Viscous Flow In Glass During Phase Seperation

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 borosilicate but differ greatly in phase-seperation characteristics. Electron micrographs were used to analyse the development of microstructure during the suspected phase separation. In both glasses, structure development is primarily responsible for the viscosity increase.