Infra-red spectroscopy was used to measure structural changes, water surface concentrations and effective hydroxyl diffusion coefficients in silica glass during isothermal hydration heat-treatment at temperatures which ranged from 80 to 1150C, under 0.467atm of water vapor. The observed structural changes were found to be identical to relaxation during annealing, and it was shown that infra-red spectrometry could be used to measure glass fictive temperatures. It was found that the presence of minute amounts of water had a marked accelerating effect upon structural relaxation. In turn, this relaxation affected the water content of the glass. Thus, slow relaxation at low temperatures hindered glass-water reaction or caused a slow increase in the reaction equilibrium constant. Expansion of the glass during water entry permitted an increase in the molecular water solubility. Finally, healing of the glass during bulk relaxation caused a decrease in the hydroxyl solubility. These processes occurred at differing rates and caused an anomalous increase and then decrease, with time, in both the surface hydroxyl concentration in thick specimens and the total hydroxyl uptake in thin specimens. This suggested that hydroxyl solubility measurements which had been performed previously below 850C were not true equilibrium solubilities. An observed kink at 550C in the Arrhenius plot of the effective OH diffusivity was attributed to a time-dependent phenomenon which was explained in terms of a slow glass-water reaction during relaxation. The diffusion of water into silica glass was therefore suggested to be limited by 2 extremes. These were a high-temperature (above 850C) and long-term extreme where relaxation and reaction were faster than diffusion and water diffused according to the Doremus model, and a low-temperature and short-term extreme in which relaxation and reaction were slower than diffusion; and water penetration was limited only by the diffusion coefficient of molecular water in the glass.

K.M.Davis, M.Tomozawa: Journal of Non-Crystalline Solids, 1995, 185[3], 203-20