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CLAYS OF NEW YORK 809 



Artificial glazes are used to some extent on the better grade of 

 stoneware made at the present day. 



Stoneware is sometimes coated with a slip of white clay. 



Burning stoneware 



Stoneware is commonly burned in round kilns. The articles 

 are piled one on top of the other till the kiln is filled, but they 

 are set in such a way as not to interfere with an even draft through- 

 out the kiln, and the larger pieces are placed in the center. If 

 both salt glazed and slip glazed wares are burned in the kiln at the 

 same time, the latter have to be protected from the salt vapor in 

 some way. 



The time of burning depends partly on the size of the kiln, and 

 partly on the clay. It may be as short as 30 hours or as long as 90. 



The temperature attained in burning stoneware also depends on 

 the clay. Experiments made in Ohio show that the temperature 

 ranges from about 1850° to 2000° F. Other experiments made 

 by the writer indicate that in the case of the N"ew Jersey semi-fire 

 clays the temperature in stoneware kilns reached 2300° T. at 

 times. 



The kilns used in burning stoneware are either up-draft or 

 down-draft, both round and rectangular. 



Glazing white earthenware and porcelain 



In this grade of ware, the glazing and burning are not done m 

 one operation, as in the case of the stoneware, but, on the con- 

 trary, the ware after molding is first burned to a comparatively low 

 temperature, after which it is dipped in the glaze and burned a 

 second time. In the case of white earthenware the temperature 

 of the second burning is lower than that of the first, while in the 

 case of the porcelain it is higher. 



The production of a glaze on the surface of either porcelain or 

 earthenware, free from the numerous defects to which such mate- 

 rials are very liable, is often attended by considerable difficulty. 



The glaze on pottery consists of a fusible mixture which is 

 applied to the surface of the ware, either when it is still in a green 



