Green Earth
In Nature, green occurs as an entirely non-crystalline earthy mass, which is probably a decomposition product of augite. It has a close, earthy fracture, a colour between seladon and olive green, and a slightly greasy appearance. In point of chemical composition it consists of silica, alumina, magnesia, sodium, potassium, ferrous oxide and water, the usual representative formula being ROS1O2 H2O, in which RO symbolises a metallic oxide.
The colour is due to ferrous oxide; and if left exposed to the air for a long time, or subjected to powerful calcination, the great affinity of ferrous oxide for oxygen causes the colour to turn red and red-brown.
Green earth is found in many localities, e. g. Bohemia, Hungary, the Tyrol and Cyprus, the finest, however, occurring near Verona, on which account it is known as Veronese earth.
Malachite
The commercial pigment consists of powdered malachite, a mineral which usually occurs in compact masses of a handsome emerald green colour, though isolated lumps exhibit considerable variation in shade, some of them being dark green and others very pale. In chemical composition, malachite is closely allied to azurite, consisting of cupric oxide, carbon dioxide and water, and the difference is entirely one of percentage proportions. The formula is CuCo3, Cu(OH)2, or Cu2(OH)2CO3, corresponding to 71·90% of cupric oxide, 19·94% of carbon dioxide and 8·16% of water.
Powdered malachite (even the dark green varieties) is always rather light in colour, and for this reason is not much used. Furthermore, the mineral is rather hard (3·5), and is consequently difficult to grind; in addition to which the mineral is fairly expensive, on account of its employment as a source of copper, particularly fine pieces being also used as ornaments or for making works of art. Moreover, like all copper compounds, it is very sensitive to the action of sulphuretted hydrogen, and liable to discoloration in course of time.
(F) Brown Earth Pigments
Numerous minerals are adapted for the manufacture of brown pigments. On the basis of chemical composition, they may be classed in two groups; those consisting of ferric hydroxide, and those in which the brown colour is due to organic substances.
The first group comprises the minerals which have already been mentioned in connection with the red earth pigments, bole and brown ochre (umber), Terra di Siena, Cologne earth and a number of other earths rich in ferric hydroxide belonging to this category. The second, or organic group, includes compounds that are very rich in carbon and are therefore of a very dark colour, the shades ranging from light brown to black, e. g. the true umbers and asphaltum.