is suitable for pottery-making. The clays from which 

 such coarse substances as bricks and flower-pots are 

 made, for example, contain too many other impure 

 ties besides iron to make them available for pottery. 

 Brick clays are common in almost every country and 

 climate. Not so the "blue" or "ball" clays. The 

 available beds of these are comparatively few, some of 

 them lying from sixty to a hundred feet below the 

 surface of the ground. But even when covered to this 

 depth, the substance is of sufficient value to pay for 

 its excavation and removal. As it comes from the 

 beds it is of a bluish color, due to organic matter; 

 but when this is removed by moderate heat, the clay 

 becomes practically pure white. As found in nature 

 the stratum of clay is from three to six feet thick, 

 usually covered by a layer of sand. For shipment, 

 the clay is cut into blocks of a size convenient for han- 

 dling, which, when dried, have the appearance of gray 

 stone. 



The chemical composition of blue or ball clay, 

 according to Muspratt, is as follows, although differ- 

 ent specimens would show variations from this: 



Silica 46 . 38 Lime i . 20 



Alumina 38 .04 Magnesia (trace) 



Protoxide of Iron .... i .04 Water 13-44 



The mass of any clay varies with the amount of 

 water it contains. When dried, some clays lose as 

 much as thirty per cent, in weight. On the other hand, 

 if clay is stirred in great quantities of water, its parti- 

 cles are so small and so light that a homogeneous 

 mixture having the consistency of thin syrup can be 



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