=Imitation Silver.=—The imitation silver pigments are made from an alloy of tin and bismuth, or from a tin amalgam. The latter is most easily made by melting tin in a porcelain dish and adding one quarter of its weight of mercury; the mixture is then well stirred, and allowed to cool. It quickly solidifies to a crystalline mass, which is tolerably brittle and can be powdered without difficulty. To obtain a substance of a true silvery appearance the amalgam must be converted to a powder of a certain degree of fineness. If the powdering is carried too far the product loses a great part of its metallic lustre, and acquires a dull grey colour. The bismuth alloy is made by fusing 100 parts of tin, adding 100 parts of bismuth, and then 10 parts of mercury. It is not absolutely necessary to add mercury, but the addition has the advantage that the solid alloy is far more easily powdered. The imitation silver made by this process has a white metallic colour approaching that of genuine silver, but not equal to it, especially in lustre. It is, however, largely used in the industries on account of its low price; for example, for paper hangings. CHAPTER XLVI. 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.