Fire-clay is found in the greatest abundance and perfection for manufacturing purposes in,
1. Slate-clay. (Thon-schiefer, Germ.) Its colour is gray or grayish-yellow. Massive, dull, or glimmering from admixture of particles of mica. Fracture slaty, approaching sometimes to earthy. Fragments tabular. Soft, sectile, and easily broken. Sp. gr. = 2·6. Adheres to the tongue, and breaks down in water. It occurs along with [pit coal]; which see. Slate-clay is ground, and reduced into a paste with water, for making fire-bricks; for which purpose it should be as free as possible from lime and iron.
2. Common clay or loam.—This is an impure coarse pottery clay, mixed with iron ochre, and occasionally with mica. It has many of the external characters of plastic clay. It is soft to the touch, and forms, with water, a somewhat tenacious paste; but is in general less compact, more friable, than the plastic clays, which are more readily diffusible in water. It does not possess the property of acquiring in water that commencement of translucency which the purer clays exhibit. Although soft to the touch, the common clay wants unctuosity, properly so called. The best example of this argillaceous substance is afforded in the London clay formation, which consists chiefly of bluish or blackish clay, mostly very tough. Those of its strata which effervesce with acids partake of the nature of marl. This clay is fusible at a strong heat, in consequence of the iron and lime which it contains. It is employed in the manufacture of bricks, tiles, and coarse pottery ware.
3. Potter’s clay, or Plastic clay.—This species is compact, soft, or even unctuous to the touch, and polishes with the pressure of the finger; it forms, with water, a tenacious, very ductile, and somewhat translucent paste. It is infusible in a porcelain kiln, but assumes in it a great degree of hardness. Werner calls it pipe-clay. Good plastic clay remains white, or if gray before, becomes white in the porcelain kiln.
The geological position of the plastic clay is beneath the London clay, and above the sand which covers the chalk formation. The plastic clay of the Paris basin is described as consisting of two beds separated by a bed of sand. The lower bed is the proper plastic clay. The plastic clay of Abondant, near the forest of Dreux, analysed by Vauquelin, gave—
Silica, 43·5; alumina, 33·2; lime, 0·35; iron, 1; water, 18.
This clay is employed as a fire clay for making the bungs or seggars, or coarse earthenware cases, in which china ware is fired.
The plastic clay of Dorsetshire and Devonshire supplies the great Staffordshire potteries. It is gray coloured, less unctuous than that of Dreux, and consequently more friable. It becomes white in the pottery kiln, and is infusible at that heat. It causes no effervescence with nitric acid, but falls down quickly in it, and becomes higher coloured. Its refractoriness allows of a harder glaze being applied to the ware formed from it without risk of the heat requisite for making the glaze flow, affecting the biscuit either in shape or colour. “Most of the plastic clays of France,” says M. Brongniart, “employed for the same ware, have the disadvantage of reddening a little in a somewhat strong heat; and hence it becomes necessary to coat them with a soft glaze, fusible by means of excess of lead at a low heat, in order to preserve the white appearance of the biscuit. Such a glaze has a dull aspect, and cracks readily into innumerable fissures by alternations of hot and cold water.” Hence one reason of the vast inferiority of the French stone-ware to the English.
4. Porcelain clay or Kaolin earth.—The Kaolins possess very characteristic properties. They are friable in the hand, meagre to the touch, and difficultly form a paste with water. When freed from the coarse and evidently foreign particles interspersed through them, they are absolutely infusible in the porcelain kiln, and retain their white colour unaltered. They harden with heat like other clays, and perhaps in a greater degree; but they do not acquire an equal condensation or solidity, at least when they are perfectly pure. The Kaolins in general appear to consist of alumina and silica in nearly equal proportions. Most of the Kaolin clays contain some spangles of mica which betray their origin from disintegrated granite.
This origin may be regarded as one of their most distinctive features. Almost all the porcelain clays are evidently derived from the decomposition of the felspars, granites, and principally those rocks of felspar and quartz, called graphic granite. Hence, they are to be found only in primitive mountain districts, among banks or blocks of granite, forming thin seams or partings between them. In the same partings, quartz and mica occur, being relics of the granite; while some seams of Kaolin retain the external form of felspar.