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 ten sion of potassium silicate and soda lime silica melts at 1400 degrees C was examined using the sessile and pendant drop arrangements, diffuse reflecta nce Fourier transform spectrometry and weight loss measurements. Volatiliza tion of alkali from the melts was modeled as a combined mechanism that incl uded diffusion of volatile species from bulk to surface and a chemical deco mposition reaction of alkali oxide on the surface. It was also found that t he volatilization of alkali could lead to differences in composition and st ructure of bulk and surface of the same melt depending upon the rate of dif fusion of the alkali through the melt; fast diffusion of alkali results in a surface tension that remains unchanged for long periods whereas blocked d iffusion results in a surface tension that changes with time initially rapi dly leveling off to an equilibrium surface profile. (C) 2000 Elsevier Scien ce B.V. All rights reserved.