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.

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:—

Bronzes from English, French and Bavarian works contain the following percentages of copper:—

English Bronzes.

French Bronzes.

Bavarian Bronzes.

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.