An insight into pyroplasticity of porcelain stoneware tiles

Pyroplasticity is active when sintering acts through viscous flow of an abundant amorphous phase, as typical of porcelain stoneware. It has become increasingly important in ceramic tile making with the development of large sizes, rectangular shapes, low thickness, and with use of vigorous fluxes and micronized fillers. Firing deformations are thought to depend on the amount, size, shape and mutual arrangement of coarse grains, as well as on the viscosity of the liquid phase that forms at high temperature. The objective of the present work was to identify the main variables responsible for the pyroplastic index and to suggest alternatives to reduce it. An insight into the pyroplasticity of porcelain stoneware tiles was gained by substituting raw materials in a typical industrial-like body composition. The six compositions were prepared by wet milling and sieve granulation. Bodies and green compacts were characterized by determining the chemical composition, particles size and green bulk density. Fired samples were characterized by determining the water absorption, bulk density, open and closed porosity, and phase composition (XRD-Rietveld). Firing deformation was determined by both the three-point flexural test at maximum density temperature (expressed as the pyroplasticity index and uniaxial viscosity) and by the Fleximeter (maximum deflection). Results showed that pyroplasticity was significantly affected by the nature of the vitreous phase at the temperature of the maximum densification rate (viscosity and flow point) and was not correlated with the volume of the vitreous phase.