush, lubricated
with a trace only of black lead. The brush must be used dry.
The article should then be lacquered.

§ 162. Steel bronzing solution.

Arsenious oxide 20 oz.

Copper sulphate (powdered) . . . . 10 „

Ammonium chloride 2 ,,

Common hydrochloric acid .... 1 gallon.

330 METAL-COLOURING AND BRONZING

Mix the above ingredients together, and allow to stand one day
before using. This produces an excellent steel -coloured bronze
upon brass almost instantaneously in the cold solution.
§ 165. Colouring brass. Use

Copper sulphate
Water

Washing soda .
Water

i£»} • <«>

1 J OZ. )

1 pint f

0>)

Add the solution (6) to the solution (a), and after well stirring,
throw the mixture on to a calico strainer and allow the liquid to
drain away. Then wash the precipitate with water. Now transfer
the precipitate to a suitable vessel and dissolve it in one fluid ounce
of ammonia solution ('880), and dilute to one pint.

§ 185. Two colours may be produced in the solution given in
§ 185, p. 230, as follows :—

1. Obtain first a blue or a steel colour, as may be preferred, all
over the work.

2. Scour down to the metal on the relief parts and re-immerse
in the solution. The parts previously steel-colour will remain
unaltered, and any colour the solution is capable of may be formed
on the scoured surface. If the unscoured surface is blue this will
pass into steel-colour.

§ 188. A black colour on zinc is obtained from the following : —

Copper chloride 1 oz.

Copper nitrate 1 „

Ammonium chloride 1 „

Hydrochloric acid 1 „

Water 1 gallon.

The above mixture is applied with a brush, and allowed to dry.
It is then brushed with a stove-brush, then dry scratch-brushed
after heating in a stove.

§ 229. In addition to the information given in § 229, p. 266, the
following is recommended : — First produce a black colour, then beat
the surface with a hard bristle brush, charged with wet powdered
pumice, until an agreeable gray colour is produced. Relieve the

APPENDIX 331

high parts of the design by scouring lightly with sand upon a bit
of chamois leather. Finish with silver lacquer.

N.B. — The surface appearance of work coloured black by means of
sulphides, or by heating after immersion in copper nitrate solution,
may often be much improved by "dry" scratch-brushing, as
described under § 61, but separate brushes should be set apart for
Florentine and black bronzing respectively.

. ( • t

INDEX

Absorbing, 6

Accidents, remedies for, 325
Acetic acid, 116, 155, 286
Acid dips, 74, 83

silicate, 27
Acidity, 56
Acids, 26

Actinic changes, 122-4
jErugo nob&is, 58
Affinities, 55
Air, 28
Aitkin, 79
Alcohol, 100, 234
Alkali, 51
Alkaline earth, 50
Alloy, 52
Alum, 179
Aluminates, 49
Aluminium, 49, 87
Amalgams, 37
Ammonia, 3, 4, 143, 159, 173-7,

215, 218, 235
Ammonium, 51

carbonate, 177-9, 200, 241

chloride, 173-7, 183-5,
197

molybdate, 235

sulphide, 159, 167, 180,
207, 221, 247, 268
Anode, 284
Antimonious oxide, 40
Antimony, 14, 213

chloride, 234, 248, 253

Antimony sulphide, 161, 213,

221
Antique bronze, 97

green, 62

patina, 166-9, 173
Aquafortis, 76-8
Arsenic, 18, 41, 153, 163-9

oxide, 163, 179, 211
Arsenious oxide, 241, 248
Asphaltum Tarnish, 320
Artists, 7
Atomic weight, 20
Atoms, 19, 22

Bacco, 302-6
Barfif, 43
Barium, 50

oxide, 50

sulphide, 164, 266
Base, 59
Basic, 26

silicate, 27
Basicity, 56
Bavarian, 220
Becquerel, 298
Benzine, 74
Berlin, 170

Commission, 172
Bismuth, 5, 38

chloride, 250

flowers of, 38

glance, 89

334

METAL-COLOURING AND BRONZING

Bismuth nitrate, 205

ochre, 38
Blacking, 78
Blende, 47
Blue-stone, 805
Bluish-green patina, 179, 184
Bobs, 93
Bone oil, 171
Bottger, 133, 161, 210-12, 240-8,

316
Brass, 53, 187

black colour on, 164, 205

brown colour on, 220

colour of, 188

colouring of, 191

composition of, 189

coppering of, 306

dipping of, 75-7

impurities in, 187

loss in bronzing, 147
Brassing, 279, 306

solution, 280
Britannia metal, 293
Bromine, 181-6
Bronze, 54-9

Barbedienne, 222

deposition of, 285

filleting, 319

green, 166

imitating on plaster, 321

powder, 315-8
Bronzing, 1, 317

chemicals for, 117

nature of, 67

object of, 58

of plaster articles, 321

paste, 102

salt, 252

solutions, 16

with iron oxide, 112
Buchner, 96, 135, 175, 180-5,
190, 214, 253

Cadmium, 48
Caesium, 52

Calamine, 47
Calcium, 50
Carbon, 28

dioxide, 29

monoxide, 30
Carlemann, 118
Cast iron, brassing of, 283

coppering of, 278
Caustic potash, 122

soda, 122
Centrifugal force, 89
Charcoal fire, 109, 149
Chemical bronzing, 60

change, 18

compounds, 18

elements, 18

metal-colouring, 60, 94
Chemicals, impurities in, 117
Chlorides, 124, 155
Chlorine, 30
Chrome ironstone, 44
Chromic oxide, 44
Chromium, 44
Cinnabar, 37
Cleaning, 73

by potash and soda, 73

of copper alloys, 75
Cobalt, 46

ammonium sulphate, 288

bloom, 46

glance, 46
Cold-short, 42
Colour, 5, 11, 69

scale, 97
Coloured compounds, 72

shadows, 8
Colouring, 65

copper, English method,
103
Colours, neutral, 12
Combustion, 2, 25
Common salt, 79, 177, 183, 197,

203
Complementary, 8
Contrast, 11

INDEX

835

Copper, 5, 14-19, 34-6

acetate, 101, 130, 131,

143-149, 155, 175, 177,

184, 198, 203-9, 226,

239, 255, 276
basic carbonate, 96, 97,

183
basic chloride, 96
brown colours on, 146, 164
carbonate, 214
chloride, 118, 122-4, 132,

193-9, 202, 240, 250-5,

285
colouring of, 95, 113
cyanide, 282
deposition on iron, 300
hydrate, 801

light brown colour on, 113
loss in bronzing, 125, 147
nitrate, 129, 135, 140, 163,

176, 180-5, 191, 215,

227, 239, 241
oxy chloride, 120
oxygen compounds of, 76
phosphate, 285
red oxide of, 71
sulphate, 128, 131, 146-8,

152, 196-9, 203, 218,

224, 236, 253-5, 281,

299, 307
sulphide, 133, 159
sulphur compounds of, 76
tartrate, 242, 802, 308
Coppering, 194, 217
bath, 276

by separate current, 275
Cream of tartar, 177, 184, 197,

209, 303-8
Cryolite, 49
Cupric chloride, 118
oxide, 95, 118
sulphide, 118
Cuprous chloride, 118
oxide, 35, 96, 118
sulphide, 35, 60, 95, 118

Cyanide, 292

D'Aroet, 322
Daylight, 128
Dead dip, 81

dipping, 79
Design, principles of, 11, 12
Dienst, 107
Dipping, 76, 145

hright, 77
Dips, 76
Discord, 11
Dittrich, 192
Dolomite, 48
Dullo, 234
Dutch metal, 314

Electricity, 271
Electro-chemical, 59, 60

chemical metal -colouring,
61, 271

chrome, 296

coppering, 278

deposition, 60, 191

metallurgy, 66

negative, 43, 272

positive, 48, 271
Elkington, 295
Eisner, 168, 205, 278, 301
Equations, 24
Ether, 74

Ferric chloride, 120, 194, 195,

251-6
Filter-paper, 115
Filtration, 115
Fischer, 272
Fleitmann, 45
Florentine bronze, 62, 272

tint, 219
Fluor-spar, 112
French, 64, 65

bronzing, 220

836

METAL-COLOURING AND BRONZING

French castings, 189

recipe, 184
Fulminating gold, 294

Galena, 47
Gallic acid, 253
Garnierite, 46
Gassiot, 298
Gauduin's solution, 278
German silver, 45, 54, 289

silver, dipping of, 77
Germans, 63
Gilder's wax, 263
Gilding, 270

metal, dipping of, 77

solution, 294
Gilt work, colouring of, 263
Glaze, 119
Gold, 5, 14, 19, 31, 67

alloys, 151

chloride, 31, 207, 293-6

colouring of, 257-9, 260

cyanide, 294

deposition of, 293

electro-deposited, 262

green, 262

harmonising power of, 67

leaf, 820

plating, 294

recovery of, 294

red, 262

size, 314-9
Gore, 274-6, 310
Gountier, 285
Grager, 800
Graphite, 28, 100
Green bronze, 166

patina, 171, 185
Grinding, 92
Gun-barrels, 252

Haldane, 104, 132
Harmony, 11
Hess, 254
Homogeneity, 56

Hot-short, 42

Hydrochloric acid, 31, 76, 77, 80
Hydrofluoric acid, 86
Hydrogen, 19, 27

potassium carbonate, 295
Hydroxyl, 3

India, 16

Indian red, 141

Interference, 9

Iodine, 18

Iridescence, 149, 153

Iridescent colours, 85, 123, 223,
336

Iron, 42

black colour on, 249, 250
black oxide, 43, 44, 72
blue colour on, 248
bronze colour on, 252
bronzing of, 245, 252
brown colour on, 252
cleaning of, 85
colour on plaster, 323
coppering of, 306
gray colour on, 247
oxide, bronzing with, 99
red oxide, 71, 198, 236

Japan, 16

Japanese, 15, 16, 64, 151

bronze, 151
Japanners, 316

Kaysbb, 184, 185, 192
Kish, 29

Kiu-shibu-ichi, 152
Eletzinski, 235
Enaffl, 235
Kupfer-nickel, 41-5

Lacquering, 59

stove, 131, 141
Lacquers, 12
Lafleur, 62, 272

INDEX

337

Langbein, 101-5, 222, 285-7, 296
Lead, 5, 37

acetate, 230, 249, 296

oxide, 38

peroxide, 297

sulphide, 38
Ledebur, 14
Leigh ton, 17
Light, 6, 7, 62

composition of, 10
Lime, 28
Liquates, 57
Lithium, 52
Liver of sulphur, 267
Ludersdorff, 243, 303-7

Magnesite, 48
Magnesium, 45-8
Malins, 79
Manganese, 44

black oxide, 44
nitrate, 237
Mannheim gold, 187
Marsh-gas, 27
Mechanical metal-colouring, 61,

314
Melbourne Exhibition, 104
Mercuric chloride, 159, 183, 209,
212

oxide, 37

sulphide, 37, 105, 160, 210
Mercury, 36, 105, 159
Meriteus, 250
Metal-colouring, 57, 63

Chinese method, 104

English method, 103
Metallo-chromy, 298
Metals, 4, 12, 18, 20, 58

colours of, 71

physical condition of, 69
Mispickel, 41
Mixed metals, 187
Moire, 81
AfoirS MetaUique, 40, 256

Molecules, 22
Monuments, 171
Mother-of-pearl, 10

Neumann, 237
Newton's rings, 97
Nickel, 15, 44

ammonium sulphate, 235,
286-9

arsenide, 45

cobalt alloy, 287

deposition of, 286

glance, 45

pyrites, 41-5

sulphide, 45
Nickeling, 288
Nitric acid, 3, 4, 79, 256
Nitrogen, 3, 19, 27
Nitrous acid, 3, 4

vapours, 78
Nobili, 296
Nobili's figures, 60

rings, 297
Noble metals, 61
Nomenclature, 22
Non-metals, 18, 21

OLBFIANT-gaS, 27

Olive oil, 172, 214
Oreide, 187
Ormolu colour, 257

dead, 258

red, 258

yellow, 258
Orpiment, 41
Osmium, 20

Oxalic acid, 135, 200, 286
Oxidation, 2, 25, 124
Oxide, 1, 3, 26
Oxidised, 2
Oxidising, 264

agent, 25
Oxygen, 2, 19, 26
Ozone, 3, 4

888

METAL-COLOURING AND BRONZING

Paint, 12
Palladium, 27
Paraffin, 74
Parcel coppering, 243
Paris Mint, 106, 198
Parkes and Martin, 247
Patchy, 114, 145, 167
Patina, 59, 166-8, 170, 180
Paul, 309
Pewter, 293
Philadelphia, 220
Philosophical instruments, 233
Phosphates, 30
Phosphorus, 19, 30

pentoxide, 30
Photographic negatives, 125
Photographs, 118
Pickle, 78
Pickling, 14

solutions, 152
Pigments, 7
Platinating, 204
Platinised, 138
Platinum, 32, 137

chloride, 137, 203, 240,
266
Poisoning by acids, 326

by alkalies, 327

by cyanide, 326

by salts, 327
Polishing, 91
Potassium, 51-6

bichromate, 80, 81

bisulphate, 86

cyanide, 75, 276, 281

ferrocyanide, 248, 285

oxalate, 178

stannate, 309

sulphantimoniate, 161

sulphide, 139, 140, 157,
160-7, 185, 207, 208, 210,
251, 267
Precipitate, 115
Prince's metal, 187
Prismatic colours, 297

Priwoznik, 118, 120
Prussian blue, 323
Puscher, 236, 241-9
Pyrolusite, 44

Quantitative experiments, 193
Quicking, 159, 292

Red bronze on plaster, 323

Reducing agent, 25

Reduction, 25

Reflect, 6, 10

Refracted, 10

Regulus of Venus, 14

Reichardt, 99

Roberts- Austen, 151

Roby, 49

Roseleur, 276, 281-3, 311

Rouge, 93

Rubidium, 52

Rust, 59

Saintb-Clairb Dbville, 237

Salt, 71, 115

Schwarz, 192

Scratch-brush, 86-8

Shaku-do, 151

Sheen, 123, 153

Shibu-ichi, 151

Silica, 28

Silicates, 28

Silicon, 28

Silver, 14-19, 32, 68

alloys, 151

chloride, 33

colouring of, 263

glance, 34

nitrate, 145

oxidised, 68

oxidising of, 264

pink tint on, 270

plating solution, 292

recovery of, 295

solders, 34

INDEX

839

;nts.

237

Silver, sulphide, 33, 85

tarnish on, 85
Simple immersion, 274
Similor, 187
Smoke-bronze, 223
Soap-bubble, 1, 10
Society of Arts, 151, 170
Soda lime, 286
Sodium, 51-6

bisulphite, 276, 283

hydrate, 202, 213, 226-8

phosphate, 241, 296

pyrophosphate, 285, 296,
311

6tannate, 309

tartrate, 286

thiosulphate, 33, 192
Solders, 53
Solutions, 70
Soot, 77, 78, 103
Specific gravity, 20
Spelter, 46, 232
Stains, removal of, 328
Stannic sulphide, 254
Stannous chloride, 310

sulphide, 40
Steel, black colour on, 249, 250

blue colour on, 248

bronzing of, 245

colours on, 97

coppering of, 306

polished, 86

sheet, 239
Stibnite, 41
Stolba, 288
Stolzel, 304
Strontium, 50
Struck, 293
Subjective colours, 8
Sulphides, 30
Sulphur, 30

dioxide, 30

trioxide, 30
Sulphuretted hydrogen, 4, 169,
182, 207, 269

Sulphuric acid, 76-8, 232-5
Superimposed, 125
Symbols, 20 .

Tacky, 317

Tartaric acid, 133, 150, 202, 226,

228
Temper, '246
Thenard, 322

Thin plates, colours of, 96
Tin, 14, 15, 39

coating metals with, 308

colouring of, 254

grain, 39

refined, 40
Tinning, 218
Tinstone, 40
Tombac, 187
Tripoli, 93
Turpentine, 317

Varnishes, 12, 317

Varnishing, 59

Velvet black, 160

Velvety coating, 157

Verdigris, 97, 152

Vermilion, 37

Vinegar, 115, 155, 169, 173-9,

184, 200, 227
Volatilisation, 57

Walckbr, 200
Water, 27

distilled, 250

gilding, 295

river, 117

spring, 117

well, 117
Watt, 275, 280-9
Wave, 9

length, 9, 97
Weiss Kopp, 286
Wet- colouring, 107
White bronze, 197
Whitening bath, 78

340

METAL-COLOURING AND BRONZING

Wiels, Mr. Fred, 286
Wuttig, 129, 197

Ybllow bronze, 181
Yellowish-green patina, 177, 178,
180-4

Zapon, 132, 140, 157, 160
Zeitschrijlfurlnstrumentenkunde^

144
Zinc, 46, 283

black colour on, 233, 305
bronzing of, 232
brown colour on, 240

Zinc, chloride, 199, 283

cleaning of, 74

coating metals with, 312

coppering of, 277, 299,
305

cyanide, 282

different colours on, 242

gray coating on, 241

oxide, 47, 48

parcel coppering of, 243

sulphate, 79, 81, 183, 281
282, 307

sulphide, 47
Zincing, 218, 312

THE END

Printed by R. & R. Clark, Limited, Edinburgh.

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B89074771387A

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