CLAY.

305

dull, or glimmering from admixture of particles of mica. Fracture slaty, approachingsometimes 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; whichsee. 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. Itis soft to the touch, and forms, with water, a somewhat tenacious paste ; but is in gen-eral less compact, more friable, than the plastic clays, which are more readily diffusiblem water. It does not possess the property of acquiring in water that commencement oftranslucency which the purer clays exhibit. Although soft to the touch, the commonclay wants unctuosity, properly so called. The best example of this argillaceous sub-stance is afforded in the London clay formation, which consists chiefly of bluish or black-ish clay, mostly very tough. Those of its strata which effervesce with acids partake°f the nature of marl. This clay is fusible at a strong heat, in consequence of the irona nd lime which it contains. It is employed in the manufacture of bricks, tiles, and coarsepottery ware.

3. Potter's clay, or Plastic clay .—This species is compact, soft, or even unctuous tothe touch, and polishes with the pressure of the finger; it forms, with water, a tenacious,Ver y ductile, and somewhat translucent paste. It is infusible in a porcelain kiln, but as-sumes in it a great degree of hardness. Werner calls it pipe-clay. Good plastic clayre mains 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 thesand which covers the chalk formation. The plastic clay of the Paris basin is describedas consisting of two beds separated by a bed of sand. The lower bed is the proper plas-iic clay. The plastic clay of Abondant, near the forest of Dreux , analyzed by Vauquelin,gave—

®jlica, 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 earthen-w are cases, in which china ware is fired.

. 1 he plastic clay of Dorsetshire and Devonshire supplies the great Staffordshire potte-IUi' ^ * s 8 ra l' colored, less unctuous than that of Dreux , and consequently more friable.

becomes white in the pottery kiln, an£ is infusible at that heat. It causes no efferves-cence with nitric acid, but falls down quickly in it, and becomes higher colored. Its re-j.jb^miness allows of a harder glaze being applied to the ware formed from it withoutor heat requisite for making the glaze flow, affecting the biscuit either in shape

the C °* 0r ‘ *' Most of the plastic clays of France, ” says M. Brongniart, “ employed fora nd? me ware > have the disadvantage of reddening a little in a somewhat strong heat;of ] cnee it becomes necessary to coat them with a soft glaze, fusible by means of excessgj ea n a t a low heat, in order to preserve the white appearance of the biscuit. Such ahot 6 , a hull aspect, and cracks readily into innumerable fissures by alternations ofto ti? n< L C0 'h water.” Hence one reason of the vast inferiority of the French stone-ware. f e English .

Thmr orce b?* n c ^ a y> 0T Kaolin earth. —The Kaolins possess very characteristic properties.IVhen^ 6 liable in the hand, meager to the touch, and difficultly, form a paste with water,they a r ^om the coarse and evidently foreign particles interspersed through them,They h absolutely infusible in the porcelain kiln, and retain their white color unaltered,acquire en with heat like other clays, and perhaps in a greater degree; but they do notKaoli ns equal condensation or solidity, at least when they are perfectly pure. TheMost of "it genera l appear to consist of alumina and silica in nearly equal proportions.feintev,.i‘„ e , Kaolin clays contain some spangles of mica, which betray their origin fromThis • . gra nite.

the porce] n ^ m ma y b e regarded as one of their most distinctive features. Almost allites, and if 1 - 11 c }ays are evidently derived from the decomposition of the feldspars, gran-they are tob' C f )a ^ ihosc rocks of feldspar and quartz, called graphic granite. Hence,ite, iormin» >,• n< ^ on *^ * n Primitive mountain districts, among banks or blocks cf gran-

niica occur" t, • sea ™ s or partings between them. In the same partings, quartz andform of feldspar ^ re ^ cs l hc granite; while some seams of Kaolin retain the external

bn China ancM^ 6 kaolins have been found—

"^hite; but are Ja P a n. The specimens imported from these countries appear pretty°i" Trance. more unctuous to the touch, and more micaceous than therrorcelain clays..bn Saxony , -pi^ rr i*

*hsht yellow' or fl noun employed in the porcelain manufactories of that country has abat this tint is . or ’ which disappears in the kiln, proving, as Wallerius observed,

bn France lS ” 0 ‘ 0 . w ; n S t° any metallic matter.

> in. -inex-la-Perche, about 10 leagues from Limoges. The Kaolin20