Clay

Clay, an earthy hydrous silicate of aluminium. It is very fine-grained, and absorbs large quantities of water, which it holds interstitially. This renders it plastic, or capable of being moulded into and retaining any shape, and also makes it impermeable to additional water. It thus forms a tenacious soil, holding wet on its surface, and is known to farmers as "cold," since much of the sun's heat that falls on it will be absorbed in evaporating the water. When gradually dried, clay falls to an impalpable dust; but when burnt it undergoes partial fusion into the most imperishable substances, porcelain, earthenware, pottery, and brick. Clays seem always to have resulted, from the chemical decomposition of felspathic matter, but may be deposited by either fresh or salt water, and may result from the decay of volcanic pumice, etc., or as an insoluble residue after the removal of calcium carbonate from limestones (at least mainly) organic. Kaolin (q.v.) is a very pure clay, free from iron or alkali, used for china, resulting especially from the decay of the felspar in granite. Pipe-clay, as in the Eocene rocks of Corfe, Alum Bay, and Bovey Tracy, is a similar white clay which shrinks too much for pottery. Fire-clay, also free from iron and alkali, as in the beds below coal-seams known as "seat," or "bottom," clay, contains free silica and resists intense heat without melting. Most clays contain potash, iron-oxides, and some magnesia, the iron-salt colouring them blue before exposure to air, then brown (on its hydration), and the familiar red of brick, when burnt. Clays belong to almost all geological periods; but those of the older series are mostly compressed into slates or shales. Among the most important clays are the Lias, Oxford, Kimmeridge, Gault, and London clays, all marine and at first blue; the Weald clay, blue and freshwater; the Plastic, Mottled, or Woolwich and Reading clay, estuarine; the Boulder-clay, with fragments of other rocks, of glacial origin, often chalky and almost white; the brown "Clay with flints," the soil resulting from the surface decay of the Chalk; the terra rossa, or similar red-brown earth on other flintless limestones; and the red cave-earth, formed similarly within caverns. In no rocks are fossils more delicately preserved than in clays.