China-clays of Cornwall. 97 



cate an absence of these deteriorating qualities, until some 

 cheap mode of separating these constituents from the other- 

 wise vitrifiable granites of our county be found, the China- 

 stone at present in use must retain its pre-eminence, consist- 

 ing as it does of a pure double silicate of potash and alumina, 

 which, when fused, forms a pearl-white translucent mass, 

 firm and resonant, consisting of an opaque body of nearly 

 perfectly formed kaolin, surrounded by and diffused through 

 the 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 forma- 

 tion 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 pur- 

 poses, the most important of which are, 1st, — in the forma- 

 tion of clay bodies to form biscuit ware ; 2dly, — to strengthen 

 clays rendered poor by the absence of potash ; and, 3dly f — 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 contain- 

 ing different proportions, 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 em- 

 ploy the ashes of ferns, which, from the amount of carbonate 

 of potash they contain, gave to it that richness and blending 

 of the body with the glaze for which it has been long remark- 

 able : 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 trans- 

 parency of the ware without rendering the glaze liable to 

 craze or peel off, as would be the case were lime alone em- 



VOL. LVI. NO. CXI. — JANUARY 1854. Q 



