Viscosity is an important process parameter in vitrification, but it can be a difficult property to measure for the full range of compositions and temperatures of a glass. Mathematical modelling is used to help augment and extrapolate experimental viscometry studies. The Vogel-Fulcher-Tamman (VFT) equation is the most prevalent model used by industry for approximating the viscosity/temperature relationship of glasses. VFT was initially developed as an empirical relationship, without reference to any mechanistic rationale for viscous flow. More recently, various new proposals about the microscopic underpinnings of rheological processes have given rise to alternative viscosity equations. In this presentation we explore some of the current theoretical developments, and compare and contrast the efficacies of their fits to experimental data. In particular, we concentrate on the superior interpolative and extrapolative properties of contemporary models with respect to nuclear waste glasses. We also examine correlations and composition dependencies in the parameters of the MYEGA model.
