The mineral known as clay is, in all cases, a product of the decomposition of other minerals, mainly felspar. This substance is a double silicate of alumina and potash, K2O.Al2O3.(SiO2)6. Pure kaolin is Al2O3(SiO2)2 + 2H2O, or 46·50% silica, 39·56% alumina, 13·9% water.
Clay may be supposed to have been formed by the conversion of felspar, under the action of air and water, into silicate of alumina, the silicate of potash being dissolved out. Being insoluble, the silicate of alumina would be transported by the water, in a very fine state of division, and finally deposited as a sediment, which in course of time became a solid mass. This, when again brought into contact with water, forms a very plastic pulp which, when dried and baked, forms a solid mass, brick, which is no longer affected by water. Perfectly pure clay forms a white mass, which, under the name of China clay or kaolin, is used for making porcelain, and is only occasionally met with in large quantities.
Pure kaolin is characterised by its great chemical indifference, being decomposed only by strong alkalis and sulphuric acid. At the high temperature of the pottery kiln, kaolin sinters to a very compact mass, but cannot be fused, except when small quantities are subjected to the intense heat of the oxyhydrogen flame, whereupon it fuses to a colourless glass of great hardness.
In an impure state, silicate of alumina occurs frequently in Nature, and then forms the minerals known under the generic names of clay, loam, marl, etc. These impure clays contain varying proportions of extraneous minerals which produce changes in the physical and chemical properties. They are grey, blue or yellow in colour, the grey and blue varieties mostly containing appreciable quantities of ferrous oxide, whilst the yellow kinds contain ferric oxide. When fired, all of them become yellow or red, the ferrous oxide being transformed into ferric oxide by the heat. Some fairly white clays are high in lime, which makes them fusible at high temperatures. In some very impure kinds, even the comparatively low heat of the brick-kiln is sufficient to cause partial fusion. For colour-making, the white clays, especially kaolin and pipeclay, form a highly important material, being procurable at very low prices and fairly easy to prepare.
The white clays are either used as pigments by themselves, or for mixing with other colours of low specific gravity.
(B) Yellow Earths
The number of yellow earths is large, but most of them exhibit a certain similarity in chemical composition, the pigmentary principle in the majority being either ferric oxide or ferric hydroxide. The former is yellow, the latter brown, and the colour of the minerals resembles that of the preponderating iron compound.
Brown Ironstone
The mineral known as brown ironstone consists of ferric hydroxide, and usually forms compact masses, no decided crystals having, so far, been observed. The lumps have an irregular or earthy fracture, a hardness of 5–5·5, and a sp. gr. between 3·40 and 3·95. The colour ranges, in the different varieties, from yellowish (rusty) brown, through cinnamon to blackish-brown. The chemical composition of the pure lumps may be expressed by the symbol 2Fe2O3 + 3H2O; but a little manganese oxide and silica is generally present even in the pure kinds.
The chief varieties of this mineral are:—