Malachite Green
Although the pigment sold under this name is nearly always an artificial product, it cannot be omitted from a work dealing with the earth colours, because, in former times, it was prepared exclusively from the mineral malachite. Owing to the fact that artificial malachite green is one of the cheapest of colours, the troublesome work involved in the mechanical preparation of the native pigment has been almost entirely abandoned, and the malachite itself is now utilised to greater advantage as a source of copper.
Malachite green (or mountain green) is found in nearly every case where copper ores exist, and is still—though very rarely indeed—prepared, in a few places, from the mineral, the dark-coloured lumps being picked out because the lighter-coloured ones would furnish much too pale a colour.
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.