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.
Native Ferric Oxide as a Pigment
In nature, ferric oxide forms extensive deposits, which, by reason of the light red colour characteristic of certain varieties of ferric oxide, are largely employed in painting. These colours may be classed among the oldest known to mankind, ferric oxide pigments having been used frequently in the most ancient paintings.
The most important varieties of ferric oxide for our purpose are: iron glance, with its modifications, micaceous iron ore and frothy hematite; red hematite, and raddle.
Iron Glance
This substance forms handsome black crystals of very high lustre, which, when small and scaly, constitute micaceous iron ore. Both, when rubbed down, furnish a dark red powder of no particular beauty. Micaceous iron ore forms the transition stage into frothy hematite, or iron cream, the sole difference being that the crystals of the latter are much smaller, and the scales finer, the iron-black colour passing gradually into cherry red. At the same time, the lustre, though still high, loses most of the metallic sheen exhibited by micaceous iron ore.
Hematite
The variety known as hematite or bloodstone, sometimes occurring as shiny nodules, is distinguished by its handsome red colour. Some of the lumps are composed of long, thin crystals grouped about a common centre so as to form a globular mass. Despite its bright colour, the hardness of hematite (between 3 and 5) prevents it from being used as a pigment, the value of the product not being commensurate with the cost of reduction.