_Black._--There is really only one member of this class, which, however, is frequently used, viz. that form of carbon occurring as hexagonal crystals and known as graphite. Another natural black natural product, occasionally used as a painters’ colour is the so-called black chalk. However, since black pigments can be produced very cheaply by artificial means, the natural colours find only a limited application; and only in one instance is graphite used alone, viz. for making blacklead pencils. As already mentioned, certain chemical industries furnish by-products which are of very little value in themselves, and many of them, indeed, may be classed as worthless, since chemical manufacturers naturally endeavour to get everything possible out of their materials in the course of manufacture. Some of these by-products, however, can advantageously be used as pigments, a good example of this being afforded by the iron oxide formed as a by-product in the manufacture of fuming sulphuric acid (Nordhausen oil of vitriol), by the old process, from green vitriol (ferrous sulphate). In itself, this oxide is practically valueless, but, by very simple treatment, it can be converted into very valuable pigments which have a market value far in excess of the original material. Although it has hitherto been the custom to confine the term earth colours to such as occur ready-formed in Nature and only require simple mechanical treatment to make them ready for immediate use as pigments, the author is nevertheless of opinion that a book dealing exhaustively with earth colours should also make some mention of all the mineral colouring matters which can be easily made into pigments by simple processes, such as calcination or bringing into association with other substances. In accordance with this view, the present work will describe all the pigments that are obtainable in this manner. Most of the earth colours consist of decomposition products of certain minerals; and this applies particularly to such of them as contain iron oxide. According as the decomposition of the original mineral has been more or less extensive, the natural product exhibits different properties; and the manufacturer must consequently endeavour to treat them in such a manner as to ensure that the pigment obtained will be as uniform as possible in shade and permanence. In order to accomplish this it is essential to have an accurate knowledge of the origin of the raw material under treatment, and of its chemical and physical properties. In view of this, the author considers it necessary to deal more fully with the pigmentary earths forming the raw materials of the earth colours, before passing on to the preparation of the colours themselves. CHAPTER II THE RAW MATERIALS FOR EARTH COLOURS