High temperature viscosity of porcelain stoneware bodies

The viscosity of porcelain stoneware at high temperatures is crucial to understanding the vitrification path, the viscous flow sintering kinetics, and the pyroplastic deformation of this material. The final viscosity of porcelain stoneware has to be determined considering both the viscosity of the liquid phase formed by the melting of feldspars – and other minerals – and the viscosity of the body made up of a suspension of crystals dispersed in the melt. A fundamental theoretical background along with semi-empirical constitutive laws on viscous flow sintering, glass densification, as well as on the high viscosity of liquids and melts already exists. Different approaches are needed to measure/estimate the two viscosities and the parameterization depends on both the chemical composition of the liquid phase and persistence of crystal phases in the melt. In this work, a first attempt to predict the viscosity of a porcelain stoneware liquid phase is proposed by means of a detailed overview of preexisting models for high temperature viscosities of glasses and melts. Although models developed for glasses take into account a large number of oxides and they can be applied to melts characterized by a wide compositional range, the maximum concentration of alumina expected by these models is too low compared with that of the systems investigated here. On the other hand, the models proposed for granitic melts, although based on a lower number of oxides, take into account alumina levels closer to those of the systems of interest. In this contribution it is demonstrated that the latter models can be used to predict the viscosity at high temperature of porcelain-like bodies. Comparative examples are provided for porcelain stoneware tiles, vitreous china, and porcelain bodies.