58 SOILS. 



clay, brick-clay, and many others of more or less local use 

 only. As these materials practically concern the farmer in 

 very many cases, they may properly find a brief discussion 

 here. 



The variety-names enumerated above in the order of the 

 actual contents of the materials in true clay substance ("col- 

 loidal clay "), are partly based upon that fact, partly upon the 

 degree of plasticity attained by that substance, and essentially 

 upon the nature and amount of foreign admixtures associated 

 with it. Thus, porcelain clay is chalky kaolinite, sometimes 

 associated with enough of pure white plastic clay to render it 

 workable in the potter's lathe, but more commonly requiring 

 to be molded in porous molds; it is very refractory to heat. 

 Pipe-clay is also white, but more plastic and usually less re- 

 factory. Fire-clay is a refractory pipe-clay commingled with 

 some coarse infusible material, such as quartz sand (or the 

 same clay burnt and crushed), in order to prevent excessive 

 contraction and change of shape in drying and burning. Pot- 

 ters' clay is a much less pure, and from that cause more fusible 

 clay, which when burnt forms at a moderate heat a semi-fused, 

 more or less hard mass, such as crockery and pottery ware. 

 Brick-clay is a still more impure clay, or loam, containing con- 

 siderable sand and usually iron oxid, and largely falls already 

 within the limits of tillable soils or subsoils, rendered fusible 

 by the presence of relatively considerable amounts of iron, 

 magnesia and lime. 



Iron colors natural clays either red, yellow, green or blue ; the latter 

 two colors turning to yellow or red on exposure to the air, and to red 

 on burning. Black color is usually due to carbon, such clays often 

 turning white on heating. 



Clays containing much lime are usually of a gray or whitish tint, and 

 like the soft crumbly limestones are often called marls, and are used as 

 such for land improvement. But it should be understood that the 

 colors of clays, mostly derived from some iron compound, have little to 

 do with their uses in the arts, except that no deeply colored clay (black 

 excepted) is refractory in the fire. 



