CLAY. Any natural earthy mixture, which possesses plasticity and ductility when kneaded up with water, is in com mon langi.age called a clay. All mine ralogists, however, have comprehended the appellation, not only clays, properly so called, but a few other mine ral substances nearly allied to some of, the clays, and which become plastic by decomposition. Clay, however, is by no means strictly a mineral species, being in most cases the result of the decompo sition of other minerals. It seems ad visable, therefore, to consider the pro perty of plasticity as an essential cha racter, and to exclude from the class of clays all earthy bodies that are destitute of it.
Mineralogists have generally arranged all the plastic clays under two species, rather from the economical uses to which they are applied, than according to their external characters, composition, or geo logical situation. The first species is the white infusible porcelain clay, and the second contains all the rest com pounded together, under the general appellation potter's clay. We have, however, a different arrangement in Aikin's dictionary, which we shall lay be fore the reader.
Essential character: plastic by inti mate mixture with water.
1. Porcelain clay. Its colour is gene rally reddish white, also greyish and yel lowish white; it has no lustre, no trans parency. It occurs either friable or com pact; stains the fingers ; adheres to the tongue; is soft but meagre to the feel; is easily broken. Specific gravity about 2.3. It falls to pieces in water, and by kneading becomes ddctile, though not in a very great degree. The Cornish por celain clay'certainly originates from the decomposition of felspar, and contains particles of quartz, mica, and talc, from which it is separated by eleutriation. The Chinese kaolin also contains mica, and is probably of the same origin as the Cor nish. The same remark may be applied to the French, &c. It is, however, by no means certain, that all porcelain clay is derived from felspar, as it varies consi derably in its composition and fusibility; all the kinds indeed are infusible at any temperature less than a white heat ; but some, especially the Japanese, are re fractory in the most powerful furnaces.
The Cornish clay, according to Wedge wood, consists of 60 per cent. alumina, nd 40 silex 2. Steatitic clay. Its colour is a light flesh red, passing into cream colour ; its texture is minutely foliated ; it has a slight somewhat greasy lustre, and takes a polish from the nail. It stains the fin gers, is very friable, and has a smooth unctuous feel. When laid on the tongue, it dissolve:- into a smooth pulp, without any gritty particles. It is very plastie, and has a strong argillaceous odour. It occurs in nodules, in a hard cellular horn stone, that forms large mountainous masses near Conway, in North Wales, and origi nates from the decomposition of indurat ed steatite.
3. Clay from slate. Its colour is ash grey, passing into ochre-yellow : its tex tare is foliated : it has a smooth unctuous feel, and its siliceous particles are so small, as to occasion scarcely any gritti ness between the teeth. It occurs in thin beds on the tops of the softer kinds of slate-rock, and from its impervious ness to water is always found lining the bottoms of the peat-mosses, with which this kind of mountains is generally cover ed, and in these situations it is of- a white ash colour, being deprived of its iron and carbon by the acid of the peat. It also occurs in thicker beds at the foot of the mountains, but is of a darker colour, and less plastic.
4. Clay from shale. its colour varies from greyish blue to bluish black: its texture is foliated : it has a smooth unc tuous feel, yokes a polish from the nail, is excessively tenacious and ductile, and has but a slight degree of grittiness. It occurs abundantly in all collieries, and is produced by the spontaneous decompo sition of the shale with which the beds of coal are covered. A sandy clay, of a greyer colour, and more refractory na ture, is procured from the decomposition of the Indurated clay that forms the floor of the coal, and is provincially call ed clunch. The Stourbridge clay, from which crucibles, glass-house pots, &c. are made, is of this kind.