The number of minerals that can be directly used as red earth pigments is comparatively small, and by far the greater proportion consist of ferruginous colours, a few of which are obtained by the mechanical treatment of native iron ores or clays coloured red by ferric oxide, the majority, however, being formed by burning certain materials of another colour. To these belong nearly all the materials mentioned in connection with the ochres and the brown iron colours, together with a few by-products of the chemical industry.
In addition to the foregoing, which have ferric oxide for their pigmentary principle, is the native mercury sulphide, occurring, as scarlet, crystalline masses, under the name of cinnabar (vermilion). The only reason for including natural vermilion with the earth colours is to make the list complete, the largest proportion of this pigment being prepared by artificial methods. The product sold as “Chinese” vermilion may, in former times, have really been introduced from China into Europe, and prepared there by grinding and levigating the best-coloured lumps of the natural cinnabar; but, at the present time, all the vermilion made—in Europe at least—is from sulphur and mercury, by artificial processes, and the name Chinese vermilion is merely retained to designate a particularly fine grade.
On the basis of occurrence and chemical properties, the red earths can be classified into several groups. The first comprises natural products requiring only mechanical preparation, such as the minerals known as hematite, micaceous iron ore, Elbaite, etc., and the special modification of red ironstone termed raddle. All these minerals consist almost entirely of ferric oxide in a pure state. The mineral, bole (red chalk, terra sigillata, Lemnos earth), is chemically allied to the ochres, being, like them, composed of alumina, frequently accompanied by lime and small quantities of magnesia, but differing in that ferric oxide is always present in bole, whereas the ochres always contain ferric hydroxide.
The second group consists of the artificial reds obtained by burning or calcining raw materials, whose ferric hydroxide is more or less transformed by heat into ferric oxide, such as vitriol ochre, pit ochre and alum sludge.
Of late years the artificial earth colours have attained a high degree of importance. They are obtained in large quantities in the manufacture of sulphuric acid from green vitriol. Formerly, it is true, they were also used as pigments under the name of caput mortuum or colcothar, but were not held in much esteem; and it is only within recent times that it has been discovered that these inferior by-products can be converted into very handsome and brilliant colours, which now form important articles of commerce.
Bole
Bole, Lemnos earth, terra sigillata, etc., is, for the most part, a product of the decomposition of highly ferruginous minerals, and occurs, in the form of lumps, having a conchoidal fracture, in pockets or detritus. The lumps have a sp. gr. of 2·2–2·5, are Isabella brown to dark brown in colour, and give a slightly greasy-looking streak. There are two distinct varieties of bole: the one adhering firmly to the tongue, whilst the other lacks this property and, when placed in water, crumbles down to powder in emitting a peculiar noise.
The composition of the boles varies, but all of them may be regarded as alumino ferric silicates combined with water. Most of the specimens examined from different deposits contain 24–25% of water, 41–42% of silica, and 20–25% of alumina, the remainder consisting of ferric oxide with small traces of manganese oxide.
Some varieties, however, are exceptional and contain only 30–31% of silica and 17–21% of water, e. g. those from Orawitza and Sinope. Lemnos earth, the true terra sigillata, is mostly silica (66%) with 8% of water, and contains a smaller percentage of ferric oxide than the others. It is also of a distinct colour, lighter than the true boles and having a greyish or yellowish tinge.
The behaviour of the different kinds on burning is just as diverse as their chemical composition. Whilst some kinds are infusible at even the highest temperatures, and merely change into hard, red masses; others, again, fuse at a moderate heat. This difference is due to chemical composition, those high in silica being generally less refractory than those in which alumina preponderates.