292 EARTHS. 



Haven are made by using a fire clay, brought from Amboy, and 

 found near the pipe clay ; an equal measure of rather coarse silice- 

 ous sand is added, and they are baked in a potter's oven, with 

 less heat than is employed for stone ware. Such bricks endure the 

 intense heat raised in the cylindrical furnace stoves, in which the 

 anthracite, and particularly the Lehigh coal is burned. On the side 

 exposed to the fire, they become vitrified, and the impurities of the 

 coal, consisting of earths, and oxide of iron, attach themselves to the 

 bricks, in the form of a slag, and if the accumulated matter is not 

 frequently detached, it eventually chokes the furnace. 



The common bricks are burned in huge piles, called, in this coun- 

 Iry, Kilns, in England, Clamps. They are constructed of the 

 moulded and sun-dried bricks, laid up with interstices, for the flame 

 and hot air, and there are cavities left at the bottom, crossing the 

 structure, in an arched form ; in these the dried wood is laid, and 

 the fire being kindled, is gradually increased, for the first twelve 

 hours, after which it is kept at a uniform height for several days and 

 nights, until the bricks are sufficiently hardened. Some are exter- 

 nally vitrified, or covered with a glaze, which is nothing but the 

 melted materials of the bricks, and is not desirable, as good bricks 

 can be made without vitrification. Some bricks are soft, and ab- 

 sorbent of water, and will split with the frost : others are firm, and 

 will endure a great length of time. There is a great diversity in the 

 elays of different places, as regards the goodness of the bricks made 

 from them. Bricks, after being partially dried in the sun, are some- 

 times pressed hi iron machines, which forces out water and air, and 

 makes them more firm and handsome. 



Terracotta, or Terre cuite, (burnt earth,) is used by the moderns, 

 as it was by the ancients, in making ornamental designs, " vases, 

 imitations, and architectural decorations. n The finer kinds of clay 

 are employed, and they are with great facility moulded into any de- 

 sired form. 



Reamur's Porcelain* This curious production might have been 

 mentioned under glass, of which it is only an alteration,- effected by 

 the action of continued heat to the point of softening, and followed 

 by slow cooling, when the glass loses its transparency, and under- 

 goes a kind of crystallization. The change is most easily effected 

 upon green bottle glass ; it is found to be owing to the loss of the 

 alkali by the heat, and that the glass thus changed will endure sud- 

 den changes of temperature, as well as the best porcelain. It is 

 usually prepared by filling a common green glass bottle with white 

 sand and gypsum ; it is buried and pressed down in this mixture, in 

 a covered and luted crucible, and baked in a potter's kiln, during 

 the usual time of firing the ware, at the end of which period, it will 

 be found changed into a kind of porcelain. Bigelow's Tech. 



