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. 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. GRAPHITE Graphite, also known as plumbago or blacklead, consists of carbon. It is usually spoken of as pure carbon, but from a very large number of carefully conducted analyses, it would appear that native graphite is never quite pure, even the finest grades of the mineral containing 96·8% of carbon at the most. The accompanying substances--which in some cases form nearly 50% of the whole--are of divergent composition and consist of iron, silica, lime, magnesia and alkalis. Even the combustible constituent of graphite is not pure carbon, but always contains a certain--though small--proportion of volatile substances. These slight traces of volatile matter are of considerable importance in connection with the hypothesis on the origin of the mineral.