GEOLOGY. 21 



siace, whose fire, is supported by atmospheric air only. So, 

 also, pure alumine, or a mixture of alumine and lime, is infu- 

 sible, but, a compound of these three earths, becomes fusible — 

 most easily, when the proportions are, alumine one part, lime 

 one part, and sand three parts. But if the proportions of sand 

 be increased it becomes infusible. 



According to M. Alexandre Beogniart, Director of the 

 celebrated procelain factory at Sevres in France, the general 

 process, in all kinds of pottery, made with washed clays, may 

 be reduced to the following six steps : subject to modification, 

 in different varieties. 



1. The washing of clays, by agitation in water. The 

 sand subsides, while the finer particles of clay remain suspend- 

 ed in water, which is decanted, and the sandy particles re- 

 main at the bottom, and are thrown away, while the water, in 

 which the finer particles remain, is left to precipitate its clay. 

 Thus the sand is separated from the finer parts of the clay. 

 Having gotten rid of the sand, the next process, is, 



2. The composition of the paste. This paste, is the clay 

 that gives it consistency, Avhen moist, and hardness when bak- 

 ed; but to this clay, is added, either silex or hard baked clay, 

 in a state of powder, to prevent cracks while drying and bak- 

 ing, to give firmness and solidity, and to enable the pottery to 

 resist the frequent changes of temperature. Sometimes a 

 fusible ingredient is added to the paste. 



3. This paste, after remaining for a time in water, is beat- 

 en and kneaded, to render it ductile, and of uniform consisten- 

 cy, and, to drive out the air, that may be in it. 



4. The forming the several articles on a lathe or in a 

 mould. 



5. When the articles are sufficiently dried, they are baked 

 in a furnace or kdn; but, in most instances, they are put in 

 cases of infusible clay. The baking should be conducted slow- 

 ly, and as the heat becomes more intense, the ware becomes 

 more dense and compact, but at the same time, more liable to 

 injury by alternate exposure to heat and cold. In this state, 



