Differences between surface and bulk properties of glass melts I. Compositional differences and influence of volatilization on composition and other physical properties

The influence of volatilization on the composition, density and surface tension of potassium silicate and soda lime silica melts at 1400°C was examined using the sessile and pendant drop arrangements, diffuse reflectance Fourier transform spectrometry and weight loss measurements. Volatilization of alkali from the melts was modeled as a combined mechanism that included diffusion of volatile species from bulk to surface and a chemical decomposition reaction of alkali oxide on the surface. It was also found that the volatilization of alkali could lead to differences in composition and structure of bulk and surface of the same melt depending upon the rate of diffusion of the alkali through the melt; fast diffusion of alkali results in a surface tension that remains unchanged for long periods whereas blocked diffusion results in a surface tension that changes with time initially rapidly leveling off to an equilibrium surface profile.