Effect of fining with sodium chloride on the phase separation of a soda lime silica glass

The effect of the technologically important procedure of fining with NaCl on the liquid-liquid phase separation of the 13Na(2)O-11CaO-76SiO(2) (mol%) glass in the metastable region of the miscibility gap has been studied by measuring the progressive changes of the small angle X-ray scattering (SAXS) intensities that characterize the phase separation process at a constant temperature of 600 degrees C. It has proved that a small quantity of chloride remains in the glass after fining. This small content of chloride causes a dramatic change of kinetics and equilibrium conditions for the phase separation process in the glass in comparison with a pure soda lime silica glass: the phase separation is accelerated considerably and the maximum volume fraction of the silica droplet phase is increased. The effect of chloride is interpreted as a shift of the miscibility gap position by 45 K up to higher temperatures. This explanation is confirmed by measurements of the binodal temperatures of the pure glass and the glass contaminated with chloride. Viscosity measurements show that chloride does not significantly alter the conditions for diffusion in the glass. Thus, the effect of chloride on the phase separation is mainly attributed to a rise of the miscibility gap.