Reactive sintering of porcelain stoneware bodies: phase evolution and related liquid phase physical properties

Porcelain stoneware is sintered by partial vitrification, through viscous flow of an abundant liquid phase formed at high temperature. The present contribution will overview the evolution of phase composition of porcelain stoneware during firing at different temperatures and soaking times. The firings were conducted in two distinct ways: dynamic (i.e. with a ramp simulating the industrial heating cycle in a roller oven) and static, by inserting the sample into the chamber furnace directly at the maximum temperature. Each mixture was characterized from the chemical point of view and, once fired, its phase composition was determined by quantitative XRPD (Rietveld method). The transformations affecting the minerals of the starting mixture determine a continuous variation of the phase composition during the heating treatment: feldspars melt quickly (K-feldspar>plagioclase) - largely melted at 1100°C - while quartz is only partially dissolved at the highest temperature. The liquid phase changes its chemical composition according to the dynamic equilibrium established with both the residual minerals (quartz, feldspar) and the new crystalline phases formed during the firing (mullite). Variations of the chemical composition of the liquid phase reflected on its physical properties, particularly on viscosity and surface tension, which define the densification kinetics in the sintering process.