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LXVII. Oti the Chemical Composition of the Substances em- 

 ployed in Pottery. By Mr. R. A. Couper*. 



A LL kinds of earthenware are composed of two parts, viz. 

 -^^ the body and the glaze. 



The body is the principal part of the vessel, being the base 

 or foundation, as indicated by the term itself. The glaze is 

 a thin transparent layer of glass which covers the body and 

 fills up its pores, giving it a smooth surface with a polished 

 and a finished appearance. 



I. The substances principally employed to form the body 

 of earthenware are, clays of different kinds, flint and Cornish 

 stone. 



Clay which constitutes the base of the body of earthenware 

 is distinguished from siliceous earth by becoming plastic when 

 mixed with water, and being very soft and not gritty to the 

 feel ; also when burned, it keeps its form, and becomes firm 

 and solid; whereas siliceous earth crumbles into a powder 

 when burned. Clay when intensely heated, as in porcelain 

 manufactories, does 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 felspar 

 deprived of its alkali. 



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 felspar, and is then received into catchpools 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. ir. 



Silica 46-32 46*29 



Alumina 39*74 40*09 



Protoxide of iron . . '27 '27 



Lime '36 '50 



Magnesia .... •44< 



Water and some alkali 12*67 12*67 



99*80 99*82 



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

 The more common clays, which are found naturally depo- 



* R ad before the Philosophical Society of Glasgow, April 28, 1847, 

 and communicated by Dr. R. D. Thomson. 



2 F2 



