highly diluted hydrochloric acid and the acid completely removed with
water. Then prepare an ammoniacal copper solution from 3 parts copper
sulphate, 3 parts spirits of sal ammoniac, and 50 parts water. If
possible the zinc articles are dipped into this solution or else the
surface is coated a few times quickly and uniformly with a flat, soft
brush, leaving to dry between the coats. When sufficient copper has
precipitated on the zinc, brush off the object superficially.
«Cobaltizing of Metals.»—Following are various processes for
cobaltizing on copper or other metals previously coppered: I.—Cobalt,
50 parts, by weight; sal ammoniac, 25 parts; liquid ammonia, 15 parts;
distilled water, 1,000 parts. Dissolve the cobalt and the sal ammoniac
in the distilled water, and add the liquid ammonia.
II.—Pure potash in alcohol, 50 parts, by weight; cobalt chloride, 10
parts; distilled water, 1,000 parts. Dissolve the cobalt in half the
distilled water and the potash in the other half and unite the two.
III.—Potassium sulphocyanide, 13 parts, by weight; cobalt chloride, 10
parts; pure potash in alcohol, 2 parts; distilled water, 1,000 parts.
Proceed as described above. All these baths are used hot and require a
strong current.
«Nickel Plating with the Battery.»—The nickel bath is prepared
according to the following formula:
I.—Nickel and ammonium sulphate 10 parts
Boracic acid 4 parts
Distilled water 175 parts
A sheet of nickel is used as an anode.
Perfect cleanliness of the surface to be coated is essential to
success. With nickel especially is this the case, as traces of oxide
will cause it to show dark streaks. Finger marks will in any case
render the deposit liable to peel off.
Cleansing is generally accomplished either by boiling in strong
solution of potassium hydrate, or, when possible, by heating to redness
in a blow-pipe flame to burn off any adhesive grease, and then soaking
in a pickle of dilute sulphuric acid to remove any oxide formed during
the heating. In either case it is necessary to subject the article
to a process of scratch brushing afterwards; that is, long-continued
friction with wire brushes under water, which not only removes any
still adhering oxide, but renders the surface bright.
To certain metals, as iron, nickel, and zinc, metallic deposits do not
readily adhere. This difficulty is overcome by first coating them with
copper in a bath composed as follows:
II.—Potassium cyanide 2 parts
Copper acetate, in crystals 2 parts
Sodium carbonate, in crystals 2 parts
Sodium bisulphite 2 parts
Water 100 parts
Moisten the copper acetate with a small quantity of water and add the
sodium carbonate dissolved in 20 parts of water. When reaction is
complete, all the copper acetate being converted into carbonate, add
the sodium bisulphite, dissolved in another 20 parts of water; lastly,
add the potassium cyanide, dissolved in the remainder of the water. The
finished product should be a colorless liquid.
If a dynamo is not available for the production of a current, a
Daniell’s battery is to be recommended, and the “tank” for a small
operation may be a glass jar. The jar is crossed by copper rods in
connection with the battery; the metal to be deposited is suspended
from the rod in connection with the positive pole, and is called the
anode. The articles to be coated are suspended by thin copper wires
from the rod in connection with the negative pole; these form the
cathode. The worker should bear in mind that it is very difficult to
apply a thick coating of nickel without its peeling.
«Replating with Battery.»—It is well known to electro-metallurgists
that metals deposited by electricity do not adhere so firmly to their
kind as to other metals. Thus gold will adhere more tenaciously {574}
to silver, copper, or brass, than it will to gold or to a gilt surface,
and silver will attach itself more closely to copper or brass than to
a silver-plated surface. Consequently, it is the practice to remove,
by stripping or polishing the silver from old plated articles before
electroplating them. If this were not done, the deposited coating would
in all probability “strip,” as it is termed, when the burnisher is
applied to it—that is, the newly deposited metal would peel off the
underlying silver. It must be understood that these remarks apply to
cases in which a good, heavy deposit of silver is required, for, of
course, the mere film would not present any remarkable peculiarity.
«Silver Plating.»—The term silver deposit designates a coating of
silver which is deposited upon glass, porcelain, china, or other
substances. This deposit may be made to take the form of any desired
design, and to the observer it has the appearance (in the case of
glass) of having been melted on.
Practically all of the plated articles are made by painting the design
upon the glass or other surface by means of a mixture of powdered
silver, a flux and a liquid to make the mixture in the form of a paint
so that it may be readily spread over the surface. This design is then
fired in a muffle until the flux melts and causes the silver to become
firmly attached to the glass. A thin silver deposit is thus produced,
which is a conductor of electricity, and upon which any thickness of
silver deposit may be produced by electroplating in the usual cyanide
silver-plating bath.
To be successful in securing a lasting deposit a suitable flux must be
used. This flux must melt at a lower temperature than the glass upon
which it is put, in order to prevent the softening of the articles by
the necessary heat and the accompanying distortion. Second, a suitable
muffle must be had for firing the glass articles upon which the design
has been painted. Not only must a muffle be used in which the heat can
be absolutely controlled, but one which allows the slow cooling of the
articles. If this is not done they are apt to crack while cooling.
The manufacture of the flux is the most critical part of the silver
deposit process. Without a good flux the operation will not be a
success. This flux is frequently called an enamel or frit. After a
series of experiments it was found that the most suitable flux is
a borate of lead. This is easily prepared, fuses before the glass
softens, and adheres tenaciously to the glass surface.
To make it, proceed as follows: Dissolve 1/4 pound of acetate of
lead (sugar of lead) in 1 quart of water and heat to boiling. Dissolve
1/4 pound of borax in 1 quart of hot water and add to the sugar of
lead solution. Borate of lead follows as a white precipitate. This is
filtered out and washed until free from impurities. It is then dried.
The precipitated borate of lead is then melted in a porcelain or clay
crucible. When in the melted condition it should be poured into a
basin of cold water. This serves to granulate and render it easily
pulverized. After it has been poured into water it is removed and
dried. Before using in the paint it is necessary that this fused borate
of lead be ground in a mortar as fine as possible. Unless this is done
the deposit will not be smooth.
The silver to be used should be finely powdered silver, which can be
purchased in the same manner as bronze powders.
The mixture used for painting the design upon the glass is composed of
2 parts of the powdered silver, and 1 part of the fused borate of lead.
Place the parts in a mortar and add just enough oil of lavender to make
the mass of a paint-like consistency. The whole is then ground with the
pestle until it is as fine as possible. The amount of oil of lavender
which is used must not be too great, as it will then be found that a
thick layer cannot be obtained upon the glass.
The glass to be treated must be cleaned by scouring with wet pumice
stone and washing soda. The glass should be rinsed and dried. The
design is then painted on the glass with a brush, painting as thick as
possible and yet leaving a smooth, even surface. The glass should be
allowed to dry for 24 hours, when it is ready for firing.
When placed in the gas muffle, the glass should be subjected to a
temperature of a very low red heat. The borate of lead will melt at
this temperature, and after holding this heat a short time to enable
the borate of lead to melt and attach itself, the muffle is allowed to
cool.
After cooling, the articles are removed and scratch brushed and placed
in a silver bath for an electro deposit of silver of a thickness
desired.
Before the plating the glass article is dipped into a cyanide dip,
or, if found necessary, scoured lightly with pumice {575} stone and
cyanide, and then given a dip in the customary blue dip or mercury
solution, so as to quickly cover all parts of the surface. It next
passes to the regular cyanide silver solution, and is allowed to remain
until the desired deposit is obtained.
A little potassium cyanide and some mono-basic potassium citrate in
powder form is added from time to time to the bath generally used,
which is prepared by dissolving freshly precipitated silver cyanide in
a potassium cyanide solution. After this the glass is rinsed and dried,
and may be finished by buffing.
«Steel Plating.»—The following is a solution for dipping steel articles
before electroplating: Nitrate of silver, 1 part; nitrate of mercury, 1
part; nitric acid (specific gravity, 1.384), 4 parts; water, 120 parts.
The article, free from grease, is dipped in the pickle for a second or
two.
The following electroplating bath is used: Pure crystallized ferrous
sulphate, 40 parts, by weight, and ammonium chloride, 100 parts, by
weight, in 1,000 parts, by weight, of water. It is of advantage to add
to this 100 parts, by weight, of ammonium citrate, in order to prevent
the precipitation of basic iron salts, especially at the anode.
«Tin Plating by Electric Bath.»—Most solutions give a dead-white film
of tin, and this has to be brightened by friction of some sort, either
by scratch brushing, burnishing, polishing, or rubbing with whiting.
The bright tin plates are made bright by rolling with polished steel
rollers. Small articles may be bright-tinned by immersion in melted
tin, after their surfaces have been made chemically clean and bright,
all of which processes entail much time and labor. Benzoic acid,
boric acid, or gelatin may be tried with a well-regulated current
and the solution in good working order, but all will depend upon the
exact working of the solution, the same conditions being set up as
are present in the deposition of other metals. These substances may
be separately tried, in the proportion of 1 ounce to each gallon of
the tin solution, by boiling the latter and adding either one during
the boiling, as they dissolve much easier with the tin salts than in
water separately. Tin articles are usually brightened and polished
with Vienna lime or whiting, the first being used with linen rags and
the latter with chamois leather. Tin baths must be used hot, not below
75° F., with a suitable current according to their composition. Too
strong a current produces a bad color, and the deposit does not adhere
well. A current of from 2 to 6 volts will be sufficient. Small tinned
articles are brightened by being shaken in a leather bag containing a
quantity of bran or by revolving in a barrel with the same substance;
but large objects have to be brightened by other means, such as scratch
brushing and mopping to give an acceptable finish to the deposited
metal.
«GILDING AND GOLD PLATING:»
Genuine gilding readily takes up mercury, while imitation gilding
does not or only very slowly. Any coating of varnish present should,
however, be removed before conducting the test. Mercurous nitrate has
no action on genuine gold, but on spurious gilding a white spot will
form which quickly turns dark. A solution of neutral copper chloride
does not act upon genuine gold, but on alloys containing copper a black
spot will result. Gold fringe, etc., retains its luster in spirit of
wine, if the gilding is genuine; if not, the gilding will burn and
oxidize. Imitation gilding might be termed “snuff gilding,” as in
Germany it consists of dissolved brass, snuff, saltpeter, hydrochloric
acid, etc., and is used for tin toys. An expert will immediately see
the difference, as genuine gilding has a different, more compact pore
formation and a better color. There are also some gold varnishes which
are just as good.
The effect of motion while an article is receiving the deposit is most
clearly seen during the operation of gilding. If a watch dial, for
instance, be placed in the gilding bath and allowed to remain for a few
moments undisturbed and the solution of gold has been much worked, it
is probable that the dial will acquire a dark fox-red color; but if it
be quickly moved about, it instantly changes color and will sometimes
even assume a pale straw color. In fact, the color of a deposit may be
regulated greatly by motion of the article in the bath—a fact which the
operator should study with much attention, when gilding.
The inside of a vessel is gilded by filling the vessel with the
gilding solution, suspending a gold anode in the liquid, and passing
the current. The lips of cream jugs and the upper parts of vessels of
irregular outline are gilded by passing the current from a gold anode
through a rag wetted with the gilding solution and laid upon the part.
Sometimes, when gilding the insides of mugs, tankards, etc., which are
richly {576} chased or embossed, it will be found that the hollow parts
do not receive the deposit at all, or very partially. When this is the
case, the article must be rinsed and well scratch brushed, and a little
more cyanide added to the solution. The anode must be slightly kept in
motion and the battery power increased until the hollow surfaces are
coated. Frequent scratch brushing aids the deposit to a great extent by
imparting a slight film of brass to the surface.
In gilding chains, brooches, pins, rings, and other articles which have
been repaired, i. e., hard soldered, sometimes, it is found that the
gold will not deposit freely upon the soldered parts; when such is the
case, a little extra scratch brushing applied to the part will assist
the operation greatly and it has sometimes been found that dry scratch
brushing for an instant—that is, without the stream of beer usually
employed—renders the surface a better and more uniform conductor and
consequently it will more readily receive the deposit. In fact, dry
scratch brushing is very useful in many cases in which it is desirable
to impart an artificial coating of brass upon an article to which
silver or gold will not readily adhere. In scratch brushing without the
employment of beer or some other liquid, however, great care must be
taken not to continue the operation too long, as the minute particles
of metal given off by the scratch brush would be likely to prove
prejudicial to the health of the operator, were he to inhale them to
any great extent.
The following solutions are for gilding without a battery: I.—In 1,000
parts of distilled water dissolve in the following order:
Crystalline sodium pyrophosphate 80 parts
Twelve per cent solution of hydrocyanic acid 8 parts
Crystalline gold chloride 2 parts
Heat to a boiling temperature, and dip the article, previously
thoroughly cleaned, therein.
II.—Dissolve in boiling distilled water, 1 part of chloride of gold and
4 parts of cyanide of potassium. Plunge the objects into this solution,
while still hot, and leave them therein for several hours, keeping them
attached to a copper wire or a very clean strip of zinc. They will
become covered with a handsome gold coating.
«Aluminum Gilding.»—I.—Dissolve 6 parts of gold in aqua regia and
dilute the solution with distilled water; on the other hand, put 30
parts of lime in 150 parts of distilled water; at the end of 2 hours
add the gold solution to the lime, shake all and allow to settle for
5 to 6 hours, decant and wash the precipitate, which is lime aurate.
Place this aurate of lime in 1,000 parts of distilled water, with 20
parts of hyposulphite of soda; put all on the fire for 8 to 10 minutes,
without allowing to boil; remove and filter. The filtered liquor serves
for gilding in the cold, by plunging into this bath the aluminum
articles previously pickled by passing through caustic potash and
nitric acid. This gilding is obtained without the aid of the battery.
II.—The gold bath is prepared with gold dissolved in the usual way, and
the addition of salts, as follows: Gold, 20 parts, by weight; sulphate
of soda, 20 parts; phosphate of soda, 660 parts; cyanuret of potassium,
40 parts; water, 1,000 parts. The bath ought to be of the temperature
of 68° to 77° F.
«Amalgam Gold Plating.»—Gold amalgam is chiefly used as a plating for
silver, copper, or brass. The article to be plated is washed over with
diluted nitric acid or potash lye and prepared chalk, to remove any
tarnish or rust that might prevent the amalgam from adhering. After
having been polished perfectly bright, the amalgam is applied as evenly
as possible, usually with a fine scratch brush. It is then set upon
a grate over a charcoal fire, or placed into an oven and heated to
that degree at which mercury exhales. The gold, when the mercury has
evaporated, presents a dull yellow color. Cover it with a coating of
pulverized niter and alum in equal parts, mixed to a paste with water,
and heat again till it is melted, then plunge into water. Burnish up
with a steel or bloodstone burnisher.
«Brass Gilding.»—On brass, which is an electropositive metal, an
electromagnetic metal, such as gold, can be deposited very cheaply from
the dilute solutions of its salts. The deposit is naturally very thin,
but still quite adhesive. In preparing it, the proportions stated below
have to be accurately observed, otherwise no uniform, coherent coating
will result, but one that is uneven and spotted.
I.—In 750 parts, by weight, of water dissolve: Phosphate of soda, 5
parts, and caustic potash, 3 parts, and in 250 parts of water, gold
chloride, 1 part, and potassium cyanide, 16 parts. Mix both {577}
solutions well and cause the mixture to boil, whereupon the brass
articles to be gilded are immersed. The gold in the mixture can be
utilized almost entirely. When the solution does not gild well any more
a little potassium cyanide is added, and it is used for pre-gilding
the articles, which can then be gilded again in a fresh solution. This
solution is very weak. A stronger one can be prepared mechanically by
dissolving 2 to 3 parts of gold chloride in very little water to which
1 part of saltpeter is added. Into this solution dip linen rags, let
them dry in a dark place, and cause them to char into tinder, which is
rubbed up in a porcelain dish. Into the powder so made, dip a soft,
slightly charred cork, moistened with a little vinegar, or else use
only the finger, and rub the gold powder upon the brass articles.
II.—To Give Brass a Golden Color, it is dipped until the desired shade
is obtained into a solution of about 175° F., produced as follows:
Boil 4 parts of caustic soda, 4 parts of milk sugar, and 100 parts of
water for 15 minutes; next add 4 parts of blue vitriol, dissolved in as
little water as possible.
«Copper and Brass Gilding.»—The solutions used to gild copper can
be generally used also for brass articles. Copper gilding acquires
importance because in order to gild iron, steel, tin, and zinc, they
must first be coated with copper, if the boiling method is to be
employed. Following is Langbein’s bath for copper and brass:
Dissolve 1 part, by weight, of chloride of gold and 16 parts, by
weight, of potassium cyanide in 250 parts, by weight, of water;
dissolve also and separately, 5 parts, by weight, of sodium phosphate
and 3 parts, by weight, of caustic potash in 750 parts, by weight, of
cold water. Mix these solutions and bring them to a boil. If the action
subsides, add from 3 to 5 parts, by weight, more potassium cyanide.
The polished iron and steel objects must first be copper-plated by
dipping them into a solution of 5 parts, by weight, of blue vitriol and
2 parts, by weight, of sulphuric acid in 1,000 parts, by weight, of
water. They may now be dipped into a hot solution containing 6 parts,
by weight, of gold chloride and 22 1/2 parts, by weight, of soda
crystals in 75 parts, by weight, of water. This coating of gold may be
polished.
«Cold Chemical Gilding.»—The chemical gilding by the wet process
is accomplished by E. E. Stahl with the aid of three baths: A gold
bath, a neutralization bath, and a reduction bath. The gold bath is
prepared from pure hydrochloric acid, 200 parts; nitric acid, 100
parts; and pure gold. The gold solution evaporated to crystallization
is made to contain 1 1/2 per cent of gold by diluting with water.
The neutralization bath consists of soda lye of 6°, of pure sodium
hydroxide, and distilled water. The reduction bath contains a mixture
of equal parts of 90 per cent alcohol and distilled water, wherein
pure hydrogen has been dissolved. The gilding proper is conducted by
first entering the article in the gold bath, next briskly moving it
about in the neutralization bath, and finally adding the reducing bath
with further strong agitation of the liquid. The residues from the
gilding are melted with 3 parts each of potash, powdered borax, and
potash niter, thus recovering the superfluous gold. The gilding or
silvering respectively produces a deposit of gold or silver of very
slight thickness and of the luster of polishing gold. Besides the
metal solution an “anti-reducer” is needed, consisting of 50 grams of
rectified and rosinified turpentine oil and 10 grams of powdered roll
sulphur. From this is obtained, by boiling, a syrupy balsam, to which
is added, before use, lavender oil, well-ground basic bismuth nitrate,
and the solution for gilding or silvering. The last takes place by a
hydrochloric solution of aluminum with the above balsam.
«Colored Gilding.»—A variety of shades of green and red gold can be
obtained by the electro-chemical process, which method may be employed
for the decoration of various objects of art. In order to produce
red gold in the different shades, a plate of pure copper is hung
into a rather concentrated gold bath (5 to 6 parts, by weight, per
1,000 parts of liquid), which is connected with the battery in such a
manner that gold is deposited on the article immersed in the bath. By
the action of the electric current copper is dissolved as well from
the copper plate and is separated simultaneously with the gold, so
that, after a certain time, a deposit containing a gold copper alloy,
conforming in color to the quantities of gold and copper contained
in it, is obtained by the electric process. When the desired shade
of color of the deposit is reached the copper plate is taken out and
replaced by another consisting of the copper gold alloy, likewise
produced by electrodeposition, and the articles are now gilt in this
liquid. In some large manufactories of gold articles this last coloring
is used even for pure {578} gold articles, to give them a popular
color. To produce green gold (alloy of gold and silver), a silver plate
is first employed, which is dipped into the gold bath and from which
enough silver is dissolved until the separating alloy shows the desired
shade. The silver plate is then exchanged for a gold-silver plate of
the respective color, and the articles are gilt with green gold.
«Gilding German Silver.»—In gilding German silver the solution may be
worked at a low temperature, the solution being weakened and a small
surface of anode exposed. German silver has the power of reducing gold
from its solution in cyanide (especially if the solution be strong)
without the aid of the battery; therefore, the solution should be
weaker, in fact, so weak that the German silver will not deposit the
gold _per se_; otherwise the deposit will take place so rapidly that
the gold will peel off when being burnished or even scratch brushed.
«Gilding of Glass.»—I.—In order to produce a good gilding on glass, the
gold salt employed must be free from acid. Prepare three solutions,
viz.:
_a._ 20 parts acid-free gold chloride in 150 parts of distilled water.
_b._ 5 parts dry sodium hydrate in 80 parts of distilled water.