The earth colours might be divided into such as occur ready-formed in Nature, and require only mechanical preparation, and which either require special treatment (e. g. calcining), or are artificial products (like the iron oxide mentioned above). Since, however, such a classification would not advantage our knowledge of the nature of this class of colours, it appears useless and superfluous, and we will therefore simply confine ourselves to arranging the earth pigments according to their colour—white, yellow, red, etc.
Adopting this classification, the following minerals and chemical products may be considered as earth colours:—
White.—These include the varieties of calcium carbonate, such as chalk, marble, precipitated chalk, calcium phosphate, calcium sulphate (in the form of gypsum, alabaster, muriacite and the precipitated gypsum produced as a by-product in many chemical works), heavy spar, the different varieties of clay, and magnesia.
Yellow.—This group comprises ferric hydroxide (hydrated oxide of iron) in the form of the various minerals known as ochre; all the preparations chiefly composed of this hydroxide, and all those prepared by artificial means. A very important member of this group is orpiment; the other arsenical compounds frequently met with native, being however, on account of their poisonous properties, no longer used as pigments.
Red.—Chief among the red earth colours are those consisting of ferric oxide (iron oxide), under various names. The only other member of the group is the far rarer vermilion.
Blue.—The blue earth colours are few in number and of no particular beauty; but they are of importance on account of their cheapness and because all the artificial blue pigments are rather expensive. Two products in particular merit attention in this connection, namely, ultramarine, and the mineral known as blue ochre or blue ironstone. The latter, as a matter of fact, cannot be used for anything else than a painters’ colour, and can be obtained at a low price; whereas ultramarine also forms a valuable raw material for the recovery of copper, and is therefore dearer.
Green.—This group, again, contains only two members, viz. malachite green (chrysocolla), and the green earths (seladonite), known as Verona, etc., green. These occur fairly often in Nature, and the green earths in particular find a wide industrial application by reason of their low price. Malachite green is very similar, in chemical constitution, to ultramarine; and both form sources of copper and are consequently expensive.
It should be mentioned that both ultramarine and malachite green can only be profitably made into pigments where the minerals can be obtained cheaply, since both of them can be manufactured where artificial pigments are produced, and are put on the market under the same names as the native articles. The very low price of the green earths makes them highly popular as colouring matters in certain branches of industry, and they are very largely used by wall-paper manufacturers.
Brown.—This is a large group, and the pigments composing it are specially distinguished for their beauty and depth of colour, on which account they are used in the finest paintings. Here, again, it is ferric oxide, in combination with water—and therefore ferric hydroxide—that furnishes a large number of the members of the group. Like the renowned Siena earth, the artists’ colours known as Vandyck brown, bole, Lemnos earth, umber, etc., mainly consist of more or less pure ferric hydroxide. These minerals are, moreover, specially important to the colour manufacturer, inasmuch as most of them enable a large number of different shades to be obtained by a simple method of treatment consisting merely of the application of heat in a suitable manner; and these colours are among the most excellent we possess, by reason of their beauty and permanence. Amongst this series must also be classed native manganese brown, which chiefly consists of a mixture of manganese oxide and the hydrated peroxide of the same metal.
Black.—There is really only one member of this class, which, however, is frequently used, viz. that form of carbon occurring as hexagonal crystals and known as graphite. Another natural black natural product, occasionally used as a painters’ colour is the so-called black chalk. However, since black pigments can be produced very cheaply by artificial means, the natural colours find only a limited application; and only in one instance is graphite used alone, viz. for making blacklead pencils.