Crazing Resistance of Hard Porcelain Ware

Studies on the Glaze Fitness of Porcelain Ware, No. 11

Abstract

The crazing resistance of porcelain ware was investigated by measuring the stress in the glaze with respect to the firing condition and the composition of the body. The base body was composed of 40% pottery stone (Toseki), 30% kaolin, 15% ball clay (Gairome), 10% feldspar and 5% quartz, and was glost-fired at SK12RF for 42h in a mass-production tunnel kiln. Investigations were also carried out to find a proper body composition which was completely secure from crazing due to overfiring or refiring.
With an increase in the amount of residual quartz in the body, the thermal expansion coefficient of the body increased, and the stress in the glaze changed from tensile to compressive. The temperature difference ΔT(°C) of the quenching test was less than 130°C when the stress in the glaze was tensile and showed an increase of up to 200°C with an increase in the compressive stress in the glaze.
With an increase in the thickness of the glaze, the crazing resistance decreased, and the amount of residual quartz in the body should be kept larger to keep the crazing resistance the same.
The stress in the glaze of the porcelain of the base composition decreased to about +100kg/cm² by being glost-fired twice, and the porcelain became increasingly subject to crazing. This was caused by the dissolution of quartz in the glass phase of the body.
When 5 to 8% of quartz was added to the base body, the stress in the glaze showed a little improvement in the normal glost-firing condition, but it showed a tensile stress from +100kg/cm² to +150kg/cm² when glost-fired twice. These compositions were proved to be ineffective in preventing the crazing caused by overfiring or refiring.
When 3 to 6% of feldspar of the base body was substituted by the same amount of quartz, the stress in the glaze showed a little improvement when glost-fired in the normal condition, and it showed nearly the same compressive stress even when glost-fired twice.
It was concluded that the partial substitution of feldspar in the base body by the same amount of quartz was very effective in preventing the crazing caused by overfiring or refiring, and thus could improve the crazing resistance in the case of mass-production and sinkin decoration technique. However such a substitution tends to decrease the translucency, the optimum composition should be considered taking the translucency and the crazing resistance into account.