Effect of pyrophyllite on vitrification and on physical properties of triaxial porcelain

Quartz was progressively replaced by pyrophyllite in a conventional porcelain mix with a composition of 50% clay, 25% quartz and 25% feldspar. The addition resulted in early vitrification and decreased thermal expansion of the sintered specimen. Addition of up to 15% pyrophyllite decreased the fired shrinkage by 6% and improved the fired strength by around 29% compared to the standard body. The gradual increase in flexural strength with incorporation of pyrophyllite was primarily due to the elimination of stresses in the structure with a decreasing quartz content as well as to the increasing amount of secondary mullite distributed throughout the matrix forming an interlocking network. However, the firing temperature and the generation of the correct amount of properly sized mullite needles are vital in achieving the desired strength. Pyrophyllite was found to dissolve in the melt in preference to quartz. Beyond the optimum proportion of pyrophyllite, a large volume of glass formed as well as large elongated pores distributed in the matrix resulting in deterioration of mechanical properties.