Effect of the Cl− content on the formation and dissolution of precipitates in a soda–lime–silica glass

The glass with composition 13Na2O–11CaO–76SiO2 (mol%) undergoes subliquidus phase separation via a binodal mechanism. Below the binodal temperature, Tb, the glass separates into two amorphous phases, silica-enriched droplets and a silica-poor matrix. Small-angle X-ray scattering (SAXS) is used to study the formation process of the droplet phase at 600 °C, as well as, the inverse process where the precipitates dissolve after an increase in temperature. A small amount of 0.47 mol% Cl− in the glass causes a drastic change of the kinetics and thermodynamics of the phase separation process in comparison with the pure glass: the phase separation is accelerated and the maximum volume fraction of the silica droplet phase is increased. The results of dissolution experiments demonstrate that due to the Cl− content Tb increases by about 60 K. So, the main effect of Cl− in the glass is interpreted as a shift of the miscibility gap to higher temperatures resulting from a higher thermodynamic driving force.