CLAY, any natural mixture of earths which breaks down or disin tegrates in water, and affords a plastic ductile mixture. It depends upon this property, rather than colour or composition, whether an earthy body belongs to the class of clays. There are many varieties of clay used for different purposes.
Pipe-Clay is of a grayish-white colour, has an earthy fracture, and a smooth greasy feel; it adheres to the tongue, and is very plastic, tenacious, and infusible. Its name shows the purpose to which it is applied. It is found near Poole in Dorsetehire.
Potter's-Clay is of various colours, and disintegrates by exposure to the air; when mixed with sand it is made into bricks and tiles. It is found in Hampshire, Berkshire, Devonshire, and is largely used in the Staffordshire Potteries. The Hampshire clay yielded by analysis— Silica 51 Alumina . 25 Limo with some Oxide of Manganese and wdter.
Clay has the general properties above described, but is of a dark colour, owing apparently to all admixture of carbonaceous matter. It is most extensively employed in the manufacture of crucibles, and especially for those used in glass-making. It is ex tremely refractory in the fire. It yielded by analysis— Silica . ..... . 57 Alumina . 30 Moisture A trace of Iron and Carbonaceous 31atter.
It appears to have originated from the disintegration of shale.
Brick-Clay, or Loam, varies much in appearance, texture, and composition; its colour is dependent upon the proportion of oxide of iron which it contains. It lies in abundance upon the London Clay, and frequently rest,' upon an interposed bed of sand. The organic remains found in it are few, but it sometimes contains the teeth of elephants.
London Clay is a very extensive deposit of a bluish clay, except near the surface, where it has often the usual clay colour. It forms
the greater part of 3liddlesex, the whole of Essex and Suffolk, and part of and frequently rises almost to the surface. Some of the lower beds aro yellowish-white or variegated. This clay occa sionally includes beds of sandstone, and of a coarse argillaceous limestone, of which Parker's Roman Cement is made. It contains also frequently the bones of the crocodile, turtle, kc.
Plastic Clay skirt,' the London Clay within the London Chalk basin, and it appears also at the Isle of Wight. This formation consists of a variable number of sand, clay, and pebble-beds irregularly alter nating, lying immediately upon the chalk ; it contains sonic appearance of coal, decidedly of vegetable origin, pyrites, oyster-shells, and the branches of tree& The sand-beds of the Plaetie-Clay formation are the grand reservoir of soft-water from which the deep wells in and around London arc supplied.
Kaolin, or Porcelain-Clay, is of various shades of white; it is dull and opaque; occurs friable or compact; feels soft to the fingers and adheres to the tongue. It is infusible, and its specific gravity Is A large tract of this clay, which includes crystals of felspar, quartz, and mica, occurs near St Austell in Cornwall. The porcelain manufactures of Worcester are supplied from it. According to Wedgwood It consists of 60 parts alumina and 40 silica. It probably arises from the decomposition of felspar. This clay occurs in France, Saxony, and Austria. Various other kinds of clay are met with in different situations ; their nature and composition depend upon thane of the rocks from the disintegration of which they have been formed; thus slate, steatite, and trap, each yields a different kind of clay.