Metallic pigments do not always consist of metals; the name is also applied to compounds which possess a pronounced metallic lustre; mosaic gold, the preparation of which has been previously described, is an example of such a pigment. Another variety of metallic pigment is made by heating finely-powdered alloys; a layer of oxide is thus produced upon the surface of the metallic particles, this layer produces a shade of the colour of the metal or alloy. Metallic pigments are only unalterable when they are composed of metals which are not changed by the action of the air. As a matter of fact, this property is not possessed by any of the metals used for this purpose; even gold and silver are blackened by the action of the sulphuretted hydrogen in the air. This alteration proceeds, however, very slowly when the metal is enveloped by a layer of a binding medium, which is always the case when the metallic powder is used for painting. There are many manuscripts, the initial letters of which have been coloured by gold or silver, in which the metals retain their peculiar lustre after the lapse of centuries.

Metallic paints made from yellow alloys have never much durability: they always contain copper, which readily alters; articles painted with imitation gold soon lose their lustre, and in the course of time become green.

Shell-Gold.—This very expensive artists’ paint is made by rubbing gold-beaters’ refuse with gum solution upon a stone slab until a completely homogeneous mixture results, which is then rapidly thickened over the fire and generally allowed to dry in small shells. The preparation so made is known as shell-gold: it is sold at very high prices.

The operation of grinding the gold is very lengthy; it considerably increases the cost of the already expensive material. The process may be considerably shortened if the gold is obtained in a very finely divided state by a chemical operation. For this process coins or broken jewellery is heated in hydrochloric acid, and nitric acid gradually added. The gold dissolves in the mixture. If it was alloyed with silver a white precipitate of silver chloride is formed, which is filtered off after largely diluting with distilled water. The solution is boiled for some time to remove excess of nitric acid. A solution of ferrous sulphate is then added; the liquid at once becomes bluish black, and in a short time deposits a brown precipitate composed of chemically pure gold, which is so finely divided that it is almost without the characteristic glitter of gold. The precipitate is filtered off, dried and preserved in stoppered bottles. In order to prepare shell-gold from this gold powder it is simply necessary to grind it with thick gum solution in a porcelain mortar; under the pressure of the pestle the gold rapidly acquires its natural glitter. The grinding need only be continued until the gold and gum solution are uniformly mixed. The mixture so made should be at once filled into the shells, since on standing the gold would soon separate from the gum solution in consequence of its high specific gravity.

Shell-Silver.—Genuine shell-silver can be made by rubbing silver leaf with gum solution upon a slab exactly as genuine shell-gold. In this case also the labour may be considerably lessened by converting the silver into a state of fine division by a chemical process. Silver is dissolved in nitric acid, which must be quite free from hydrochloric acid, or insoluble silver chloride would be formed. In the operation brown suffocating fumes are evolved which attack the respiratory organs; it should therefore be conducted in the open air or under a flue with a good draught. The silver solution, which is generally coloured blue by admixed copper, is diluted with a large quantity of distilled water. A sheet of copper is then dipped into the liquid and rapidly moved about in it. The silver then separates as a dark grey powder; after washing it is chemically pure, and when ground with gum solution it produces genuine shell-silver.

Shell-silver and shell-gold are rarely used in painting on account of their cost; they are chiefly used for illuminated manuscripts. When these metallic pigments are to be used in oil painting, in place of gum they must be ground with a liquid which mixes with boiled oil or essential oils. For this purpose copaiba balsam is to be recommended.

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.