Steatite or Soapstone
Steatite so closely resembles talc in most of its properties, that the two minerals were long regarded as identical. Whereas, however, talc is scarcely acted upon at all by the strongest acids, steatite is completely decomposed by prolonged boiling therewith, although both minerals have exactly the same composition.
As a pigment, steatite is far more important than talc, and, as French chalk, is largely used for drawing or writing. To prepare it for this purpose pure white steatite requires no preliminary treatment, beyond cutting the large lumps up into quadrangular prisms, which are mounted in wood, like lead pencil, and used for writing on the blackboard. The powder produced in the cutting process is made up into pastel crayons. With this object, the powder is mixed with a sufficient quantity of some mineral pigment to produce a mass of the desired shade, and is kneaded to a stiff paste with water containing an adhesive such as gum, glue or tragacanth mucilage. The mass is shaped into prisms, which, when dry, are cut into pencils and mounted in wood. Steatite being like talc, without action on even the most delicate colours, can be used as a diluent in the preparation of light shades.
CHAPTER V
YELLOW EARTH COLOURS
All the yellow earth colours, without exception, have ferric oxide as their colouring principle, the differences in shade being entirely due to the varying proportion in which that oxide is present. The various names under which they are known date back to a period when the chemical nature of these colours was still unknown, and have been mostly derived from the locality of origin.
The yellow earths can therefore be divided into two groups, according to their chemical character. The first group, in which the ferric oxide is present as hydroxide, comprises all the ochres, Siena earth, and a number of others which are obtained from native ochre by special treatment. In the colours of the second group, ferric oxide is still the colouring principle, but is combined with other substances in place of water.
It is, as a matter of fact, incorrect to rank the ochres in general as yellow earths, because they can be made to yield nearly every variety of colour from the palest yellow to the deepest red, brown and violet. These colours merit the particular attention of the colour-maker and the painter, being distinguished by very low cost of production, unusual permanence and beauty of tone. In the interests of that highly important matter to the artist, namely the production of colours of unlimited permanence, it is desirable that colour manufacturers should bestow greater care on the manufacture of these colours than has hitherto been the case. An extremely favourable point about nearly all these pigments is that they can be very cheaply prepared by artificial means, so that the manufacturer is in a position to turn out a large number of the handsomest and most durable colours with a small amount of expense and labour.