Fluxing action of illite and microcline in a triaxial porcelain body

Comparative studies were made on two types of triaxial porcelain bodies; one used conventional potash feldspar (ST0) and the other used illite (IT0) as the fluxing agents. Scanning electron micrographs showed that densification and mullite formation in the IT0 body had already occurred at 1000 °C suggesting that there was enough liquid to assist densification and mullite crystal growth. In the ST0 body, mullite crystals started to emerge around 1100 °C, while potash feldspar completely melted around 1200 °C. Full vitrification of the ST0 body required the firing temperature of 1300 °C, which was around 50 °C higher than the IT0 body. The IT0 body showed superior strength to the ST0 body at all studied firing temperature ranges and thus was possibly due to (i) a better densification, (ii) a higher amount of interlocking mullite crystals and (iii) the smaller amounts of residual quartz.