Porcelain and Earthenware. 247 



each, where the waters fall. Clays are found in a natural state of 

 various degrees of purity ; seldom if ever without some admixture 

 of foreign matters. That is best which burns whitest, and is capa- 

 ble of combining with the largest quantity of flint or sand without 

 cracking ; that being the limit beyond which it cannot be advantage- 

 ously united. 



The proportion of silex to alumine in Chinese porcelain is 74 per 

 cent, of silex, to 16 of alumine ; yet although the silex predominates, 

 the argillaceous substance gives the character to the compound ; im- 

 parts the cohesive and ductile properties, which make it capable of 

 being turned and moulded into forms; and after being subjected to 

 a red heat renders it indestructible by the action of the atmosphere. 

 According to Vauquelin, European porcelains of good quality contain 

 at least two thirds silex, and alumine from a fifth to a third. A very 

 small amount of magnesia in the mixture, lessens the tendency which 

 the other earths have to contract in baking. Too large a quantity of 

 magnesia however, is to be avoided, as it renders the composition 

 too fusible. A small addition of lime, in place of magnesia produces 



corresponding effects. 



The best clay found in Europe is the felspar obtained from the de- 

 composition of granite rocks. * It is sometimes found in lumps in 

 the clefts of mountains, but may be obtained by pounding or grind- 

 ing, and washing over, the white or grey granite ; by which means 

 the quartz and mica are separated, and the felspar obtained in a fine 

 powder of extreme whiteness. This is done by throwing the ground 

 stones into a running stream, the quartz and mica subside, while the 

 argillaceous parts run off in a thick cream upon the surface of the 

 water. " At the end of these rivulets are catch pools, where the 

 waters are arrested, and time given them to deposite the pure clay, 

 when the water is drawn off, and the solid matter is taken out in 

 square blocks and dried for use."f This clay approximates in some 



degree to the kaolin of China. 



Pure silex is procured from calcined flint quenched in water while 

 hot, which breaks them through their whole substance. They are 



* Pure argillaceous earth maybe obtained, by dissolving alum in water, and then 

 decomposing it with an alkali. Cornish clays are very smooth and ductile and ex- 

 tremely white, and by Wedgewood's analysis, contain sixty parts alumine, and twen- 

 ty silex. He adds fc< the granite itself is sometimes used with the clay, on account 

 of its binding quality, knitting the other materials more closely together by its fusi- 

 bility." Its fusibility is caused by the alkali contained in its felspar, while in its 

 undecomposed state. t Paikes' Essay. 



