proceeding to the bronzing. The process used is always the same; the
different shades are, however, too numerous to cover all of them in
one explanation. The bronzing of zinc clocks is most frequently done
on a brown ground, by mixing graphite, lampblack, and sanguine stirred
in water in which a little Flanders Dutch glue is dissolved. The
application is made by means of a brush. When it is dry a {568} spirit
varnish is applied; next, before the varnish is perfectly dry, a little
powdered bronze or sanguine or powdered bronze mixed with sanguine or
with graphite, according to the desired shades. For green bronze, mix
green sanders with chrome yellow stirred with spirit in which a little
varnish is put. When the bronzing is dry, put on the varnish and the
powdered bronze as above described. After all has dried, pass the brush
over a piece of wax, then over the bronzed article, being careful to
charge the brush frequently with wax.
«COLORING OF METALS:»
«Direct Coloration of Iron and Steel by Cupric Selenite.»—Iron
precipitates copper and selenium from their salts. Immersed in a
solution of cupric selenite, acidulated with a few drops of nitric
acid, it precipitates these two metals on its surface in the form of a
dull black deposit, but slightly adherent. But, if the object is washed
with water, then with alcohol, and rapidly dried over a gas burner, the
deposit becomes adherent. If rubbed with a cloth, this deposit turns a
blue black or a brilliant black, according to the composition of the
bath.
The selenite of copper is a greenish salt insoluble in water, and but
slightly soluble in water acidulated with nitric or sulphuric acid. It
is preferable to mix a solution of cupric sulphate with a solution of
selenious acid, and to acidulate with nitric acid, in order to prevent
the precipitation of the selenite of copper.
This process, originated by Paul Malherbe, is quite convenient for
blackening or bluing small objects of iron or steel, such as metallic
pens or other small pieces. It does not succeed so well for objects of
cast iron; and the selenious acid is costly, which is an obstacle to
its employment on large metallic surfaces.
The baths are quickly impoverished, for insoluble yellow selenite of
iron is deposited.
Brilliant Black Coloration.—Selenious acid, 6 parts; cupric sulphate,
10 parts; water, 1,000 parts; nitric acid, 4 to 6 parts.
Blue-Black Coloration.—Selenious acid, 10 parts; cupric sulphate, 10
parts; water, 1,000 parts; nitric acid, 4 to 6 parts.
By immersing the object for a short time the surface of the metal can
be colored in succession yellow, rose, purple, violet and blue.
«Coloration of Copper and Brass with Cupric Selenite.»—When an object
of copper or brass is immersed in a solution of selenite of copper
acidulated with nitric acid, the following colors are obtained,
according to the time of the immersion: Yellow, orange, rose, purple,
violet, and blue, which is the last color which can be obtained. In
general, the solution should be slightly acid; otherwise the color is
fugacious and punctate.
_a._ _b._
Selenious acid 6.5 2.9 parts
Sulphate of copper 12.5 20.0 parts
Nitric acid 2.0 2.5 parts
Water 1,000.0 1,000.0 parts
«Production of Rainbow Colors on Metals» (iron, copper, brass, zinc,
etc.)—I.—The following process of irisation is due to Puscher. It
allows of covering the metals with a thick layer of metallic sulphide,
similar to that met with in nature—in galena, for example.
These compounds are quite solid and are not attacked by concentrated
acids and alkalies, while dilute reagents are without action. In 5
minutes thousands of objects of brass can be colored with the brightest
hues. If they have been previously cleaned chemically, the colors
deposited on the surface adhere with such strength that they can be
worked with the burnisher.
Forty-five parts of sodium hyposulphite are dissolved in 500 parts of
water; a solution of 15 parts of neutral acetate of lead in 500 parts
of water is poured in. The clear mixture, which is composed of a double
salt of hyposulphite of lead and of sodium, possesses, when heated to
212° F., the property of decomposing slowly and of depositing brown
flakes of lead sulphide. If an article of gold, silver, copper, brass,
tombac, iron, or zinc is put into this bath while the precipitation
is taking place, the object will be covered with a film of lead
sulphide, which will give varied and brilliant colors, according to its
thickness. For a uniform coloration, it is necessary that the pieces
should be heated quite uniformly. However, iron assumes under this
treatment only a blue color, and zinc a bronze color. On articles of
copper the first gold color which appears is defective. Lead and tin
are not colored.
By substituting for the neutral acetate of lead an equal quantity of
cupric sulphate and proceeding in a similar way, brass or imitation
gold is covered with a very beautiful red, succeeded by an imperfect
green, and finally a magnificent brown, with iridescent points of
greenish red. The latter coating is fairly permanent.
Zinc is not colored in this solution, and {569} precipitates in it a
quantity of flakes of greenish brown (cupric sulphide), but if about
one-third of the preceding solution of lead acetate is added, a solid
black color is developed, which, when covered with a light coating of
wax, gains much in intensity and solidity. It is also useful to apply a
slight coating of wax to the other colors.
II.—Beautiful designs may be obtained, imitating marble, with sheets
of copper plunged into a solution of lead, thickened by the addition
of gum tragacanth, and heated to 212° F. Afterwards they are treated
with the ordinary lead solution. The compounds of antimony, for example
the tartrate of antimony and potash, afford similar colorations, but
require a longer time for their development. The solutions mentioned do
not change, even after a long period, and may be employed several times.
III.—By mixing a solution of cupric sulphate with a solution of sodium
hyposulphite, a double hyposulphite of sodium and of copper is obtained.
If in the solution of this double salt an article of nickel or of
copper, cleaned with nitric acid, then with soda, is immersed, the
following colors will appear in a few seconds: Brilliant red, green,
rose, blue, and violet. To isolate a color, it is sufficient to take
out the object and wash it with water. The colors obtained on nickel
present a moiré appearance, similar to that of silk fabrics.
IV.—Tin sulphate affords with sodium hyposulphite a double salt, which
is reduced by heat, with production of tin sulphide. The action of this
double salt on metallic surfaces is the same as that of the double
salts of copper and lead. Mixed with a solution of cupric sulphate,
all the colors of the spectrum will be readily obtained.
V.—Coloration of Silver.—The objects of copper or brass are first
covered with a layer of silver, when they are dipped in the following
solution at the temperature of 205° to 212° F.: Water, 3,000 parts;
sodium hyposulphite, 300 parts; lead acetate, 100 parts.
VI.—Iron precipitates bismuth from its chlorhydric solution. On heating
this deposit, the colors of the rainbow are obtained.
«Coloration by Electrolysis.»—I.—Colored Rings by Electrolysis (Nobili,
Becquerel).—In order to obtain the Nobili rings it is necessary to
concentrate the current coming from one of the poles of the battery
through a platinum wire, whose point alone is immersed in the liquid
to be decomposed, while the other pole is connected with a plate of
metal in the same liquid. This plate is placed perpendicularly to the
direction of the wire, and at about 0.04 inches from the point.
Solutions of sulphate of copper, sulphate of zinc, sulphate of
manganese, acetate of lead, acetate of copper, acetate of potassium,
tartrate of antimony and potash, phosphoric acid, oxalic acid,
carbonate of soda, chloride of manganese, and manganous acetate, may be
employed.
II.—A process, due to M. O. Mathey, allows of coloring metals by
precipitating on their surface a transparent metallic peroxide. The
phenomenon of electro-chemical coloration on metals is the same as that
which takes place when an object of polished steel is exposed to heat.
It first assumes a yellow color, from a very thin coating of ferric
oxide formed on its surface. By continuing the heating, this coating of
oxide increases in thickness, and appears red, then violet, then blue.
Here, the coloration is due to the increase in the thickness of a thin
coating of a metallic oxide precipitated by an alkaline solution.
The oxides of lead, tin, zinc, chromium, aluminum, molybdenum,
tungsten, etc., dissolved in potash, may be employed; also protoxide of
iron, zinc, cadmium, cobalt, dissolved in ammonia.
Lead Solution.—Potash, 400 parts; litharge or massicot, 125 parts. Boil
10 minutes, filter, dilute until the solution marks 25° Bé.
Iron Solution.—Dissolve ferrous sulphate in boiling water, and preserve
sheltered from air. When desired for use, pour a quantity into a
vessel and add ammonia until the precipitate is redissolved. This
solution, oxidizing rapidly in the air, cannot be used for more than an
hour.
III.—Electro-chemical coloration succeeds very well on metals which are
not oxidizable, such as gold and platinum, but not well on silver. This
process is employed for coloring watch hands and screws. The object is
placed at the positive pole, under a thickness of 1 1/4 inches of the
liquid, and the negative electrode is brought to the surface of the
bath. In a few seconds all the colors possible are obtained. Generally,
a ruby-red tint is sought for.
IV.—Coloration of Nickel.—The nickel piece is placed at the positive
pole in a solution of lead acetate. A netting {570} of copper wires is
arranged at the negative pole according to the contours of the design,
and at a short distance from the object. The coloration obtained is
uniform if the distance of the copper wires from the object is equal at
all points.
«Coloring of Brass.»—I.—(_a_) Brown bronze: Acid solution of nitrate of
silver and bismuth or nitric acid. (_b_) Light bronze: Acid solution
of nitrate of silver and of copper, (_c_) Black: Solution of nitrate
of copper. In all cases, however, the brass is colored black, if
after having been treated with the acid solution, it is placed for
a very short time in a solution of potassium sulphide, of ammonium
sulphydrate, or of hydrogen sulphide.
II.—The brass is immersed in a dilute solution of mercurous
nitrate; the layer of mercury formed on the brass is converted into
black sulphide, if washed several times in potassium sulphide. By
substituting for the potassium sulphide the sulphide of antimony or
that of arsenic, beautiful bronze colors are obtained, varying from
light brown to dark brown.
III.—Clean the brass perfectly. Afterwards rub with sal ammoniac
dissolved in vinegar. Strong vinegar, 1,000 parts; sal ammoniac, 30
parts; alum, 15 parts; arsenious anhydride, 8 parts.
IV.—A solution of chloride of platinum is employed, which leaves a very
light coating of platinum on the metal, and the surface is bronzed. A
steel tint or gray color is obtained, of which the shade depends on the
metal. If this is burnished, it takes a blue or steel gray shade, which
varies with the duration of the chemical action, the concentration,
and the temperature of the bath. A dilute solution of platinum is
prepared thus: Chloride of platinum, 1 part; water, 5,000 parts.
Another solution, more concentrated at the temperature of 104° F., is
kept ready. The objects to be bronzed are attached to a copper wire
and immersed for a few seconds in a hot solution of tartar, 30 parts
to 5,000 parts of water. On coming from this bath they are washed 2 or
3 times with ordinary water, and a last time with distilled water, and
then put in the solution of platinum chloride, stirring them from time
to time. When a suitable change of color has been secured, the objects
are passed to the concentrated solution of platinum chloride (40°).
They are stirred, and taken out when the wished-for color has been
reached. They are then washed 2 or 3 times, and dried in wood sawdust.
V.—To give to brass a dull black color, as that used for optical
instruments, the metal is cleaned carefully at first, and covered with
a very dilute mixture of neutral nitrate of tin, 1 part; chloride of
gold, 2 parts. At the end of 10 minutes this covering is removed with a
moist brush. If an excess of acid has not been employed, the surface of
the metal will be found to be of a fine dull black.
The nitrate of tin is prepared by decomposing the chloride of this
metal with ammonia and afterwards dissolving in nitric acid the oxide
of tin formed.
VI.—For obtaining a deposit of bismuth the brass is immersed in a
boiling bath, prepared by adding 50 to 60 parts of bismuth to nitric
acid diluted with 1,000 parts of water, and containing 32 parts of
tartaric acid.
VII.—The electrolysis of a cold solution of 25 to 30 parts per 1,000
parts of the double chloride of bismuth and ammonium produces on brass
or on copper a brilliant adherent deposit of bismuth, whose appearance
resembles that of old silver.
«Production of Rainbow Hues.»—Various colors.—I.—Dissolve tartrate of
antimony and of potash, 30 parts; tartaric acid, 30 parts; water, 1,000
parts. Add hydrochloric acid, 90 to 120 parts; pulverized antimony,
90 to 120 parts. Immerse the object of brass in this boiling liquid,
and it will be covered with a film, which, as it thickens, reflects
quite a series of beautiful tints, first appearing iridescent, then the
color of gold, copper, or violet, and finally of a grayish blue. These
colors are adherent, and do not change in the air.
II.—The sulphide of tin may be deposited on metallic surfaces,
especially on brass, communicating shades varying with the thickness
of the deposit. For this purpose, Puscher prepares the following
solutions: Dissolve tartaric acid, 20 parts, in water, 1,000 parts; add
a salt of tin, 20 parts; water, 125 parts. Boil the mixture, allow it
to repose, and filter. Afterwards pour the clear portion a little at
a time, shaking continually, into a solution of hyposulphite of soda,
80 parts; water, 250 parts. On boiling, sulphide of tin is formed,
with precipitation of sulphur. On plunging the pieces of brass in the
liquid, they are covered, according to the period of immersion, with
varied shades, passing from gold yellow to red, to crimson, to blue,
and finally to light brown.
III.—The metal is treated with the {571} following composition:
Solution A.—Cotton, well washed, 50 parts; salicylic acid, 2 parts,
dissolved in sulphuric acid, 1,000 parts, and bichromate of potash,
100 parts. Solution B.—Brass, 20 parts; nitric acid, density 1.51, 350
parts; nitrate of soda, 10 parts. Mix the two solutions, and dilute
with 1,500 parts of water. These proportions may be modified according
to the nature of the brass to be treated. This preparation is spread on
the metal, which immediately changes color. When the desired tint is
obtained, the piece is quickly plunged in an alkaline solution; a soda
salt, 50 parts; water, 1,000 parts. The article is afterwards washed,
and dried with a piece of cloth. Beautiful red tints are obtained by
placing the objects between 2 plates, or better yet, 2 pieces of iron
wire-cloth.
IV.—Put in a flask 100 parts of cupric carbonate and 750 parts of
ammonia and shake. This liquid should be kept in well-stoppered
bottles. When it has lost its strength, this may be renewed by pouring
in a little ammonia. The objects to be colored should be well cleaned.
They are suspended in the liquid and moved back and forth. After a few
minutes of immersion, they are washed with water and dried in wood
sawdust. Generally, a deep-blue color is obtained.
V.—Plunge a sheet of perfectly clean brass in a dilute solution of
neutral acetate of copper, and at the ordinary temperature, and in a
short time it will be found covered with a fine gold yellow.
VI.—Immerse the brass several times in a very dilute solution of cupric
chloride, and the color will be deadened and bronzed a greenish gray.
A plate of brass heated to 302° F. is colored violet by rubbing its
surface gently with cotton soaked with cupric chloride.
VII.—On heating brass, perfectly polished, until it can be no longer
held in the hand, and then covering it rapidly and uniformly with a
solution of antimony chloride by means of a wad of cotton, a fine
violet tint is communicated.
VIII.—For greenish shades, a bath may be made use of, composed of
water, 100 parts; cupric sulphate, 8 parts; sal ammoniac, 2 parts.
IX.—For orange-brown and cinnamon-brown shades: Water, 1,000 parts;
potassium chlorate, 10 parts; cupric sulphate, 10 parts.
X.—For obtaining rose-colored hues, then violet, then blue: Water, 400
parts; cupric sulphate, 30 parts; sodium hyposulphite, 20 parts; cream
of tartar, 10 parts.
XI.—For yellow, orange, or rose-colored shades, then blue, immerse the
objects for a longer or shorter time in the following bath: Water, 400
parts, ammoniacal ferrous sulphate, 20 parts; sodium hyposulphite,
40 parts; cupric sulphite, 30 parts; cream of tartar, 10 parts. By
prolonging the boiling, the blue tint gives place to yellow, and
finally to a fine gray.
XII.—A yellowish brown may be obtained with water, 50 parts; potassium
chlorate, 5 parts; nickel carbonate, 2 parts; sal nickel, 5 parts.
XIII.—A dark brown is obtained with water, 50 parts; sal nickel, 10
parts; potassium chlorate, 5 parts.
XIV.—A yellowish brown is obtained with water, 350 parts; a
crystallized sodium salt, 10 parts; orpiment, 5 parts.
XV.—Metallic moire is obtained by mixing two liquids: (_a_) Cream of
tartar, 5 parts; cupric sulphate, 5 parts; water, 250 parts. (_b_)
Water, 125 parts; sodium hyposulphite, 15 parts.
XVI.—A beautiful color is formed with one of the following baths: (_a_)
Water, 140 parts; ammonia, 5 parts; potassium sulphide, 1 part. (_b_)
Water, 100 parts; ammonium sulphydrate, 2 parts.
«Bronzing of Brass.»—The object is boiled with zinc grains and water
saturated with ammoniacal chlorhydrate. A little zinc chloride may be
added to facilitate the operation, which is completed as above.
It may also be terminated by plunging the object in the following
solution: Water, 2,000 parts; vinegar, 100 parts; sal ammoniac, 475
parts; pulverized verdigris, 500 parts.
«ELECTRODEPOSITION PROCESSES.»
The electrodeposition process is that used in electroplating and
electrotyping. It consists in preparing a bath in which a metal salt
is in solution, the articles to be plated being suspended so that they
hang in the solution, but are insulated. The bath being provided with
an anode and cathode for the passing of an electric current, and the
article being connected with the cathode or negative pole, the salts
are deposited on its surface (on the unprotected parts of its surface),
and thus receive a coating or plating of the metal in solution. {572}
When a soft metal is deposited upon a hard metal or the latter upon a
metal softer than itself, the exterior metal should be polished and
not burnished, and for this reason: If silver is deposited upon lead,
for instance, the great pressure which is required in burnishing to
produce the necessary polish would cause the softer metal to expand,
and consequently a separation of the two metals would result. On the
other hand, silver being softer than steel, if the burnisher is applied
to silver-coated steel the exterior metal will expand and separate from
the subjacent metal.
Many articles which are to receive deposits require to have portions
of their surfaces topped off, to prevent the deposit spreading over
those parts; for instance, in taking a copy of one side of a bronze
medallion, the opposite side must be coated with some kind of varnish,
wax, or fat, to prevent deposition; or, in gilding the inside of a
cream jug which has been silvered on the outside, varnish must be
applied all around the outer side of the edge, for the same reason.
For gilding and other hot solutions, copal varnish is generally used;
but for cold liquids and common work, an ordinary varnish, such as
engravers use for similar purposes, will do very well. In the absence
of other substances, a solution of sealing wax, dissolved in naphtha,
may be employed.
«Plating of Aluminum.»—The light metal may be plated with almost any
other metal, but copper is most commonly employed. Two formulas for
coppering aluminum follow:
I.—Make a bath of cupric sulphate, 30 parts; cream of tartar, 30
parts; soda, 25 parts; water, 1,000 parts. After well scouring the
objects to be coppered, immerse in the bath. The coppering may also be
effected by means of the battery with the following mixture: Sodium
phosphate, 50 parts; potassium cyanide, 50 parts; copper cyanide, 50
parts; distilled water, 1,000 parts.
II.—First clean the aluminum in a warm solution of an alkaline
carbonate, thus making its surface rough and porous; next wash it
thoroughly in running water, and dip it into a hot solution of
hydrochloric acid of about 5 per cent strength. Wash it again in clean
water, and then place it in a somewhat concentrated acid solution of
copper sulphate, until a uniform metallic deposit is formed; it is then
again thoroughly washed and returned to the copper sulphate bath, when
an electric current is passed until a coating of copper of the required
thickness is obtained.
«Brassing.»—The following recipe is recommended for the bath: Copper
acetate, 50 parts, by weight; dry zinc chloride, 25 parts, by weight;
crystallized sodium sulphite, 250 parts, by weight; ammonium carbonate,
35 parts, by weight; potassium cyanide, 110 parts, by weight. Dissolve
in 3,000 parts of water.
«Coppering.»—I.—This is the Dessolle process for the galvanic
application of copper. The special advantage claimed is that strong
currents can be used, and a deposit obtained of 0.004 inch in 1 1/2
hours. After having cleaned the object to be coppered, with sand or in
an acid bath, a first coat is deposited in an ordinary electrolytic
bath; then the object is placed in a final bath, in which the
electrolyte is projected on the electrode, so as to remove all bubbles
of gas or other impurities tending to attach themselves to the surface.
The electrolyte employed is simply a solution of cupric sulphate in
very dilute sulphuric acid. For the preliminary bath the double cyanide
of potassium and copper is made use of.
II.—Those baths which contain cyanide work best, and may be used for
all metals. The amount of the latter must not form too large an excess.
The addition of a sulphide is very dangerous. It is of advantage
that the final bath contain an excess of alkali, but only as ammonia
or ammonium carbonate. For a copper salt the acetate is preferable.
According to this, the solution A is prepared in the warm, and solution
B is added with heating. Solution A: Neutral copper acetate, 30 parts,
by weight; crystallized sodium sulphite, 30 parts, by weight; ammonium
carbonate, 5 parts, by weight; water, 500 parts, by weight. Solution B:
Potassium cyanide (98 to 99 per cent), 35 parts, by weight; and water,
500 parts, by weight.
«Coppering Glass.»—I.—Glass vessels may be coated with copper by
electrolytic process, by simply varnishing the outer surface of the
vessel, and when the varnish is nearly dry, brushing plumbago well over
it. A conducting wire is then attached to the varnished surface, which
may be conveniently done by employing a small piece of softened gutta
percha or beeswax, taking care to employ the plumbago to the part which
unites the wire to the plumbagoed surface.
II.—Dissolve gutta percha in essence of turpentine or benzine; apply a
coat of the solution on the glass in the places to {573} be coppered
and allow to dry; next rub it with graphite and place in the electric
bath. The rubber solution is spread with a brush.
«Coppering Plaster Models, etc.»—Busts and similar objects may be
coated by saturating them with linseed oil, or better, with beeswax,
then well blackleading, or treating them with phosphorous, silver and
gold solutions, attaching a number of guiding wires, connected with
all the most hollow and distant parts, and then immersing them in the
sulphate of copper solution and causing just sufficient copper to be
deposited upon them, by the battery process, to protect them, but not
to obliterate the fine lines or features.