The intimately mixed material is formed into thin rods, which are dried and then baked, the heat driving out the water in the clay and transforming it into a solid mass. An addition to this main application of graphite, the mineral is also used for making crucibles, chiefly for melting the noble metals. Crucibles of this kind are largely manufactured near Passau, Bavaria, and similar crucibles are made in England from Ceylon graphite. Another important use for graphite is as a coating for iron articles to protect them from rust. For this purpose, however, only the inferior kinds are employed; and these can also be made up into excellent cements capable, in particular, of offering considerable resistance to the action of heat and chemicals. To complete the tale of the applications of graphite, its employment as a lubricating agent for machinery, especially for reducing friction in machines made of wood, may be mentioned. Latterly also, the finest levigated graphite has come into use, in admixture with solid fats or mineral oils, for lubricating large engines, for which purpose it is excellently adapted. BLACK CHALK Black chalk, slate black, Spanish chalk, crayon, etc., is not a chalk at all, in the mineralogical sense, but consists of clay shale of varying colour. Some kinds of this shale are pure black, almost velvet black, and these are considered the best. Others have a more greyish or bluish tinge and are of low value as pigments. The purer the black, the finer the grain of the material, and therefore the greater its value to the colour-maker. The variety obtained from Spain is generally admitted to be the best, and for this reason the name of Spanish chalk has been applied to all similar minerals. In all cases the black colour of Spanish chalk is due to carbon; but the particular modification of carbon present has not yet been accurately identified. According to some, it is chiefly graphite, whereas others ascribe the colour to amorphous carbon. Apparently, the material found in different deposits contains either one or the other of these modifications of carbon. Deposits of black chalk are fairly plentiful, but in many of them the material is so contaminated with extraneous minerals that a somewhat troublesome method of preparation is needed to fit them for the purpose of the draughtsman. With this object, the native product must be ground extremely fine, and the powder levigated; and owing to the expense of these processes, they are now seldom used, it being possible to obtain a good black chalk far more cheaply than by levigating the natural material.