17- Mr. Couper on the Chemical Composition of Pottery. 



in porcelain manufactories, docs not regain its plasticity, which it loses in 

 the burning, although pounded very fine, in which state it is technically 

 termed potsherd. 



Clay is obtained naturally from Cornwall, Dorset, and Devonshire, and 

 is the finer particles of decomposed feldspar, deprived of its alcali. 



(1.) The finest clay (termed China Clay) used in Britain is obtained 

 artificially from Cornwall, by running a stream of water over decomposed 

 granite, which carries with it the finer particles of feldspar, and is then 

 received into catch-pools or ponds, where it is allowed to subside. The 

 water is then run off, leaving a fine sediment, which is removed and 

 exposed to the atmosphere for four or five months, when it is ready for 

 export. By analysis of this clay, previously dried at 212°, I found it to 

 consist of — 



I. IL 



Silica, 46-32 46*29 



Alumina, 39'74 4009 



Protoride of iron, "27 ■ — 



Lime, 36 50 



Magnesia, "44 — 



Water, 1267 — 



99-80 



For the second analysis I am indebted to Mr. John Brown. 



The more common clays, which are found naturally deposited, are 

 supposed to have been produced in a similar manner to the China clay; 

 the rains having washed from the hills the decomposed rock into a lake 

 or estuary, where it has subsided and gradually displaced the water, and 

 become in the course of time perfectly firm and solid, forming fields of 

 clay. The clay is found in layers or strata lying over each other ; each 

 layer possessing some distinctive property from the other, which renders 

 each clay fitted for a peculiar purpose. 



(2.) Sandy Clay, (stiff or ball clay,) is the upper layer of clay, and 

 is used by itself for making salt glazed ware ; it is well adapted for this 

 kind of ware, in consequence of the considerable quantity of silica or 

 sand which it contains. By analysis of this clay I found it to be com- 

 posed of — 



Silica, 6668 



Alumina, 2608 



Protoxide of iron, 126 



Lime, 84 



Magnesia, a trace. 



Water, 514 



100- 

 Being previously dried at 212°, spec, gravity = 2*558. 



