302 CLAYS AND CLAY INDUSTRY. 



on the surface of the ware. Many clays are capable of taking" a 

 good salt glaze, some take a poor one, and others do not glaze 

 at all. 



From experiments recently made by L. E. Barringer 1 it seems 

 that a clay may be either too' aluminous or too siliceous to be suc- 

 cessfully salt glazed, but that if the process of salt glazing is 

 properly carried out, clays in which the proportion 2 of silica to 

 alumina is more than 4.6 to 1 and less than 12.5 to 1, are capable 

 of receiving a glaze. The degree of fineness of the free silica in 

 the clay makes a little difference. The finer the sand the lighter 

 the color o>f the glaze. 



Barringer also found that, contrary to what was usually sup- 

 posed, a considerable quantity of soluble salts, as much as 3 per 

 cent., could be present in a clay without seriously interfering with 

 the salt glazing, when conducted at cone 8. 



Glazing white earthenware and china. — In this grade of ware 

 the glazing and burning are not done in one operation, as in stone- 

 ware, but the ware is first burned to steel hardness, then dipped 

 in the glaze, and burned a second time. In the case of white 

 earthenware, the second burning is done at a lower temperature, 

 and in the case of china it is done at a higher temperature than 

 the first. The glazes for white earthenware and porcelain are 

 complex compounds of an artificial character. They consist of a 

 mixture of acids and bases combined according to a definite 

 formula, in such proportions that they will melt to a glass at the 

 temperature reached in burning. A glaze thus produced must 

 furthermore agree with the body in its shrinkage, and coefficient 

 of expansion, in order to prevent various defects, such as crazing, 

 peeling, etc. A discussion of the composition and methods of cal- 

 culating glaze formulas hardly lies within the province of this 

 report, and those wishing to become acquainted with this subject, 

 are referred to a most excellent little manual of Ceramic Calcula- 

 tions issued by the American Ceramic Society. 3 



1 Trans. Amer. Ceramic Society, Vol. IV. p. 223. 



2 Molecular ratio. 



3 Purchasable for $1.00 from S. G. Burt, Rookwood Pottery, Cincinnati, O. 



