246 Porcelain and Earthenware. 



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ciples, that it is cause of surprise that the Chinese could have brought 

 it to perfection, without the aid of Science, The agency of water 

 on most of the operations of nature and art, being " a solvent for al- 

 kalies, and most of the acids, and earths, and by its decomposition 

 imparting oxygen to one principle, and hydrogen to another;" if 

 not well understood, will at times frustrate the expectations of the 

 manufacturer. More important to the potter if possible is a " thorough 

 knowledge of the properties of different kinds of clay. Macquer who 

 examined more than eight hundred specimens, says that he did not 

 find one entirely free from metallic matter." It requires much prac- 

 tice also, to be able to understand the nature of clays so as to em- 

 ploy them to advantage. Alumine* when pure, is a white, opaque, 

 tenacious earth, with an oily feeling and is the plastic material in all 

 the varieties of clay. If beaten up with water it forms a ductile paste, 

 easily moulded, smooth and compact in its texture, but if exposed to 

 the heat of the furnace, it contracts greatly in its dimensions, be- 

 comes rifty, exfoliates, and falls to pieces. But silexf for which it 

 has a great affinity, when combined with it in certain proportions, 

 obviates these defects : exposed to heat it becomes impenetrable, 

 resists the percolation of fluids, and is incapable of decomposition 

 from the action of the atmosphere. Pure crystallized silex is trans- 

 parent; alumine is also transparent in some of the precious gems, 

 but both earths in their powdery state are opaque. These earths 

 either saperate or in combination, are the basis of most of the gems. 

 Alumine is wholly insoluble in water — diminishes in volume and in- 

 creases in hardness proportioned to the degree, and long continuance 



of the heat to which it is subjected. Flint is silex in a state nearly 

 pure, is obtained in great abundance in chalk hills, and sometimes 

 occurs in secondary limestone. It has a glimmering lustre ; its frag- 

 ments are sharp edged, and its fracture conchoidal. The best flints 

 are translucent, of a dark grey color. Those which shew ferrugin- 

 ous spots of yellow or brown should be rejected, as they would dis- 

 color the ware. Silex is held largely in solution in certain hot min- 

 eral waters, and in volcanic fountains, probably by the aid of soda, 

 which is a solvent for it, as is well known in the manufacture of 

 glass. Klaproth detected twenty five grains of silex in a thousand 

 ounces of the mineral waters of Carlsbad in Bohemia ;§ and the 

 Geysers, or hot springs, in Iceland, hold so much silex in solu- 

 tion, that solid hollow basins are formed around the cavity of 



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Pure clay. t Rock crystal and white beach sand are examples. 



