THE DEVITRIFICATION OF SODA-LIME-SILICA GLASSES1

The isotherms and boundary curves in that portion of the ternary system Na₂O-SiO₂–CaO.SiO₂–SiO₂ of direct application to glass technology, namely, from 64 to 78% SiO₂, 0 to 20% CaO, including the whole of the field of the compound Na₂O.3CaO.6SiO₂, and portions of the adjoining fields of Na₂O.2SiO₂, Na₂O.2CaO.3SiO₂, CaO.SiO₂, tridymite, and quartz, have been determined with greater precision than in a previous study² by greatly increasing the number of mixtures studied, and thereby decreasing the interpolation necessary in the interpretation. An extended discussion is given of the devitrification of glass, in which it is shown that the liquidus temperature is the only datum point which is solely a function of glass composition, and is unaffected by the past history of the glass or by fortuitous circumstances at the time devitrification is taking place. At the liquidus the first trace of crystalline phase is in equilibrium with the glass; above the liquidus the glass will dissolve any such crystals which may have been formed at some previous time, but new crystals cannot form; while only below the liquidus is there a tendency for crystals to separate. The driving force causing this reaction is derived from the difference in free energy between the unstable glass and the stable aggregation of crystalline phases, and this force is opposed by a resistance of the nature of a viscosity. The dissociation in the liquid phase of the compounds which separate on devitrification diminishes the rate at which the devitrification equilibrium is reached, and increasing the complexity of the glass will have a similar effect. The excess in surface energy at the interface glass-crystal may have an influence at the instant of formation of the first infinitesimal crystal, hut after the crystal has grown to a dimension appreciably greater than the sphere of molecular action the excess energy at this interface ceases to be of importance. The only surface tension measurements which have been made on glass refer to the interface glass-air, and the excess energy at this interface has no necessary correlation with the devitrification process.