The vitreous phase of porcelain stoneware: Composition, evolution during sintering and physical properties

High performance ceramic tiles (ISO 13006 Group BIa, water absorption < 0.5%) are composed of porcelain stoneware: a compact and light-colored material containing a large amount of vitreous phase, which governs sintering behavior and affects geometrical, mechanical and functional properties of finished products. Ninety-three porcelain stoneware tiles were analyzed for bulk chemistry (XRF) and quantitative phase composition (XRD-Rietveld) in order to calculate both chemical composition and physical properties of the vitreous phase; their evolution during the sintering process was followed by lab simulation of industrial firing and quenching in the 1100–1200 °C range. Porcelain stoneware tiles contain 40% to 75% wt. of a vitreous phase having a quartz-feldspathic composition with an alumina excess coming from clay minerals breakdown. Vitreous phase formation by feldspars melting is a fast phenomenon, starting from ~ 1050 °C, that is mostly accomplished before viscous flow begins densification, which goes on involving a slow-rate quartz dissolution. Sintering kinetics is expected to be controlled by viscosity and surface tension of the liquid phase, which appear to depend essentially on the alumina content (hence on the mullite stability) along with the Na/K and Na/Ca ratios. At any rate, a microstructural control on sintering is claimed as the rheological behavior of the viscous phase (i.e. the matrix containing both liquid phase and fine-grained crystals of quartz and mullite) is substantially different from that of the liquid phase only.