The water frequently present in iron mines sometimes contains large quantities of sediment, which consist mainly of iron ochre and can be advantageously worked up into pigments. There is scarcely any need to mention that all substances containing ferric oxide can be used for making any of the pigments obtainable from the oxide itself, the only difference between the various raw materials being their degree of purity, so that it is not always so easy to obtain a certain desired shade from a given material in such beauty as is furnished by another material, the small quantities of impurities associated with the ferric oxide having, in many instances, an important influence on the colour.
(D) Blue Earths
Only two minerals are known which are capable of direct use as blue pigments, viz. vivianite (native Prussian blue) and copper carbonate (azurite, ultramarine), and as neither of them is particularly handsome, they are only used for unimportant work. Lapis lazuli is no longer employed.
Azurite, or Ultramarine
This mineral, which is of frequent occurrence with malachite and other cupriferous minerals, forms small crystals of a beautiful deep azure blue consisting of cupric oxide in combination with carbon dioxide and water, expressed by the formula 2CuCO3, Cu(OH)2, or Cu3(OH)2(CO3)2, and containing 69·19% of cupric oxide, 25·58% of CO2 and 5·23% of water. The colour of the powdered mineral is much paler than that of the crystals. The pigment, which is used for cheap paints, is not particularly stable, and loses much of its beauty when applied to plaster.
Vivianite
This mineral occurs in many places as crystalline masses, but also forms earthy deposits, some of which, especially in certain bogs, attain considerable thickness. The colour is between indigo and blackish blue; and the freshly won mineral often has an unsightly whitish appearance, which, however, soon changes into the pure blue. The cause of this peculiarity is due to the fact that vivianite originally consisted of hydrated ferrous phosphate, which is white, this compound being transformed, under the influence of the air, into the blue ferric phosphate.
Vivianite contains ferric oxide, phosphoric acid and water, but in variable proportions. The original composition, expressed by Fe2(PO4)2 + 8H2O2, corresponds to 43·03% of ferrous oxide, 28·29% of P2O5 and 28·68% of water; but, in the air, part of the ferrous phosphate is oxidised to basic ferric phosphate, so that the content of ferrous oxide may range from 9·75 to 42·71%, and that of ferric oxide between 1·12 and 38·20%. Vivianite is also sold as blue ochre, and is now seldom used as a painters’ colour, owing to the introduction of a large number of artificially prepared blues, which are superior to vivianite in colour and are cheaply made. However, it can still find application in localities where it is obtainable in quantity.
(E) Green Earth Pigments
The green earth pigments comprise green earth (Verona green) and malachite. Like the blue earths, they cannot lay any particular claim to beauty, but they are very cheap, and consequently are largely used where low price is the chief consideration.