202 ON THE CHINA-STONE AND CHINA CLAYS OF CORNWALL. 



glaze of silicic acid, to which its transparency is due : and not only does 

 the foregoing deteriorating substances render the article useless, but 

 should there be a very great excess of quartz crystals or silica the article 

 will not from the formation of single silicates be capable of fusion at 

 any temperature ; though this fault may be remedied by the addition of 

 either potash or soda, to which the vitrifaction not only of this but of 

 the various kinds of glass, is also due ; felspar, according to Liebig, 

 containing 17*75 per cent, of potash. 



China- stone is used in the Potteries for a number of purposes, the 

 most important of which are, 1st, — in the formation of clay bodies to 

 form biscuit ware ; 2ndly, — to strengthen clays rendered poor by the 

 absence of potash ; and, 3rdly, — in the preparation or construction of 

 glazes, for the calcined biscuit ware, when mixed with other ingredients. 



The manufactured China-stone and China-clay is termed " pottery," 

 of which there are several varieties, each containing different propor- 

 tions of China-stone, clay, and other articles. In the porcelain series 

 there is said to be but 3 per cent, of potash, but this I imagine from the 

 transparency and purity of the body, to be inaccurate. The Chinese used 

 to employ the ashes of fems, which from the amount of carbonate of pot- 

 ash they contain, gave to it that richness and blending of the body with 

 the glaze for which it has been long remarkable : bone ash was also 

 used, both by the Chinese and French, and is now employed by our 

 potters in considerable quantity, for the sake of the phosphate of lime 

 it contains, which, during the process of fusion, adds considerably to 

 the transparency of the ware without rendering the glaze liable to craze 

 or peel off, as would be the case were lime alone employed, in fact at 

 times, during a single firing, more than 5,O00Z. worth of pottery is ren- 

 dered useless by the admixture of this earth, the surface of the services 

 becoming covered with a congeries of cracks and fissures, hence great 

 care is necessary to prevent its addition. 



The terms employed to designate the kinds of calcined and fused 

 wares, are : — Pipe- clay, the least used and least important; Queen's 

 ware ; Terra Cotta ; Basaltes ; and Porcelain biscuit ; the whole <)f 

 which were introduced by Wedgwood, to whose persevering, accurate, 

 and scientific research, Ave are indebted for the position our pottery now 

 holds ; and it should not be forgotten that the rapid strides by which 

 we have gained it, and the discoveries that have of later years been 

 made in this art, have been wholly derived from a good practical ac- 

 quaintance with chemical analysis, the importance of which cannot be 

 too strongly urged, on both the potter and the producer of the raw 

 materiaL The other and more common wares are, porclain ; pottery, 

 an inferior kind of porcelain ; and earthenware; to the description of 

 which I shall for the present confine my attention, that of the before 

 mentioned wares, as w T ell as of Parian, biscuit China, &c, belonging 

 more strictly to the province of the potter than to that of the writer of 

 the present essay* 



