CLAYS. (Fa., Glaise ; GER., Then.) Clays may be defined as hydrous silicates of alumina, mingled with more or less mineral im purities, and coloured by metallic oxides and organic matters. Usually they are soft, sectile, and plastic, emitting, when breathed upon, a peculiar odour, known as " argillaceous." They chiefly occur as superficial deposits in river valleys, estuaries, dried lake beds, or in glacial drifts. Some of those found in the Tertiary formation aro sufficiently plastic for use ; but the beds of the older formations, except a few in the Carboniferous, Lias, ancl Oolite, are found to have acquired a slaty texture. There are several distinct varieties of olay, each possessing characteristics which fit it for a particular purpose.
Briek-elay.—Clays" fitted for the' manufacture of ordinary bricks, tiles, drain pipes, &c., are widely diffused ; but the most important aud extensive beds are those of the glacial or immediately post-glacial period. These clays are usually of fine texture, and coloured red, blue, grey, or yellow, according to the character of the formation whence they are derived, the colour being always due to the presence of foreign matters. Estuary silts, the Tertiary system, and the outcrops of the argilla °eons beds of older systems also yield considerable quantities of brick-clay. It consists of a coarse and irregular admixture of silicate of alumina—pure clay—with sand, iron, lime, alkalies, and a few accidental impurities. Analyses of four varieties of goad brick-day from Dunfermline, Durham, Tullarone, and Portobello, reveal the following proportions of the principal ingredients :—Silica and sand : 64.14, 61.09, 66.16, 53.95 ; alumina : 13.54, 19.91, 16.08, 25.55 ; oxide of iron, 7.57, 6.75, 8.38, 8.06; lime, 1.90, 3.36, 1.88, 0.68 ; potash and soda : 1-51, 2.83, 1.83, 1.54. The clay must be free from large stones ; but a considerable quantity of sand is not objectionable, some clays containing so much as to bring the total percentage of silica up to nearly 90. An excess of
iron, lime, or alkalies is liable to make the clay run into a glass during the process of baking. Before manufacture into bricks, &c., the clay undergoes a " tempering," or exposure to the weather, especially to frost, which much improves it. It is then carefully ground, and mixed with wat,er, sfter which it is left to dry.
China-clay.—This substance is also known as Kaolin, porcelain clay, and Cornish clay. It arises from the natural decomposition of felspar in soft disintegrating granite, gneiss, and porphyry ; the rocks which are rich in soda-felspar yielding it most abundantly. The main supplies of this country are derived from Cornwall and Devon ; in continental Europe, beds of good quality exist in France, Bavaria, Saxony, Prussia, Bohemia, Bornholm island, and Hungary ; in China, it is very plentiful; and in the United States, it occurs in many localities.
The approximate composition of china-day may be stated as silica, 47.2 ; alumina, 39.1 ; water, 13.7 per cent. Often a little iron, lime, and potash or soda are left in the prepared article, by the imperfection of the cleansing process. The most important characters are colour, plasticity, and a capacity for hardening under the influence of heat.
The china-clay industry of Cornwall and Devon has been admirably described by J. H. Collins, F.G.S., in a paper recently read before the Society of Arts.
Oceurrenee.—The natural clay rock is almost always covered with a thick layer of stones, sand, or impure and discoloured clay, known as " overburden." This capping often much resembles glacial drift ; but it never contains any scratched or glaciated stones, or travelled blocks. It varies in thickness from 3 ft. to 40 ft., and must, of course, be removed before the clay can be wrought. The clay rock, being a decomposed granite, consists of china-clay, irregular crystals of quartz, and flakes of mica, with sometimes a little sehorl and uudecomposed felspar.