PORCELAIN. 523 



latter has been deprived of half its silica, and above three- 

 fourths of its potash. The silicate of alumina left, which forms 

 the clay, is very difficult of fusion. The porcelain clay used at 

 Berlin, which is extracted from the decomposed porphyry of 

 Mori, contains, in 100 parts, 71-4 silica, 26 alumina, with small 

 quantities of peroxide of iron, potash, and lime (Mitscherlich). 

 (2) Pure silica, which is prepared by heating chert or flint to 

 redness, and quenching it in water. The mineral is afterwards 

 obtained, by grinding and subsequent levigation, in a state of 

 the greatest division. The clay and silica, both in the humid 

 state, are then carefully intermixt. A body for the best kind 

 of earthenware may be made of 'JO parts of the prepared clay 

 and 24 of ground flint. But to form porcelain an addition is 

 also made (3) of finely levigated felspar, to impart fusibility, 

 the proportion necessary being learned by experiment. Phos- 

 phate of lime, sulphate of lime, and carbonate of lime are also 

 added for the same purpose. A mixture of the prepared clay 

 and felspar is also employed, without the introduction of silica. 

 At Berlin, the porcelain earth of Mori is mixed with a quan- 

 tity of felspar, which, it is said, amounts to about 32 per cent, 

 without the addition of silica. At the royal pottery of Sevres, 

 in France, the materials employed are felspar as it is found in 

 three different states of decomposition, and which are technically 

 termed sable cailloteux, sable argileux, and kaolin, the last 

 being that in which the decomposition is most advanced, with 

 small quantities of silica (sable d'Aumont) and chalk. They 

 are combined in the following proportions : 



Kaolin . . . . - . JO 



Sable cailloteux . . .12 



Sable argileux . . . 9.18 



Sable d'Aumont . . . 5.29 



Lime 3.53 (6.3 chalk.) 



100 



The body (pate) of the Sevres porcelain, so formed, consists 

 very uniformly, according to M. Malaguti, in 100 parts, of - 



Silica . . . . 57 to 58 



Alumina . . . 34.5 to 35 



Lime . 4.5 



Potash . . 3 



It forms a highly translucent and beautiful porcelain. Felspar 



