808 



CLAY 



broken ; sp. pr.=a2-6 ; adheres to the tongue, and breaks down in water. Slate-clay 

 is ground and reduced into a paste with water for making fire-bricks ; for which pur- 

 pose it should bo as free as possible from lime and iron. 



2. FIHE-CLAT. In this country the geological position of the fire-clay, which is 

 largely employed in the manufacture of fire-bricks, glass-house pots, &c., is imme- 

 diately beneath the coal, each bed of which rests upon a stratum of greater or less 

 thickness of a clay possessing the peculiar qualities of fire-clay, and distinguished in 

 the mining districts, from the position it occupies with reference to the coal, by the 

 name of under-day. The Stourbridge clay is of this character. The following analysis 

 of Stourbridge clay was made by Mr. C. Tookey in Dr. Percy's laboratory : 



99-66 



3. COMMON CLAY OK 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 com- 

 mencement of translucency which the purer clays exhibit. Although soft to the touch, 

 the common clay wants uuctuosity, 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. 



4. POTTERS' CLAY, OR PIPE-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 degree of hardness. Werner called it pipe-clay. Good plastic 

 clay remaine white, or if grey before, becomes white in the porcelain-kiln. The clay 

 from Poole in Dorsetshire is a celebrated potters' clay, and the clay from the neigh- 

 bourhood of Newton Abbot in Devonshire is a well-known pipe-clay. The following 

 is an analysis of Poole clay, made in Dr. Percy's laboratory, by Mr. W. Weston : 



\ hygroscopic 



99-36 



Plastic Clay. The geological position of the ' Plastic clay' of geologists, (the old 

 name of the ' Woolwich and Beading beds '), is beneath the London-clay, and above tho 

 sand (Thanet 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 Vanquelin, gave Silica, 43-5 ; alumina, 33'2 ; lime, 0'35 ; iron, 1 ; 

 water, 18. 



This clay is employed as a fire-clay for making tho bungs or seggars, or coarse 

 earthenware cases, in which china-ware is fired. 



The plastic clay of Dorsetshire which supplies the great Staffordshire potteries, 

 occurs near the base of the Bagshot beds (Middle Eocene). It is grey coloured, less 

 unctuous than that of Dreux, and consequently more friable. It becomes white in the 



