Viscous Flow in Glass During Phase Separation

Abstract

The isothermal viscosities of two borosilicate glasses, one a commercial glass widely used for chemical glassware, increase by 4 to 5 orders of magnitude with heat‐treatment time near the annealing point. The glasses are basically sodium borosilicates but differ greatly in phase‐separation characteristics. Electron micrographs were used to analyze the development of microstructure during the suspected phase separation. In both glasses, structure development is primarily responsible for the viscosity increase. An analysis of the data and a theoretical interpretation of the effect are presented.