Effect of quartz on the sintering and bending strength of the porcelain bodies in quartz-feldspar-kaolin system was investigated by the observation of bulk density, 3-point bending strength test, X-ray diffraction and the microstructures using a scanning electron microscope. Quartz and feldspar were jet-milled to mean particle sizes of 5μm and 4.5μm respectively, before the wet-mixing of the raw materials. Porcelain bodies containing 45% feldspar and 55% kaolin showed full densification in long firing range. A mean bending strength of the body increased from 15kg/mm2 to 19kg/mm2 with the increase of firing temperature from 1150°C to 1300°C. Quartz particles acted as inclusions before dissolution and restrained the sintering of the body to increase the maturing temperature. These particles hardly reacted on the eutectic melts, formed from decomposed kaolin and feldspar, and showed isolated microstructures. Therefore, the bending strength of the body containing quartz never exceeded 16kg/mm2 in this experimental conditions. These results were discussed in a viewpoint of microstructural developments, utilizing a phase diagram in K2O-Al2O3-SiO2 system.
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Kobayashi et al. (1994) studied this question.