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.
In order to render the boles suitable for painting, they are put through a somewhat different treatment than the other earth colours. The freshly dug material is first sorted, the uniformly coloured lumps of fine texture being set apart and suffused with water, with which they form a pasty mass of low plasticity, which is kneaded by hand to make it homogeneous, and is then stirred up with more water. When the lumps have distributed in the water, the latter is drawn off into a second tub, and the residue is stirred up with fresh water, the treatment being repeated until the effluent no longer shows any signs of colour.
The liquid in which the finely divided bole is suspended is left to settle, and the bole subsides as a fine powder, which is dried to the condition of paste, pressed into moulds and dried completely.
Owing to its low content of ferric oxide, the colour of bole is not particularly bright, but is very permanent—a property equally shared by all the other ferric oxide pigments.