Copper 82·45 Its specific gravity was 8·482.
Zinc 10·30
Tin 4·10
Lead 3·15
------
100·00
The alloy most proper for bronze medals which are to be afterwards
struck, is composed of from 8 to 12 parts of tin and from 92 to 88 of
copper; to which if 2 or 3 parts in the hundred of zinc be added they
will make it assume a finer bronze tint. The alloy of the Kellers is
famous for this effect. The medal should be subjected to three or four
successive stamps of the press, and be softened between each blow by
being heated and plunged into cold water.
The bronze of bells or bell metal is composed in 100 parts of copper 78,
tin 22. This alloy has a fine compact grain, is very fusible and
sonorous. The other metals sometimes added are rather prejudicial, and
merely increase the profit of the founders. Some of the English bells
consist of 80 copper, 10·1 tin, 5·6 zinc, and 4·3 lead; the latter metal
when in such large quantity is apt to cause insulated drops, hurtful to
the uniformity of the alloy.
The _tam-tams and cymbals of bronze_.--The Chinese make use of bronze
instruments forged by the hammer, which are very thin, and raised up in
the middle; they are called gongs, from the word _tshoung_ which
signifies a bell. Klaproth has shown that they contain nothing but
copper and tin; in the proportions of 78 of the former metal and 22 of
the latter. Their specific gravity is 8·815. This alloy when newly cast
is as brittle as glass, but by being plunged at a cherry-red heat into
cold water and confined between two discs of iron to keep it in shape,
it becomes tough and malleable. The cymbals consist of 80 parts copper
and 20 tin.
Bronze vessels naturally brittle may be made tenacious by the same
ingenious process, for which the world is indebted to M. Darcet. Bronze
mortars for pounding have their lips tempered in the same way. Ancient
warlike weapons of bronze were variously compounded; swords were formed
of 87-1/2 copper, and 12-1/2 tin in 100 parts; the springs of balistæ
consisted of 97 copper, and 3 tin.
_Cannon metal_ consists of about 90 or 91 copper, and 10 or 9 of tin.
From the experiments of Papacino-d’Antony, made at Turin, in 1770, it
appears that the most proper alloy for great guns is from 12 to 14
parts of tin to 100 of copper; but the Comte Lamartilliere concluded
from his experiments made at Douay, in 1786, that never less than 8 nor
more than 11 of tin should be employed in 100 parts of bronze.
_Gilt ornaments of bronze._--This kind of bronze should be easy of
fusion, and take perfectly the impression of the mould. The alloy of
copper and zinc is when fused of a pasty consistence, does not make a
sharp cast, is apt to absorb too much amalgam, is liable to crack in
cooling, and is too tough or too soft for the chaser or the turner. Were
the quantity of zinc increased to make the metal harder, it would lose
the yellow colour suitable to the gilder. A fourfold combination of
copper, zinc, tin, and lead, is preferable for making such ornamental
bronze articles; and the following proportions are probably the best, as
they unite closeness of grain with the other good qualities. Copper 82,
zinc 18, tin 3 or 1, lead 1-1/2 or 3. In the alloy which contains most
lead, the tenacity is diminished and the density is increased, which is
preferable for pieces of small dimensions. Another alloy which is said
to require for its gilding only two thirds of the ordinary quantity of
gold, has the following composition: copper, 82·257; zinc, 17·481; tin,
0·238; lead, 0·024.
The antique bronze colour is given to figures and other objects made
from these alloys by the following process:--Two drams of sal-ammoniac,
and half a dram of salt of sorrel, (binoxalate of potash,) are to be
dissolved in fourteen ounce measures (English) of colourless vinegar. A
hair pencil being dipped into this solution, and pressed gently between
the fingers, is to be rubbed equally over the clean surface of the
object slightly warmed in the sun or at a stove; and the operation is to
be repeated till the wished for shade is obtained. (See GILDING.)
The bronze founder ought to melt his metals rapidly, in order to prevent
the loss of tin, zinc, and lead, by their oxidizement. Reverberatory
furnaces have been long used for this operation; the best being of an
elliptical form. The furnaces with dome tops are employed by the bell
founders, because their alloy being more fusible, they do not require so
intense a heat; but they also would find their advantage in using the
most rapid mode of fusion. The surface of the melting metals should be
covered with small charcoal, or coke; and when the zinc is added, it
should be dextrously thrust to the bottom of the melted copper.
Immediately after stirring the melted mass so as to incorporate its
ingredients, it should be poured out into the moulds. In general the
metals most easily altered by the fire, as the tin, should be put in
last. The cooling should be as quick as possible in the moulds to
prevent the risk of the metals separating from each other in the order
of their density, as they are very apt to do. The addition of a little
iron, in the form of tin-plate, to bronze, is reckoned to be
advantageous.
One part of tin, and two parts of copper, (nearly one atom of tin and
four of copper, or more exactly 100 parts of tin, and 215 copper,) form
the ordinary speculum metal of reflecting telescopes, which is of all
the alloys the whitest, the most brilliant, the hardest, and the most
brittle. The alloy of 1 part of tin, and 10 of copper, (or nearly one
atom of the former to eighteen of the latter,) is the strongest of the
whole series.
Ornamental objects of bronze, after being cast, are commonly laid upon
red-hot coals till they take a dull red heat, and are then exposed for
some time to the air. The surface is thereby freed from any greasy
matter, some portion of the zinc is dissipated, the alloy assumes more
of a coppery hue, which prepares for the subsequent gilding. The black
tinge which it sometimes gets from the fire may be removed by washing it
with a weak acid. It may be made very clean by acting upon it with
nitric acid, of specific gravity 1·324, to which a little common salt
and soot have been added, the latter being of doubtful utility; after
which it must be well washed in water, and dried with rags or saw dust.
BRONZING, is the art of giving to objects of wood, plaster, &c. such a
surface as makes them appear as if made of bronze. The term is sometimes
extended to signify the production of a metallic appearance of any kind
upon such objects. They ought first to be smeared over smoothly with a
coat of size or oil varnish, and when nearly dry, the metallic powder
made from Dutch foil, gold leaf, mosaic gold, or precipitated copper, is
to be applied with a dusting bag, and then rubbed over the surface with
a linen pad; or the metallic powders may be mixed with the drying oil
beforehand, and then applied with a brush. Sometimes fine copper, or
brass filings, or mosaic gold, are mixed previously with some pulverized
bone ash, and then applied in either way. A mixture of these powders
with mucilage of gum arabic is used to give paper or wood a bronze
appearance. The surface must be afterwards burnished. Copper powder
precipitated by clean plates of iron, from a solution of nitrate of
copper, after being well washed and dried, has been employed in this
way, either alone or mixed with pulverized bone-ash. A finish is given
to works of this nature by a coat of spirit varnish.
A white metallic appearance is given to plaster figures by rubbing over
them an amalgam of equal parts of mercury, bismuth, and tin, and
applying a coat of varnish over it. The iron-coloured bronzing is given
by black lead or plumbago, finely pulverized and washed. Busts and
other objects made of cast iron acquire a bronze aspect by being well
cleaned and plunged in solution of sulphate of copper, whereby a thin
film of this metal is left upon the iron.
Copper acquires by a certain treatment a reddish or yellowish hue, in
consequence of a little oxide being formed upon its surface. Coins and
medals may be handsomely bronzed as follows: 2 parts of verdigris and 1
part of sal ammoniac are to be dissolved in vinegar; the solution is to
be boiled, skimmed, and diluted with water till it has only a weak
metallic taste, and upon further dilution lets fall no white
precipitate. This solution is made to boil briskly, and is poured upon
the objects to be bronzed, which are previously made quite clean,
particularly free from grease, and set in another copper pan. This pan
is to be put upon the fire, that the boiling may be renewed. The pieces
under operation must be so laid that the solution has free access to
every point of their surface. The copper hereby acquires an agreeable
reddish brown hue, without losing its lustre. But if the process be too
long continued, the coat of oxide becomes thick, and makes the objects
appear scaly and dull. Hence they must be inspected every 5 minutes, and
be taken out of the solution the moment their colour arrives at the
desired shade. If the solution be too strong, the bronzing comes off
with friction, or the copper gets covered with a white powder, which
becomes green by exposure to air, and the labour is consequently lost.
The bronzed pieces are to be washed with many repeated waters, and
carefully dried, otherwise they would infallibly turn green. To give
fresh-made bronze objects an antique appearance, three quarters of an
ounce of sal ammoniac, and a dram and a half of binoxalate of potash
(salt of sorrel) are to be dissolved in a quart of vinegar, and a soft
rag or brush moistened with this solution is to be rubbed over the clean
bright metal, till its surface becomes entirely dry by the friction.
This process must be repeated several times to produce the full effect;
and the object should be kept a little warm. Copper acquires very
readily a brown colour by rubbing it with a solution of the common liver
of sulphur, or sulphuret of potash.
The Chinese are said to bronze their copper vessels by taking 2 ounces
of verdigris, 2 ounces of cinnabar, 5 ounces of sal ammoniac and 5
ounces of alum, all in powder, making them into a paste with vinegar,
and spreading this pretty thick like a pigment on the surfaces
previously brightened. The piece is then to be held a little while over
a fire, till it becomes uniformly heated. It is next cooled, washed, and
dried; after which it is treated in the same way once and again till the
wished-for colour is obtained. An addition of sulphate of copper makes
the colour incline more to chesnut brown, and of borax more to yellow.
It is obvious that the cinnabar produces a thin coat of sulphuret of
copper upon the surface of the vessel, and might probably be used with
advantage by itself.
To give the appearance of antique bronze to modern articles, we should
dissolve 1 part of sal ammoniac, 3 parts of cream of tartar, and 6 parts
of common salt in 12 parts of hot water, and mix with the solution 8
parts of a solution of nitrate of copper of specific gravity 1·160. This
compound, when applied repeatedly in a moderately damp place to bronze,
gives it in a short time a durable green coat, which becomes by degrees
very beautiful. More salt gives it a yellowish tinge, less salt a bluish
cast. A large addition of sal ammoniac accelerates the operation of the
mordant.
BROWNING _of gun-barrels and other arms._--By this process, the surface
of several articles of iron acquires a shining brown colour. This
preparation, which protects the iron from rust, and also improves its
appearance, is chiefly employed for the barrels of fowling-pieces and
soldier’s rifles, to conceal the fire-arms from the game and the enemy.
The finest kind of browning is the Damascus, in which dark and bright
lines run through the brown ground.
This operation consists in producing a very thin uniform film of oxide
or rust upon the iron, and giving a gloss to its surface by rubbing wax
over it, or coating it with a shell-lac varnish.
Several means may be employed to produce this rust speedily and well.
The effect may be obtained by inclosing the barrels in a space filled
with the vapour of muriatic acid. Moistening their surface with dilute
muriatic or nitric acid, will answer the same purpose. But the most
common material used for browning, is the butter or chloride of
antimony, which, on account of its being subservient to this purpose,
has been called _bronzing salt_. It is mixed uniformly with olive oil,
and rubbed upon the iron slightly heated; which is afterwards exposed to
the air, till the wished-for degree of browning is produced. A little
aquafortis is rubbed on after the antimony, to quicken its operation.
The brown barrel must be then carefully cleaned, washed with water,
dried, and finally polished, either by the steel burnisher, or rubbed
with white wax, or varnished with a solution of 2 ounces of shell-lac,
and 3 drams of dragon’s blood, in 2 quarts of spirit of wine.
The following process may also be recommended: Make a solution with half
an ounce of aquafortis, half an ounce of sweet spirit of nitre, 1 ounce
of spirit of wine, 2 ounces of sulphate of copper, and 1 ounce of
tincture of iron, in so much water as will fill altogether a quart
measure. The gun barrel to be browned must first of all be filed and
polished bright, and then rubbed with unslaked lime and water to clear
away all the grease. Its two ends must now be stopped with wooden rods,
which may serve as handles, and the touch-hole must be filled with wax.
The barrel is then to be rubbed with that solution, applied to linen
rags or a sponge, till the whole surface be equally moistened; it is
allowed to stand 24 hours, and is then scrubbed with a stiff brush. The
application of the liquid and the brushing may be repeated twice or
oftener, till the iron acquires a fine brown colour. After the last
brushing, the barrel must be washed with plenty of boiling water,
containing a little potash; then washed with clean water, dried, rubbed
with polishing hard wood, and coated with shell-lac varnish, for which
purpose the barrel must be heated to the boiling point of water. It is
finally polished with a piece of hard wood.
Storch recommends to make a browning solution with 1 part of sulphate of
copper, one third of a part of sulphuric ether, and 4 parts of distilled
water.
To give the damask appearance, the barrel must be rubbed over first with
very dilute aquafortis and vinegar, mixed with a solution of blue
vitriol; washed and dried, and rubbed with a hard brush to remove any
scales of copper which may be precipitated upon it from the sulphate.
Statues, vases, bas-reliefs, and other objects made of gypsum, may be
durably bronzed, and bear exposure to the weather better than after the
ordinary oil-varnish, by the following process:--Prepare a soap from
linseed oil, boiled with caustic soda lye, to which add a solution of
common salt, and concentrate it by boiling, till it becomes somewhat
granular upon the surface. It is then thrown upon a piece of linen
cloth, and strained with moderate pressure. What passes through is to be
diluted with boiling water, and again filtered. On the other hand, 4
parts of blue vitriol and 1 part of copperas are to be dissolved
separately in hot water. This solution is to be poured slowly into the
solution of soap, as long as it occasions any precipitate. This
flocculent matter is a mixture of cupreous soap and ferruginous soap,
that is, a combination of the oxides of copper and iron with the
margaric acid of the soda soap. The copper soap is green, the iron soap
is reddish brown, and both together resemble that green rust which is
characteristic of the antique bronzes. When the precipitate is
completely separated, a fresh portion of the vitriol solution is to be
poured upon it in a copper pan, and is made to boil, in order to wash
it. After some time, the liquid part must be decanted, and replaced by
warm water for the purpose of washing the metallic soaps. They are
finally treated with cold water, pressed in a linen bag, drained and
dried. In this state the compound is ready for use in the following
way:--
Three pounds of pure linseed oil are to be boiled with twelve ounces of
finely-powdered litharge, then strained through a coarse canvass cloth,
and allowed to stand in a warm place till the soap turns clear. Fifteen
ounces of this soap-varnish, mixed with 12 ounces of the above metallic
soaps, and 5 ounces of fine white wax, are to be melted together at a
gentle heat in a porcelain basin, by means of a water bath. The mixture
must be kept for some time in a melted state, to expel any moisture
which it may contain. It must be then applied, by means of a painter’s
brush, to the surface of the gypsum previously heated to the temperature
of about 200° F. By skilful management of the heat, the colour may be
evenly and smoothly laid on without filling up the minute lineaments of
the busts. When, after remaining in the cool air for a few days, the
smell of the pigment has gone off, the surface is to be rubbed with
cotton wool, or a fine linen rag, and variegated with a few streaks of
metal powder or shell gold. Small objects may be dipped in the melted
mixture, and then exposed to the heat of a fire till they are thoroughly
penetrated and evenly coated with it.
BROWN DYE. Upon this subject some general views are given in the article
DYEING, explanatory of the nature of this colour, to which I may in the
first place refer. This dye presents a vast variety of tints, from
yellow and red to black brown, and is produced either by mixtures of
red, yellow, and blue with each other, or of yellow or red with black,
or by substantive colours, such as catechu or oxide of manganese, alone.
We shall here notice only the principal shades; leaving their
modifications to the caprice or skill of the dyer.