BRONZE PIGMENTS.
It would be anticipated from the name that bronze pigments were
composed of an alloy of copper and tin; in reality the alloy is
composed of copper and zinc, _i.e._, brass. The bronze pigments are
made by a similar process to that described for genuine gold and silver
pigments. The waste produced in the manufacture of imitation gold leaf
is ground with a solution of dextrine upon a slab until the mixture is
uniform and separate metallic particles can be perceived only through
a lens. Whilst genuine gold and silver paints are always made in small
quantities, on account of the expensive nature of the material, and
machinery is not employed, the bronze pigments are in different case;
the use of mechanical arrangements is necessary for producing the fine
subdivision, otherwise the bronze would be very dear on account of the
great cost of grinding. The mechanical arrangements required to divide
the alloy are of the ordinary nature, but special machines have been
constructed for the manufacture of bronze pigments, by which the metal
is far more quickly converted into powder than by means of grinding
machinery. These machines consist of metal drums studded on the
interior with a large number of fine needles and capable of very rapid
rotation. When a metal powder already tolerably fine is brought into
these drums, it is rapidly brought to such a condition of fine division
as would be attained by hand grinding only by prolonged and laborious
exertion.
The raw material for the manufacture of bronze powders is produced
in making imitation gold and silver leaf; the waste metal obtained
in beating the sheets is used. The employment of this waste has two
advantages: the metal is already in very thin sheets and is composed of
alloys varying in colour from silver white, through gold, to a bright
copper. In making leaf metal the waste of each colour is kept carefully
separate, so that it simply requires to be broken up to produce bronze
powders of different shades.
Before this waste is brought into the drums mentioned above it must be
subjected to a preliminary grinding in a mortar with a small quantity
of a fatty oil, which serves to bind together the mass. Sufficient oil
should be used to give the mass some degree of coherence; if too much
oil is added the space between the needles of the drum would be coated
with the mass and the process in the drum would require a much longer
time. The uniform mixture of oil and bronze waste is then brought
upon a wire sieve of the finest possible mesh, and the mass is rubbed
through by means of a fine metal brush into a vessel below; thus the
larger particles are retained by the sieve and only those which are
smaller than the mesh pass through. The product of this process is
then brought into the drums, which are rapidly revolved; the small
particles of metal are thrown with great force against the side and
are converted by the fine points with which it is studded into a very
fine powder. The time required for this process depends on the rate of
revolution and on the quantity of powder treated at once. The drums are
stopped from time to time and the contents examined. When the powder
is sufficiently fine it is taken out of the drum. This is most easily
accomplished if the drum is arranged to take apart into two halves.
In most works it is usual to free the bronze powder from the admixed
oil by subjecting the mass to the greatest pressure that can be
produced by a very powerful hydraulic press. The oil which flows
from the press is always green, which shows that chemical action has
taken place. In consequence of the large surface imparted to the oil
it becomes speedily rancid, and then contains free fatty acids which
attack copper very energetically.
[Illustration: FIG. 35.]
The use of hydraulic presses may be avoided by removing the oil
by means of a solvent. Fatty oils dissolve very readily in carbon
bisulphide, but this solvent cannot be used in this case, because
commercial carbon bisulphide always contains dissolved sulphur, which
would blacken the bronze powder. Petroleum ether and benzene are very
suitable solvents for this purpose. On account of the volatility of
these inflammable liquids lights must be absolutely excluded from the
room in which they are used, and in order to avoid loss of solvent
the bronze powder must be treated in closed vessels. The safest plan
is to use a special apparatus of simple construction to dissolve
the oil. Fig. 35 shows the construction of an arrangement suitable
for this purpose. It consists of a cylindrical vessel of tin plate
surrounded by a rim into which fits the edge of the cover. When the
lid is placed on and the rim filled with water, the contents of the
vessel are closed in air-tight and cannot evaporate. The lower portion
of the vessel is conical, and is joined to a pipe in which is a tap,
and which communicates at the side by a tube with the glass vessel in
which the solvent is contained; another tube connects the neck of this
vessel with the cover. When this apparatus is used for extracting the
oil from bronze powder, a filter of strong blotting paper is placed
in the conical portion of the vessel, care being taken that it fits
accurately so that it is not torn by the weight of the bronze. The oily
bronze powder is placed on this filter, the cover set on, and the rim
filled with water. By opening the tap attached to the solvent reservoir
the liquid is allowed to enter the cylinder from below, the air in the
latter passing through the tube in the cover to the reservoir. After
some hours the oil is dissolved, the tap in the cover is then opened
and the liquid run off by opening the lowest tap. If the bronze powder
is not quite free from oil after one treatment with the solvent, the
operation is repeated with a fresh quantity.
When free from oil the powder and filter are removed from the apparatus
and dried; the dry mass forms a solid cake, which is broken up in a
mortar and by a little grinding changed into a fine powder.
The colour of the bronze powder is the same as that of the alloy used,
but the shade is always rather paler than that of the coherent metal;
regard must be paid to this circumstance in making a bronze of a
determined colour: the alloy employed must have a rather deeper colour
than the shade the bronze is to possess.
The manufacture of leaf metal and bronze powder is frequently conducted
in the same works, which also often prepare the requisite alloys; we,
therefore, give a few examples of the composition of the alloys which
produce certain shades. The more zinc the alloys contain the lower is
their melting point, the greater their brittleness and hardness and
the paler their colour. An increase in the copper causes the colour
of the alloy to approach more nearly to that of gold, and increases
the malleability, a property useful in making leaf metal, but not
desirable in making metallic powder. A zinc copper alloy which contains
between 1 and 7 per cent. of zinc has an almost pure red, or even a
dark red colour; an alloy containing 7·4 to 13·8 per cent. of zinc has
a pure golden yellow colour, between 16·6 and 25 per cent. of zinc a
yellow appears. An increase of the percentage of zinc above this point
produces the colour of brass; it is noteworthy that an alloy containing
still more zinc, 33 to 41 per cent., again shows a reddish colour,
which is most developed when the alloy contains equal parts of zinc and
copper. If the zinc is increased still further the shade gradually goes
over to white; this change is already observed in an alloy containing
51 per cent. of zinc, which shows a pure golden yellow colour, and is
very brittle. When the zinc rises to 53 per cent. the colour is reddish
white; at 56 per cent. it is yellowish white, at 64 per cent. bluish
white, and between 75 and 90 per cent. the alloy is bluish-grey.
The alloys for bronze powders of different shades have the following
composition, according to R. Wagner:—
Copper, Zinc
per cent. per cent.
Pale yellow 83 17
Red 94-90 6-10
Deep red 100
Bronzes from English, French and Bavarian works contain the following
percentages of copper:—
ENGLISH BRONZES.
Orange 9·82 per cent.
Deep yellow 82·37 ”
Pale yellow 80·42 ”
FRENCH BRONZES.
Copper red 97·32 per cent.
Orange 94·44 ”
Pale yellow 81·29 ”
BAVARIAN BRONZES.
Copper red 98·92 per cent.
Violet 98·82 ”
Orange 95·30 ”
Straw yellow 81·55 ”
Speiss yellow 82·34 ”
In each case the remainder of the alloy consists entirely of zinc.
Alloys containing from 1 to 35 per cent. of zinc are only malleable
in the cold. The malleability is at the greatest with a content of
zinc between 15 and 20 per cent.; such alloys are the most suitable
for making leaf metal. Alloys containing between 36 and 40 per cent.
of zinc may be hammered either cold or hot, whilst the former alloys
become brittle on heating. When the percentage of zinc is still further
increased the malleability decreases. The most brittle alloys contain
60 to 67 per cent. of zinc.
The alloys are made in a furnace with a good draught, for copper
liquefies at a very high temperature. To prevent loss of copper by
oxidation the molten metal should not come in contact with air; it
should be covered by a layer of red·hot coal, which prevents oxygen
from reaching it. When the copper is completely melted, which is
ascertained by stirring with a piece of wood, the whole of the zinc
is added. Some skill is required in this operation, otherwise a large
proportion of the zinc will be volatilised, and the vapours will burn
when they come in contact with air, in which case dazzling bluish white
flames are seen over the crucible. The best method is to throw the zinc
into the crucible and immediately stir it into the molten metal with
a wooden rod. The products of the dry distillation of the wood, which
are given off in great quantity at this high temperature, keep the air
from the surface of the metal and prevent the oxidation of the zinc
vapours. The zinc is thoroughly mixed with the copper by stirring with
the wooden rod, the crucible is then slowly cooled, with the precaution
that the surface of the metal is kept covered by red-hot coals so long
as the metal is fluid. When sufficiently cool the metal is poured into
shallow iron moulds, in which it quickly solidifies; it is then rolled
into sheets, which may be converted into thin leaves by hammering in a
similar manner to that in which the gold-beater makes gold leaf.
To obtain bronze powders of different shades alloys of different
colours may be used; the bronze powders may also be shaded by two
methods—either by adding certain colouring matters of very great
colouring power or by partially oxidising the finely divided metallic
powder. In the first process the finely ground colouring matter is
mechanically mixed with the bronze powder. The use of manual labour
would involve a great loss of time; even when quite small quantities of
bronze and colouring matter are mixed in a mortar it is necessary to
grind diligently for a very long time before a mixture of homogeneous
appearance is obtained. In working on a somewhat larger scale it is
therefore advisable to adopt mechanical mixing arrangements. A very
simple apparatus suffices. A sheet-iron cylinder is used which can be
revolved, and provided with a well-fitting slide. In this cylinder are
placed the bronze powder and the colouring matter until it is about
half full; then, after tightly closing the slide, it is set in slow
rotation, which is continued until a test taken out shows a uniform
colour.
When bronze powder is slowly heated in a shallow vessel the colour
begins to darken at a temperature not much above the boiling point of
water. In consequence of the fineness of the particles of the metallic
powder the copper readily takes up oxygen, and is superficially
converted into copper oxide. This oxide is of a darker colour, and thus
by this method the shade of the bronze can be deepened as desired.
This simple operation requires a certain amount of practice to produce
a product of a determined shade. The desired result is most safely
attained when the bronze is spread out quite uniformly in a thin layer
upon a metal plate, which is gently heated from below. The powder soon
begins to darken; by cooling the plate the progress of the oxidation
may be arrested at any moment.
Recently bronze powders have come into the market showing all possible
colours in the deepest shades, by the aid of which very remarkable
colour effects can be produced. These bronzes are made by dissolving
an aniline dye in a little alcohol, pouring this solution over the
powder, and mixing the dye uniformly through the whole of the bronze by
working the mass for a sufficient length of time. In this way bronze
powders are produced which possess a green, red, blue or violet lustre,
according to the colour of the dye used. These colours with metallic
lustre can also be produced by bronzing the article with a white (zinc)
bronze, and then coating it with a varnish in which the required
aniline dye is dissolved. A bronze with a fine golden red glitter is
produced by applying a golden yellow bronze and then a varnish in
which a little aniline red is dissolved. It should be observed here
that these effects, produced by a coat of varnish in which an aniline
dye is dissolved, only turn out well when the dye is used in very
small quantity, for these colours are the strongest with which we are
acquainted, and in colouring power far surpass cochineal carmine, which
is renowned for this property.
When bronze is coloured by dyes the most varied shades can be obtained
with a metallic lustre. According to Conradty a very fine blue bronze
is obtained by boiling white bronze for some hours with a weak alum
solution, washing and drying, and then mixing in a mortar with a
strong solution of aniline blue in alcohol until the solvent has
evaporated. This operation is repeated until the desired depth of shade
is obtained. The bronze is then washed with pure water. Conradty also
recommends that the coloured bronze should be ground with a little
petroleum, and then exposed to the air to allow the petroleum to
evaporate. This operation, for which no chemical reason can be given,
is quite unnecessary. If other dyes or mixtures of them are used in
place of aniline blue, bronzes of corresponding colour are obtained.
However handsome are the bronzes coloured by this process, nearly all
have the disadvantage that the colours have little permanence, and
quickly fade when exposed to light. This is especially the case when
the bronzed article is coated with an oil varnish; if, however, a
spirit varnish is used, or indeed any varnish composed of a resin and a
volatile solvent, the colour of the bronze, protected by the layer of
resin, remains quite unaltered for a long time.
=Electrolytic Copper Bronze.=—Electrolytically precipitated copper may
be used as a bronze pigment; it is most simply made by adding pieces
of metallic zinc to a solution of copper sulphate free from iron and
violently shaking the flask for a long time. The liquid becomes warm,
and the copper separates in the form of a very fine precipitate, which
is collected on a filter and washed with air-free water (boiling water
is best) and then quickly dried. The upper portions of the precipitate
in the filter, which are exposed to the air, have generally a brownish
colour due to the incipient oxidation of the finely divided metal. They
are removed, and the lower portions show the characteristic colour of
pure copper.
In the same way silver can be precipitated from a solution of silver
nitrate in a finely divided state, but the particles of the silver
powder are so very small that they reflect very little light, and
consequently the powder has an unsightly grey colour. When a surface
painted with this silver is rubbed gently with a hard body, the
metallic lustre appears.
=Tungsten Bronze Pigments= are expensive and rarely employed. They
are obtained by fusing sodium or potassium tungstate in a porcelain
crucible and gradually adding tungstic acid until the mass has an
acid reaction. Tin dioxide is then added in quantity sufficient to
neutralise the tungstic acid; the mass is cooled and finely powdered.
According as potassium or sodium tungstate is used, a violet or reddish
pigment is obtained which exhibits the peculiar metallic lustre of a
bronze powder.
Still more costly is vanadium bronze, which is made by adding ammonium
vanadate to a solution of 2 parts of copper sulphate and 1 part of
ammonium chloride with continual stirring, until the precipitate no
longer re-dissolves on stirring. The liquid is then heated for several
hours to about 35° C., when vanadium bronze separates in golden yellow
scales. These are collected on a filter, washed and dried When ground
with oil or gum solution they can be used as a red gold bronze. The
colour is unaltered by the air.