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 2CuOCO2 + H2O, that of lazulite being 3CuO(CO2)2 + H2O, 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.
Malachite blue is chiefly used for indoor work, and also as a water colour; but it is always rather pale and dull.
Vivianite
This mineral—also termed blue ochre—is a transformation product of various iron ores, and occurs native as fairly extensive deposits in some places, especially in peat bogs. It forms ill-defined crystals, which are of a low degree of hardness (2·0) and vary in specific gravity between 2·6 and 2·7. The colour of the freshly won mineral is whitish or pale blue, but soon changes to a dark blue in the air, owing to the oxidation of the ferrous phosphate, originally present, into ferric phosphate.
Vivianite can be transformed into a pigment by a simple process of crushing and levigation; but the product is never very handsome, and, at best, is only suitable for quite common paint work, though characterised by considerable stability.