Viscosity of aluminosilicate melts and glasses: Rapid measurement by hot stage microscopy

The viscosity of silicate melts represents a key parameter to control the ceramic and glass manufacturing processes, as well as magmatic to volcanic processes from the nanoto the macro-scale (e.g. crystallization, vesiculation and degassing until eruption dynamics and emplacement). The techniques used to measure the viscosity evolution as a function of the temperature (T) are generally time- and energy-consuming, requiring equilibrium conditions and providing results only in narrow viscosity ranges for each adopted viscosity technique. The reduction of testing time and the obtainment of viscosity data in the widest possible temperature interval represents a challenge for both academic and industrial targets. Hot stage microscopy (HSM) was selected as a valid experimental alternative to obtain aluminosilicate melt viscosity. With this purpose, aluminosilicate glasses with already known viscosity-temperature (η - T) relationships were selected. Cylindrical specimens of pressed powders were heated at 10 ◦C/min till melting, to determine the characteristic temperatures (CT). Shear viscosity at each CT was calculated using their known VFT (Vogel-Fulcher-Tammann)(η – T) relationships.Finally, viscosity data were calibrated by introducing chemical-dependent correction factors to quantify surface tension effect on the CT, used to calculate new subsets of VFT parameters. The comparison between the viscosity calculated using this approach and the experimental ones shows a good correspondence, significantly improving literature studies, disclosing a promising prospect of this non-contact technique.