NATURAL SOURCE OF PORCELAIN CLAY. 81 



On the surface, and for a considerable depth 

 into their substance, the rock is altered to a 

 soft matter resembling mortar. This is col- 

 lected and washed ; the water which comes from 

 the washing of it being of the colour of milk, 

 in consequence of its containing a quantity of 

 white earthy substance suspended in it, is con- 

 ducted into tanks, and in its passage through 

 several reservoirs, deposits this white earthy 

 matter at the bottom. The tanks are then 

 emptied of water, and the white deposit being 

 removed and dried in the open air, and subse- 

 quently more completely by a drying -stove, 

 constitutes the beautifully fine white clay em- 

 ployed in the manufacture of porcelain. Not less 

 than 10,000 tons of this white clay, thus derived 

 from the decomposed material of the granite 

 rock, is exported annually for the use of the 

 potteries. Its chemical composition the com- 

 position in great part of our china cups and 

 ornamental ware is alumina (the basis of com- 

 mon alum), silicic acid, a little alkali and lime, 

 and in the unburnt state, a large proportion of 

 water, together with a variable amount of sand. 

 The Chinese, as well as ourselves, employ the 

 same material for the manufacture of their ex- 

 quisite porcelain. 



The composition of granite, unchanged as 

 contrasted with that which has been thus de- 



G 



