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.
CHAPTER X
BLACK EARTH COLOURS
Only two minerals are known that can be used as black earth colours, namely black chalk or shale black, and blacklead or graphite. Whereas the former of these is of merely subordinate importance, most of the black chalks being prepared artificially, graphite is all the more so because it is employed, not only as the sole material for lead pencils, but also for making graphite crucibles, as blacklead stove polish, as a lubricant, etc. One of its numerous applications is in connection with the electro deposition of metals, its high electrical conductivity causing it to be used for coating the interior of the moulds in which this deposition is effected.