=Bistre.=—When soot is produced at a very low temperature it is very lustrous, and, in addition to carbon, contains a notable quantity of the products of dry distillation. When this soot is powdered and treated with water the latter substances are dissolved; boiling water should be used. When water is no longer coloured the soot is suspended in a large quantity of water and subjected to a process of levigation. The fine powder obtained by repeated levigation has, when dry, an ugly brown colour, but when ground it acquires an extremely warm shade. CHAPTER XXXIX. BLACK PIGMENTS. Carbon occurs in nature in many different forms—the diamond, graphite (black lead), and purified lamp black are, from the chemical point of view, one and the same substance, namely, carbon. In colour making the non-crystalline form of carbon, which is pure black, is alone used; almost all black pigments used in painting are composed of tolerably pure carbon. In whatever way and from whatever materials carbon is made for use as a pigment, the manufacturer must always endeavour to obtain it as pure as possible, and in a condition of the finest division; the depth of colour and the covering power depend on these two conditions. Carbon only shows a pure black colour when it is pure; if it contains relatively small quantities of impurities, it has a more or less brown shade. At first sight it appears to be a very simple matter to obtain black pigments from organic materials; it is simply necessary to expose them in the absence of air to a temperature high enough to decompose them, carbon is then left as a residue. In spite of the apparent simplicity of this operation there are many difficulties. The preparation of good black pigments demands considerable practice. There are two methods by which carbon can be made for use as a pigment. One has been indicated, the heating of organic materials in the absence of air, _i.e._, their dry distillation; the second, which produces the best qualities of black, consists in burning substances very rich in carbon with a very small supply of air. The product of this process, commonly known as soot, is very different in external appearance from carbon made by dry distillation. The latter forms an easily powdered mass, which only acquires lustre by grinding with oil, whilst soot consists generally of very light flocks, which, in consequence of their greasy nature, exhibit a peculiar velvety lustre.