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.
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.