Enriching the Surfaces of Silver.
By the application of the processes about to be described, the
finishing touches in their relation to articles or wares of silver
manufacture are effected. These processes, as adopted by the trade,
are various, almost every firm having a specially prepared mixture and
mode of employing it. We shall refer only to those which, from their
practical utility, are likely to be of service to those workmen who
have to do with this particular metal. The branch of the art of which
we are now treating comes only into operation when every other process
of workmanship has been completed; and some of these processes must be
executed in a perfect manner in order to arrive at the highest possible
results in this one. The best and richest surface is produced when the
metal to be operated upon is good in quality, and the workmanship of
a fair order, so far as regards smoothness, and freedom from surplus
solder-marks.
One of the oldest methods for producing a pure snowy whiteness upon
articles of silver was as follows:--Take an iron or copper annealing
pan (the latter is much to be preferred), place the work upon it in
proper order, so that it may be heated all over alike. It should,
previous to this, be immersed in a thick solution of borax, or
otherwise brushed over with it. After the work has been properly
arranged upon the pan for annealing, it must be sprinkled over with
fine charcoal dust; the pan is then placed in the muffle upon a bright
clear fire without blaze, and when the work has assumed a degree of
heat approaching to cherry redness, it is withdrawn and allowed to
cool. When this has taken place, it is removed and boiled out in a
very weak solution of sulphuric acid, commonly called oil of vitriol.
If the right colour was not then produced, the process was repeated
as many times as circumstances permitted, though usually two or
three times was found to be amply sufficient. The annealing process
required great attention, for the work being in contact with borax,
if slightly overheated, it was liable to become melted, therefore the
operation was a delicate one to perform, and was only intrusted to
such workmen as were experienced in the art. Small delicate articles
were commonly treated with the mouth blow-pipe and gas-jet, and were
placed upon a pumice-stone, or some other suitable substance capable
of withstanding the power of burning. According to the inferiority
of the silver alloy, is the difficulty of producing a good white
surface on wares of such standards. Fine silver requires very little
whitening, generally one process suffices to effect a good colour;
but inferior standards require half a dozen or more to bring up the
proper degree of whiteness; and those ranging below 9 oz. 12 dwts. to
the lb. Troy cannot be whitened at all by the means we have described,
but require the application of the modern chemical process, known as
electro-plating.
The East Indian silversmiths never touch their manufactures with
any kind of abrasive substance, from the most delicate, to the more
strongly made article. But then it should be remembered, that in India
the natives work from the pure material, a point which they rigidly
adhere to; whereas we are compelled in this country to manipulate in
all sorts of qualities; and some of these require no little trouble
and difficulty to bring back to the surface the snow-white appearance
of the pure metal. In the former case it is effected without any
difficulty whatever--in fact, the metal scarcely undergoes any change
throughout the whole of the manipulations to which it is subjected in
the various processes of manufacture. The Indian mode of procedure is
as follows:--Some juicy lemons are cut into slices; the silver articles
are briskly rubbed with these for a short time, and subsequently
covered with them, being placed in a suitable vessel for a few hours
for the completion of the process. For very delicate articles of
jewellery the natives cut a large lime nearly in two halves, into which
they insert the work; the halves are then tightly closed up again, and
placed aside for a few hours; when the article is removed, it is well
rinsed in clean water, and consigned to a vessel of nearly boiling
soap-suds, where it is well brushed, again rinsed in fresh hot water,
and finally dried on a metal plate placed over hot water; the process
is rendered complete by a little gentle rubbing with wash-leather if
the work be of a plain nature. Green tamarind pods are also employed
by them for the purpose of whitening silver, in the same manner as
just described; they are great detergents both of gold and silver
manufactures, and are largely employed by artisans in the East for the
removal of oxides and fire-marks.
Another process for the whitening of silver goods is performed in the
following manner:--Take the work, which must be cleanly prepared, and
give it a coating of the following mixture:--Finely powdered vegetable
charcoal four parts, saltpetre one part; the ingredients should be
well mixed with a little water, and may be applied to the surface of
the metal either by brushing over with a soft brush, or by dipping
the work into it. The work is then placed upon the annealing pan and
submitted to the heat of the muffle, until the wet powder has become
perfectly dry and ceased to fly about; it is then withdrawn from the
muffle, allowed to cool, and afterwards boiled out in a solution of
potash prepared for the purpose, in the proportion of about one ounce
of bi-sulphate of potash to twenty ounces of water. The boiling out
is done in a copper pan (Fig. 39), and if the work be put through the
above process two or three times a beautiful dead-white colour is the
result. It is then washed in a hot solution of soda, soap, and water,
or if preferred bright, scratched, or burnished, and the process is
finally completed by drying it in fine boxwood sawdust, which should
be made hot, but not allowed to char or burn in any way, as it would
produce a stain upon the work very difficult to remove, and thus the
finish would be considerably impaired.
[Illustration: Fig. 39. Boiling-out Pan.]
In large manufactories the process of whitening silver goods is
repeatedly required to be performed, and where such is the case, the
above methods are found not only tedious and expensive, but occupy much
unnecessary time and labour; to dispense with a portion of which, the
custom of covering the work with a chemical preparation was accordingly
departed from, and yet it was made to show its former brilliancy.
To effect this object the liquid for boiling it in was differently
prepared, which only required to be made of a proper strength to do all
that the surface mixture had done before. The following is the method
adopted in preparing the cleansing liquid. Boiling-out mixture:--To one
pound of smoking salts, add two ounces of cream of tartar; well shake
the ingredients together, so that they may be thoroughly incorporated.
The smoking salts employed for this purpose are not the ordinary
_spirits of salts_ of commerce, but a preparation of the common oil
of vitriol; therefore the one should not be taken for the other; the
spirits of salts would turn the work black, whereas, if the proper
ingredient were procured, a fine dead matte or blanched surface would
be the result of its application. The mixture employed for boiling
the work in consists of the proportion of one ounce of the above
preparation to about thirty ounces of water. The silver articles
simply require to be annealed, allowed to cool, and then boiled for
a minute or so in this solution, when the desired result will be
attained; if, however, the exact colour be not obtained the first time,
the process should be repeated a second, and if necessary, a third
time; the right colour will then be produced, if the articles are not
made of a too inferior standard.
The mode we have ourselves adopted for the colouring or whitening of
silver goods is somewhat different, and still more simple than even the
above. We will proceed to give the details of the process. A mixture
of very dilute sulphuric acid is first provided, in the proportion of
one ounce to forty ounces of water, and well mixed together; the work,
after being heated to a good red heat, is boiled in this, which soon
removes the oxide from the surface, and shows the fine white colour of
the pure silver. For fine silver work, such as Indian filigree, one
process will generally suffice, for English standard quality two, and
for low qualities three, but these latter must not by any means be too
low; if so, no colouring can take place by the method just described.
Objects of delicate workmanship are usually annealed by the gas; being
placed on a pumice-stone of light material, the flame of the gas
is blown with the mouth blow-pipe, in such a manner that the object
gradually becomes heated all over alike; and the work should be well
heated, as this facilitates the process of oxidation, and subsequently
that of whitening. The oxidation takes place at the expense of the
copper in the silver alloy, and this is only effected by raising the
articles to a very high temperature, which produces the oxidation of
the copper coming in contact with the air, and which necessarily exists
upon the surface of the alloyed goods. Whitening silver goods then
is nothing more than the removal of the base alloy from the surface,
leaving the pure metal behind with its full rich colour. Therefore to
be clear, the process of annealing in contact with cold air oxidizes
the copper upon the surface, and the pickling mixture so dissolves and
removes it, that it gradually undergoes a process of refining, and is
ultimately made to represent the finest material in all its purity.
Sometimes silver work is to be seen having a brown colour upon it;
this is produced when the acid employed for cleansing has been too
strong; it can only be remedied by another annealing and boiling-out in
a much more diluted mixture. There are various other methods employed
in the trade for the purpose of whitening silver work of the best
quality; and although annealing is always a part of the process, other
ingredients, such as salt and tartar, permanganate of potash, cyanide
of potassium, alum, &c., have been severally used for the cleansing or
whitening mixture. They may be useful in their application to plated
work (articles that have received a coating of pure metal by means of
the electro-metallurgical process), for cleansing purposes only, but
for all practical purposes the process to which we have called special
attention is to be much preferred.
Common articles of silver cannot be whitened by annealing and
boiling-out in a diluted acid; a thin film of pure silver must be
deposited upon their surface by the process of electro-deposition,
or by the action of some chemical preparation in which fine silver
forms the principal ingredient. Such preparation, however, as the
latter can be used only to plain surfaces, therefore they are not
applicable to all kinds of work. They are composed of the following
chemical ingredients:--1st, chloride of silver 1 part, cream of tartar
1 part; 2nd, chloride of silver 1 part, common salt 1-1/2 parts; 3rd,
chloride of silver 1 part, prepared chalk 1 part, pearl-ash 1 part;
4th, chloride of silver 1 part, alum 1 part, common salt 2 parts. The
chloride of silver is easily prepared by precipitating it from the
nitrate with a solution of common salt or hydrochloric acid. The
various mixtures should be worked up with water into a thin paste,
and applied to the work by rubbing with a soft cork or piece of
wash-leather, or by thoroughly stirring it about in the mixture until
it has acquired the requisite degree of whiteness. For the purpose of
silvering watch and clock faces, &c. these mixtures may be used with
advantage and entire success.
Other solutions are sometimes employed for similar purposes and are
very useful; being simple in their preparation and easy of management.
We have selected the following as being the most practical:--Take one
ounce of the _nitrate_ of silver and dissolve it in one quart of pure
distilled water, or if this cannot be procured, water which has been
boiled, by which it loses some of its impurity. When the nitrate of
silver has become thoroughly dissolved, throw into the mixture a little
powdered hyposulphite of soda, this will precipitate the silver, and
when it has taken place, a further addition of hyposulphite of soda
should be made, which will eventually re-dissolve the precipitate, and
the solution is then ready for use. To produce a good mixture, the salt
of soda should be added slightly in excess. The solution is used by
simply dipping a sponge in it and rubbing it over the surface of the
articles to be coated, and this is continued until they have assumed
the desired colour.
For improving the colour of silver and electro-plated wares, the
following mixture has been strongly recommended:--Nitrate of silver
4 pennyweights, cyanide of potassium 5 ounces, and water 1 quart;
the ingredients should be well mixed together, and applied by means
of a soft brush or sponge to the surface of the work. In using this
cyanide solution, the operator should be careful to guard against a too
frequent contact with it, as it is decidedly injurious to the hands,
especially if there be any abrasion of the skin; it being one of the
deadliest poisons known. Sufficient details of the process of silver
whitening and cleansing having now been given to assist the workman
who manipulates in this particular metal, and to enable him to select
a form of recipe in every way adapted to the kind of work in hand, we
shall now proceed to the modern process of _electro-plating_, and give
a practical description of it in its applicability to the trade of the
silversmith.
This art is decidedly of modern origin, as far as concerns its
employment for commercial purposes. The invention is supposed to be
due to the electrical and chemical researches of Mr. Spencer, of this
country, and Professor Jacobi, of Russia, both of whom claim to have
found out the art of depositing one metal upon another, somewhere about
the same period. Of course it was left to others to apply the invention
to the industrial arts, and it was not until after the discovery of the
_Constant Battery_, by Professor Daniell, about half a century ago,
that the art began gradually to extend in the direction of commercial
pursuits. The Messrs. Elkington, of Birmingham, were the first to
employ it in their manufactures, with a success which their enterprise
thoroughly merited. This took place about the year 1840, and since that
time the art of electro-plating and gilding has wonderfully developed,
in its application to the various manufactures of the country. Its
progress would be a subject highly interesting, were we to trace the
general details of it, but the part of it we are considering being the
practical mode of its employment in manufactures, we shall at once
direct our attention to it, by giving a complete description of the
process; so that the ordinary silversmith may be enabled to employ it
in his business with safety and advantage.
The first thing to be considered in electro-plating is what _Battery_
to employ, which will be the most simple, inexpensive, and effective
one. When the battery is only occasionally required for use we prefer
the Smee before any other. It is a small portable apparatus, and
consists of a high, but narrow, glass or stoneware jar, in the form of
a cylinder, capable of holding about two quarts; inside this jar is
fitted a thin plate of platinized silver fitted to a frame with two
zinc plates, one on each side of it, the zinc plates being held to
the frame by means of a binding screw. Strong copper wires are firmly
secured to these screws, which serve as the positive and negative
poles of the battery. Those parts of the plates which are not exposed
to the action of the acid solution may be advantageously coated with
sealing-wax varnish or melted paraffin wax, to protect them from the
destructive influences of the battery acid, and to prevent it from
creeping upwards, which destroys the connections. The jar is filled
with water acidulated with sulphuric acid, in the proportion of 1 of
acid to 20 of water; the frame containing the plates is then lowered
into the solution, and the battery is ready for use. In the above form
of battery for occasional use we have one simple in construction,
easy of management, of fair constancy, and when once prepared very
inexpensive, merely requiring a little free acid at times to keep
up the strength of the current. Two cells of this form of battery,
each holding two quarts of mixture, will be found sufficient for all
ordinary purposes. The zincs should be well amalgamated, and not touch
the bottom of the cells. The connections should be regularly examined,
and kept perfectly free from corrosion, which would stop the passage of
the current. For plating small delicate articles of jewellery one cell
of the above form will be found powerful enough for the purpose.
The battery that we prefer and have of late years employed, for
regular continuous working, is the Bunsen, consisting as before of a
cylindrical glass or stoneware jar of the same size and dimensions,
fitted with a well amalgamated _cylinder_ of zinc and a copper wire
secured to it; a porous cell is placed in the centre, and a bar or rod
of carbon is put into this cell with a copper wire also secured to it.
The porous cell is filled with a mixture of equal parts of nitric and
sulphuric acids, or sulphuric acid alone; we prefer the latter, as it
does not give off such fumes as does the other acid; a little of the
more powerful acid, however, is sometimes required to be added in order
to increase the action, as with this acid alone it sometimes becomes
slow. The outer cell is filled with a mixture of 1 part of sulphuric
acid to 20 parts of water, and the connections being in proper order
the battery is then ready for use. In action this form of battery is
regular and continuous, it lasts a long time upon one charge, and is
therefore inexpensive in use; if the two cells are coupled for power or
intensity, an unusual quantity of work may be got through in a given
time. This cell is admirably suited to the work of the manufacturing
silversmith, and to those who prefer doing their own plating.
The amalgamation of the zinc is effected as follows:--The cylinders are
best treated by putting some mercury into a coarse flannel bag, dipped
repeatedly into muriatic acid and applied to the surface of the zinc,
both inside and out; and when they present the bright characteristic
appearance of mercury they are sufficiently operated upon, and may be
rinsed and set aside to drain. The zinc plates may be advantageously
amalgamated by placing some mercury in a shallow dish with a little
muriatic or sulphuric acid, a hare's foot or a piece of cloth tied to
the end of a stick is then dipped into the mercury and acid, and rubbed
over the plates until they are sufficiently protected with mercury,
when they should be rinsed in clean water and set aside to drain. If
possible the process of amalgamation should always be conducted in
the open air, as the fumes which are given off, if breathed, are
highly injurious. The best possible way to amalgamate rods of zinc is
by pouring mercury into the melted metal just before casting it into
rods, in the proportion of 1-1/2 oz. of mercury to the pound of zinc.
This makes the rods exceedingly brittle, and they should therefore be
handled with care. The mercury should not be added to the zinc when the
latter is at too high a temperature, and the best manner of testing
this is by the application of a piece of paper to the molten metal,
when if it takes fire, the temperature is still too high; it should be
allowed to cool until the paper refuses to ignite, then and not till
then is the proper time for the addition of the mercury.
The copper conducting wires and binding screws must be cleaned when
they become much corroded; if not they add resistance to the current,
and it will become considerably diminished, or cease altogether. The
cleaning may be effected by simply annealing and then plunging them
while still hot into dilute sulphuric acid pickle, or dipping them into
nitric acid for about an instant.
The zincs should be taken from the battery liquids when not required
for use; and the porous cells should be removed every night and their
contents poured into a large jug kept for the purpose. The porous
cells should be placed in clean water to prevent the salts of the
battery liquid from crystallizing in the pores, which would crack and
spoil them for future use; the carbons should also be placed in water;
and when required for use again these arrangements should be reversed,
when the battery will work as well as ever.
The solution for electro-plating articles of jewellery is the next part
of the subject we have to consider, of which there are many, containing
various proportions of the metal employed in silver depositing. The
following is one of the best we have employed: take pure silver and
dissolve it in a mixture of nitric acid and water:--
Fine silver 5 dwts.
Nitric acid 4 drms.
Water 2 drms.
The silver should be put into a small Florence flask, so as to allow
the mixture of acid and water to cover it thoroughly; this mixture
on being added to the metal soon promotes a chemical action, and the
silver becomes gradually dissolved. If the acid employed has been
weak, it will necessitate a further addition of it to complete the
dissolution of the silver, or the removal of the flask to some other
and warmer place, such as a sand-bath, but care should be taken not
to apply too much heat. As the chemical action proceeds, red fumes
are formed in the flask; and the action should be allowed to go on
until these cease to rise, when the silver should by that time have
become dissolved. The mixture then consists of a solution of nitrate
of silver, and should be carefully poured into a suitable porcelain
or Wedgwood capsule; this is then heated upon a sand-bath until a
scum or pellicle appears upon the surface, when it should at once be
transferred from the sand-bath to a suitable place for cooling. During
the last operation the mixture begins to form itself into crystals,
and the liquid which appears reluctant to crystallize should be poured
away from those already formed, into another capsule, and again heated
until it has sufficiently evaporated to crystallize. When the whole
of the liquor has finally undergone this process, the crystals of
nitrate of silver must be removed to another vessel, and about one
pint of cold water added, the whole being then well stirred until they
have become thoroughly dissolved. A solution of cyanide of potassium
is next prepared in water, in the proportion of about one ounce of
cyanide to the pint of water; some of this solution is then added to
the one containing the nitrate of silver. It must, however, be added
very cautiously, for precipitation soon takes place, and if too much
be used, the precipitate becomes again dissolved. For this reason it
is advisable to take a little of the solution from the vessel, in
a wine-glass or test tube, and to add a few drops of the prepared
solution of cyanide, in order to ascertain its exact state. If the
application of this solution produces no effect upon the nitrate of
silver, the operation of precipitation is complete.
The liquid above the sediment should next be carefully poured away to
avoid any waste of silver; when this is done fresh water should be
added, well stirred with the sediment, and allowed to settle; it is
then again poured off, and the process repeated until the precipitate
has become thoroughly washed. Now add sufficient cyanide of potassium
to dissolve the precipitate and a little more, and make up two quarts
of solution with fresh clean water. It is better to filter the solution
before using it.
The solution may be made by means of the battery, and, if preferred,
the above mode of chemically preparing it may be dispensed with. The
following is the most simple method by the battery process:--Dissolve
in two quarts of water about half an ounce, and no more, of best black
cyanide; this should be done in an oval, or still better, oblong
stoneware vessel, placed in an iron one of the same shape containing
water. The stone jar should not be allowed to touch the bottom or
sides of the iron one, a space being left for holding the water. When
the cyanide has become dissolved, fill a porous cell with some of the
solution, place this cell in the other vessel containing the cyanide;
the solution should be about the same height in both vessels. Now
put a piece of sheet copper, secured to the end of the wire issuing
from the zinc of the battery, into the porous cell, and place in the
larger vessel containing the cyanide solution about one ounce of sheet
silver, properly secured to the wire issuing from the carbon of the
battery. In a short time the solution in the larger vessel will have
acquired the right proportion of silver (5 dwts. to the two quarts) for
use; when this has been effected, the porous cell should be removed
and its contents thrown away. These solutions are both worked hot, at
a temperature of not exceeding 120° Fahr. with the battery we have
described.
The solutions are heated by means of gas-jets, and the articles are
plated by being suspended to the wire proceeding from the zinc of the
battery. To the wire proceeding from the carbon is to be attached the
piece of sheet silver which dissolves and keeps up the strength of
the solution. The piece of silver or anode being lowered into the
solution, upon the immersion of the work, an almost instantaneous
deposit of fine silver takes place, the thickness of which depends
entirely upon the period of immersion.
When a solution begins to plate of an inferior kind through the
acquisition of organic matter, it will be better to abandon it
altogether and make a new one, rather than to waste valuable time in
repeated attempts at improvement, which seldom can be effected in
solutions that have been employed for all kinds of work. The silver
may be recovered from such solutions by means of the battery, by
precipitation, and by evaporation; the first process, however, we
have not always found successful, the solution in some cases refusing
to give up its silver to the action of the battery. It is put into
operation as follows:--The anode which supplies the solution with
silver is replaced by one of platinum, on which the cyanide solution
has no action whatever; a piece of clean sheet copper should be hung
upon the zinc wire of the battery, and the battery kept in action until
the whole of the silver held in the solution has become deposited upon
it; at which stage it may be removed, and the exhausted solution thrown
into the waste water tub. The piece of copper containing the silver
may be used again in the place of the silver anode until it has become
dissolved, or it may be removed by any other means, if preferred. In
the event of the above plan failing, the process of precipitation or
evaporation should be resorted to. If the former one be adopted, the
solution should be poured into a large open vessel, and considerably
diluted with water; sulphuric acid should then be carefully poured in,
a little at a time, until it produces no effervescence. The sulphuric
acid precipitates the silver, and the fumes which it creates are highly
deleterious to health, therefore the process ought to be performed in
the open air, and not in ill-ventilated workshops. When the sulphuric
acid produces no effect upon the solution, it should be allowed to
stand for a while for the precipitate to subside, when the water
above (which should be clear) may be drawn off, the precipitate well
washed, to free it from the acid, dried, and fused in a crucible with
a little potash or soda. If the latter plan be adopted, the solution
may be placed in a cast-iron kettle or saucepan, and then heated upon
a gas-jet or stove, until evaporation takes place, after which the
sediment should be removed and fused in a crucible, as before.
[Illustration: Fig. 40. Scratch-brush Lathe.]
The finishing of silver work requires some little knowledge and
skill to perform it properly; and we think that a few observations
bearing upon it will be of service to those for whom this manual is
written. After either of the processes of whitening or plating, the
work has to be scratched, unless required to be left a _dead_ white,
then this process does not take place; the scratching removes from
the surface the dull white colour produced by the above processes,
and effects a characteristic bright and uniform colour to the work
of the silversmith. Scratching is done at the lathe (Fig. 40) by the
application of a very fine brass-wire brush of circular form running
upon the spindle; a solution of weak ale runs from a barrel with a
tap to it, placed upon the framework of the lathe so as to enable the
beer running from it to fall upon the brush during the whole time of
its rotary action, and this assists the brush the more easily to glide
over the surface of the work submitted to it. A large quantity of
silversmith's work receives no other treatment than this, after the
whitening processes have taken place. Silver chains are burnished by
means of a polished steel jack chain, and the application of a little
soft soap and hot water, or otherwise scratch-brushed. The beautiful
frosted surfaces to be seen upon silver lockets, and other work of a
similar nature, are all produced by means of the scratch-brush.
Burnishing is another mode of finishing silver work. It produces a
polished surface, which reflects like a mirror, and gives the greatest
lustre: it removes marks left by the polishing mixtures, and produces
a darker surface than the other modes of finishing. The tools employed
for this process are extremely variable, and well adapted to the
different kinds of work to which they are applied; they are of two
kinds, one being formed of hard stone, and the other of polished
hardened steel; they vary with regard to shape, some being straight
with rounded points, or with curved and blunted edges, others with
large rounded surfaces, &c. Stone burnishers are made of blood-stone,
which is mounted in a wooden handle with a brass ferrule, which firmly
secures the stone to it, in which state it is used. Steel burnishers
are likewise fixed in wooden handles, which enable them to be firmly
grasped by the operator. Throughout the whole process of burnishing,
the tool should be repeatedly moistened with a solution of soap and
water; which causes it to glide more easily over the surface of
the work, prevents it from becoming too much heated, and generally
facilitates its action. In consequence of the great friction which
the burnishing tool undergoes, it soon loses its bite, when it slips
over the work as if it were greasy; its effectiveness must therefore
be restored from time to time by rubbing it upon the leather which the
workman has beside him for the purpose. It generally consists of a
piece of buff leather, impregnated with a little crocus. In very small
articles only steel burnishers are used, as they are finer in make, and
by their greater variety of form, are exceedingly well adapted to all
kinds of work; in this class of work, if any soap-suds should adhere to
the article they may be removed by the application of a little tissue
paper. Large pieces of work are rubbed with a piece of old linen, or
washed in a warm solution of soap and water, rinsed, and dried in
boxwood sawdust, which finally completes the process.
Silver work may be oxidized by any of the following processes:--
I.
Sal-ammoniac 2 parts.
Sulphate of Copper 2 "
Saltpetre 1 part.
Reduce the above ingredients to a fine powder, and dissolve it in a
little acetic acid. If the article is to be entirely oxidized, it may
be dipped for a short time into the boiling mixture; if only in parts,
it may be applied with a camel-hair pencil, the article and the mixture
both being warmed before using.
II.
Platinum 1 part.
Hydrochloric acid 2 parts.
Nitric acid 1 part.
Dissolve the platinum in the mixture of acid, evaporate to
crystallization, and when cold, dissolve again in a little sulphuric
ether. Apply the mixture with a camel-hair pencil to the parts required
to be blackened.
III.
Saltpetre 2 parts.
Common Salt 1 part.
Spirits of Salts 1 "
Reduce the salts to powder, and place it in a black-lead crucible along
with the acid, boil up, and then dip the articles into the mixture
for a short time, or otherwise apply it to the parts required to be
oxidized.
These mixtures will give the various tints of oxidation to silver work
if properly treated; but if other tints be desired, the following
chemical substances may be employed according to taste:--For
slate-coloured surface, dip the articles into a boiling solution of
sulphuret of potassium. Strong hydrosulphate of ammonia produces a
dark tint of oxidation, and if diluted with much water a light tint is
produced. Nitric acid produces a light surface. The fumes of sulphur
produce a beautiful blue-coloured surface. This operation should be
conducted in a closed box, and all parts not to be blackened should be
coated with a suitable resist varnish. After any of these processes the
articles may either be scratched, or otherwise burnished.