The treatment of malachite for the preparation of pigment presents certain difficulties owing to the comparative hardness (3·5–4) of the mineral, which is also rather heavy (sp. gr. 3·6–4·0). On the large scale, the selected mineral is first put through a stamping-mill, and then ground, very hard stones being required for this purpose. The fine product from this (usually wet) process is levigated and dried. The pit water of some copper mines contains certain quantities of blue vitriol (copper sulphate) in solution; and such pit water is generally treated for the recovery of a very pure form of copper, the so-called cementation copper. The liquor might also be worked up into malachite green, by collecting it in large tanks and precipitating the dissolved copper oxide with milk of lime, the bluish-green deposit separating in association with gypsum being transformed into a light malachite green by washing and drying. A darker green, free from gypsum, could be prepared by using a solution of carbonate of soda as precipitant. Neither the native nor the artificial malachite green is particularly handsome in colour; and both possess, in addition, the unpleasant property of gradually going off colour in the air, all the copper compounds being quite as sensitive to sulphuretted hydrogen as those of lead, and finally turning quite black under the influence of that gas. CHAPTER IX BLUE EARTH COLOURS Only three minerals are known to be suitable as pigments; and indeed, at present, only two, the third, lapis lazuli, being now of merely historical interest. Nowadays, no one would think of using this rare and expensive mineral as a pigment, since ultramarine, which has the same pigmentary properties, is extremely cheap, whereas the pigment from lapis lazuli was worth its weight in gold. The only two blue earth colours of any interest at present are malachite (copper) blue, and the blue iron earth Vivianite; and even these, though by no means rare, are little used, since artificial blues are now made which are far superior in beauty and can be obtained so cheaply that the natural pigments are put out of competition. MALACHITE BLUE (LAZULITE) Lazulite and malachite (mountain blue) are of frequent occurrence in copper mines, and the former is distinguished by its beautiful azure blue colour, which, however, suffers considerably when the crystals are reduced to powder. Both minerals are very similar in chemical composition, and consist of cupric carbonate. The formula of malachite is 2CuOCO{2} + H{2}O, that of lazulite being 3CuO(CO{2}){2} + H{2}O, so that the only difference between them is that of the relative proportions of the substances in combination. Lazulite is also rather hard (3·5–4·0), but owing to the small size and brittle character of the crystals it is not very difficult to pulverise. In the air, malachite blue behaves in much the same way as malachite green, turning black in presence of sulphuretted hydrogen.