2 Burton, Pluvihisvi in Pottery Workers. 



composed of a body or clay substance, which may 

 be a natural claj', of no matter what kind, mixed with 

 varying pro[)ortions of fusible substances such as felspar, 

 or witii hardening and infusible substances such as 

 ground sand or flint. The exact composition of this 

 body or paste, together with the nature of the actual 

 ingredients used to arrive at the required chemical 

 composition, determines the nature of all the manufacturing 

 processes through which the material passes in becoming 

 finished pottery. For instance, in the most highly 

 developed form of pottery known, i.e., Hard-paste porcelain, 

 familar to everyone in the form of Chinese vases and 

 Berlin porcelain basins and crucibles, the body or paste 

 is a mi.xture, principally, of china claj' and felspar, while 

 the glaze is, practically, pure felspar. It is evident that 

 the tem[:)erature required to melt a felspar so that it will 

 uniformly glaze a piece of pottery must be exceedingly 

 high, indeed it is generally estimated at about 1500*^0. 

 In other forms of pottery, where the firing temperature 

 can be carried to the point of incipient fusion of the clay- 

 substance without bending the pieces, a glaze is obtained 

 by flooding the kiln, at that high temperature, with 

 vapours of common salt. A reaction takes place 

 between the vapours of common salt, the water vapour 

 always present in the kiln gases, and the free silica in 

 the body of the ware, resulting in the formation of a thin 

 glassy coating on the ware, known as salt-glaze, and the 

 liberation of hydrochloric acid. Here again the firing 

 temperature must be very high, though inferior to that at 

 which hard-paste porcelain is produced. With wares such 

 as these, whether hard porcelain, such as is largely made 

 on the Continent, or salt-glazed stonewares (made as 

 largely in our own country as in any continental one), it 

 is perfectly possible to use glazes free from lead, so that, 



