Relation of Crazing of Porcelain and Stress in Porcelain Glaze, and Method of Rapid Determination of Stress in Porcelain Glaze

摘要

Crazing is one of the most serious defects in tableware. Stress in glaze due to an unfitness of the thermal expansion of body and that of glaze is the main cause for crazing. A rapid method of determining the stress in porcelain glaze was developed, in which birefringence caused by the stress in the glaze was measured using a quartz optical wedge and a gypsum test plate under a polarizing microscope. Test pieces were cut from sample wares by using a cutting tool driven by an ultrasonic vibrator.The dimensions of the test pieces were pre-determined, - so that stresses in the glaze would not be released by the cutting operation. The variation of stresses in each item was measured using test pieces cut from various positions of the same item and it was confirmed that the measurement of stress in one test piece per sample ware was sufficient for quality and production controls.Through the measurements of stresses in the glaze of a large number of crazed and non-crazed porcelain wares, it was found that stresses in the glaze of crazed wares were always tensile and were in the range of +10 to +500 kg/cm~2, while those in non-crazed wares were in the range of +200 (tensile) to -1100 kg/cm~2 (compressive). Most of the compressive stresses in the non-crazed wares were in the range of -500 to -800 kg/cm~2. When stresses in the glaze of commercial wares, both domestic and imported, were measured, most of them were found to he compressive, but a few of them, especially those of vitreous china were found to be more or less tensile.