Viscosity and Microstructure of Phase‐Separated Borosilicate Glasses

The viscosity of a sodium borosilicate glass (SiO 2 70, B 2 O 3 23, and Na 2 O 7 wt%) was measured as a function of heat‐treatment temperature and time for phase separation. The viscosity was shown to approach an equilibrium value unique to the heat‐treatment temperature. The magnitude of the equilibrium viscosity increased as the heat‐treatment temperature decreased. These results, plus electron microscopy of microstructure development, established that the viscosity of phase‐separated borosilicate glasses is primarily controlled by the composition of the continuous, high‐viscosity phase and that the effect, if any, of the microstructure size is small. In contrast to previous reports, the heat‐treatment time required to attain the equilibrium composition was extremely long.