Abstract :
Porcelain bodies contain 40-60 vol.% of glass. The varying composition of glassy phase greatly influences
mechanical properties, translucency and firing temperature of the final ceramic product. Despite the melting
temperature of at least 1100°C, of the low melting flux present in the body, glass formation begins at a
temperature lower than 1000°C. Experimental results have been explained on the basis of predictions from the
K2O-Al2O3-SiO2 phase diagram. Initially, the amount of the liquid phase increases with increase in firing
temperature due to the melting of feldspar. Silica discarded from clay relict during mullite formation at ≥1000°C
and partial quartz dissolution at commercial firing temperatures also contribute to the glassy phase. The
composition of glass close to quartz grains and within clay relicts is highly silica-rich. The alkali content is
sometimes higher in the matrix but heterogeneous in general due to inhomogeneous mixing of the initial body
ingredients and inhomogeneous crystallization in various micro-regions. The variation in the size of mullite
crystals observed in various micro-regions of the body is related to the viscosity of the constituent glass. The
cracking of quartz grains has been discussed in terms of the thermal expansion mismatch of the constituent
phases and cristobalite formation.
