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, per cent. | Zinc 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.