Now that enamels are not used exclusively for artistic purposes, but in considerable quantities for earthenware and other technical purposes, the manufacture of enamel colours has attained considerable importance; works already exist occupied almost exclusively with this special branch of manufacture. By Lacroix’ process enamel colours are made which can be applied to porcelain without further admixture with a flux; for example, a very fine blue is made by dissolving 300 parts of pure alumina and 100 parts of cobalt carbonate in nitric acid, evaporating the solution to dryness, igniting the residue and fusing it with 300 parts of quartz sand free from iron, 900 parts of crystallised boric acid and 1,800 parts of red lead. A blue glass is obtained which easily melts, but is difficult to powder; but if poured when fluid in a thin stream into cold water, it forms thin threads, which are very brittle in consequence of the rapid cooling, and may be easily converted into a soft powder. CHAPTER XLV. METALLIC PIGMENTS. 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.