The behaviour of viscosity at high temperatures is of fundamental importance for crucial aspects of the glass manufacturing process, such as melting, refining, drop temperature and forming. This is why viscosity is the primary measure of glass behaviour in the molten state.

High-temperature viscosity is measured in Pa·s or Poise in accordance with ISO 7884-2 and ASTM C965 using the principle of a rotating rheometer within a calibrated furnace: a platinum rotor is immersed in the molten glass (contained in a Pt crucible) at a specific temperature, and the torque required to maintain the rotor in rotation at a constant speed is measured; this torque is a function of the viscosity of the melt.

Viscosity measurements are carried out up to 1550°C for glass, but can be performed up to 1700°C if the sample requires such temperatures, and for viscosities up to log7 Pa·s (log8 Poise).