THE EFFECT OF HEAT‐TREATMENT AND COMPOSITION OF SEMI‐PORCELAIN BODIES ON CRAZING

ABSTRACT Limits of Investigation.—The investigation was confined to practical semi‐porcelain bodies and glazes, therefore, the results may be applicable only to this branch of the whitewares industry. Effect of Thickness of Glaze.—The thicker the glaze the greater the tendency to craze. The glaze on several pieces of dinner ware may appear to the eye to be of uniform thickness, but still have sufficient variation in thickness as to show difference in crazing behavior. It being impossible to obtain a uniform thickness of glaze by hand dipping there is a need for a mechanical dipper. An improved type of biscuit kiln which will produce ware with a uniform absorption is another need. Effect of Glaze Composition.—When sufficient change was made in the composition of a typical semi‐porcelain glaze to cause a marked decrease in its crazing tendency, it was at the expense of other desirable properties. Effect of Body Composition.—The addition of 10.0% to 20.0% of “Hercules” clay to a general ware body greatly decreased its tendency to craze. No other change in body composition was found feasible, for when the change was sufficient to cause a decrease in the crazing it was at the sacrifice of other essential properties. Effect of Variation in Absorption.—Contrary to the general opinion among potters, the tendency to craze was not increased by every increase in absorption. An absorption of about 7.0% to 12.0% showed the most resistance to crazing. Effect of the Time‐temperature of the Gloss Burn.—Tendency to craze is decreased by increasing the maturing time‐temperature portion of the gloss burn.