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 N aCl 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 be en studied by measuring the progressive changes of the small angle X-r ay scattering (SAXS) intensities that characterize the phase separatio n process at a constant temperature of 600 degrees C. It has proved th at a small quantity of chloride remains in the glass after fining. Thi s small content of chloride causes a dramatic change of kinetics and e quilibrium conditions for the phase separation process in the glass in comparison with a pure soda lime silica glass: the phase separation i s accelerated considerably and the maximum volume fraction of the sili ca droplet phase is increased. The effect of chloride is interpreted a s a shift of the miscibility gap position by 45 K up to higher tempera tures. This explanation is confirmed by measurements of the binodal te mperatures of the pure glass and the glass contaminated with chloride. Viscosity measurements show that chloride does not significantly alte r the conditions for diffusion in the glass. Thus, the effect of chlor ide on the phase separation is mainly attributed to a rise of the misc ibility gap.