6: Viscosity of Glass Forming Melts

The viscosity of a glass forming melt varies over 14 to 15 orders of magnitude during the production of a glass. As a result, several different techniques must be used to measure a complete viscosity/temperature curve. The viscosity/temperature curve has a complex shape, which is commonly described by the Vogel–Fulcher–Tamman equation or by discussion of the fragility of the melt. Compositional changes which decrease the connectivity of the vitreous network decrease the viscosity, while changes which increase the connectivity increase the viscosity. Hydroxyl, fluorine, and PbO all act as fluxes and reduce the viscosity of oxide melts. The viscosity in the glass transformation range is a strong function of the fictive temperature of the melt, and changes with time if a specimen is heat treated at a temperature that is different from its original fictive temperature. Phase separation and crystallization strongly alter the viscosity of melts, especially in the glass transformation range. The connectivity of the vitreous phases is especially important in determining the effective viscosity of phase separated specimens.