EARTHENWARE BODIES AND GLAZES1

Effect of body composition on crazing .—-Six bodies of each of 2 standard clay compositions were prepared with variable clay and flint content and after biscuiting at cone 8 were glazed with 21 earthenware glazes and glost fired at cones 4 and 6. The results indicated that the variability of the silica content of clays would not be great enough to produce crazing in a well-balanced glaze. Effect of proportion and composition of frit .—-A standard whiteware glaze was compounded in 6 different ways and several other glazes in 2 ways. It was found that the method of compounding had no effect on crazing but it affected the gloss and fusibility. Increase in the percentage of material fritted increased the gloss and fusibility. With the same percentage of frit the best glost and highest fusibility were obtained when the flint and part of the clay were included in the frit. Effect of some variations in glaze composition .—-Substitution of CaO by Na₂O, pound for pound, as well as direct addition of Na₂O, increased crazing, improved gloss and increased the fusibility. Direct addition of feldspar increased crazing slightly and diminished gloss, but did not noticeably affect the fusibility. Substitution of 1½1/2 parts of feldspar for one part of flint to maintain the same fusibility increased crazing and diminished gloss. Direct addition of CaO improved gloss, increased fusibility and slightly reduced crazing.