288 EARTHS. 



and contain one fusible ingredient, and are purer. The pottery being 

 opake, needs not tbe felspar, and it has a dull earthy fracture instead 

 of a vitreous one. 



The most common earthen ware is made of pipe clay, often con- 

 taining iron, which of course colors the ware when it is burned. A 

 clay, much used in this country, is obtained from Amboy, N. Jersey, 

 and is gray, both before and after it is burned. 



The plastic property possessed by moist clay, and by means of 

 which it is moulded, depends on the alumina ; but the pieces would 

 crack and be destroyed by shrinkage, were not the alumina correct- 

 ed by the silica, which is not prone to shrink in the fire. If natural 

 clays then have the requisite proportions of the two earths, and are 

 free from iron, they have all the properties that are essential ; and if 

 a color is produced by burning, it does not prevent the clay from 

 forming a useful ware, although it may not be beautiful. Magnesia 

 frequently enters into the composition of clays, and is a valuable in- 

 gredient, as it is a very infusible earth, and contracts but little in the 

 fire ; but if there is much lime, it will act as a flux, and produce a dis- 

 torted ware. 



As the natural clays do not always contain a sufficient portion of 

 siliceous earth, it is usual, in such cases, to mix with them siliceous 

 sand or ground flints, the clay being first blended with water into a 

 paste, and it is then uniformly mixed with the siliceous ingredient.* 



Fabrication of porcelain and pottery. There are important dif- 

 ferences between the two, and there are many varieties of operations 

 relating to both, but a few general facts may be stated. There is no 

 analogy between these processes and those by which glass is made ; 

 they are in fact directly opposite ; glass is " softened by heat, and 

 wrought at a high temperature, whereas the clay is wrought while 

 cold, and afterwards hardened by heat." Bigelow. 



There is much labor in preparing the materials, the detail of which 

 would be foreign from the object of this work, in which only a few 

 of the most important operations can be mentioned. 



Circular conical vessels are moulded upon the potter's wheel, a 

 very ancient instrument, mentioned by the earliest writers, sacred 

 and profane. A mass of the prepared clay is placed in the centre, 

 and it revolves by a movement given by the foot, or by some other 

 power ; the potter, his hands being moistened, to prevent adhesion, 

 one hand being on the outside, and the other within, gives it a circu- 

 lar form, and he employs sometimes a rude instrument, like a knife, 

 to aid in finishing the piece. Many articles, modeled in this way, 

 being too thick, are afterwards turned in the lathe, to make them 

 thinner. 



* Pottery contains silica, two thirds, alumina, from one fifth to one third, and 

 sometimes one five hundredth or one two thousandth of lime, and iron from the 

 smallest portion to 15 or 20 per cent. Vauquelin, quoted by Parkes. 



