The Volatilisation and Vapour Tension at High Temperatures of the Sodium Silicate–Silica Glasses

JSGT 1932 V16 T331-T349

Employing the method previously described by us, the volatilisation losses from a series of seven glasses containing soda and silica between the limits 48·85% Na₂O, 51·15% SiO₂ and 11·83% Na₂O, 88·17% SiO₂ have been determined at temperatures ranging from 1100° to 1400° for different periods of time. At 1400°, the losses in 20 hours from these two glasses were, respectively, 48·0 and 2·6 mg per sq cm; whilst from the first glass at 1400°, 1300°, 1200°, and 1100° the corresponding losses were 48·0, 20·0, 8·0, and 2·8 mg per sq cm. The rate of loss of Na₂O, even from the glass containing 48·85% Na₂O, was thus far less than that (236 mg per sq cm in 20 hours) from the alkali–lead oxide (30%)–silica glass previously studied. The vapour tensions of glasses containing 48·85% and 44·58% Na₂O were 2·20 mm Hg, and 1·35 mm Hg, respectively, at 1350°, 1·37 and 0·85 mm at 1300° and 0·56 and 0·35 mm at 1200°. The ratios of the vapour tensions and of the volatilisation losses of these two glasses were shown to be the same. It was shown that if the losses be plotted against Na₂O content for different periods of heating. the curves consist in each case of two intersecting straight lines indicating that the molten glasses are not constituted of Na₂O and SiO₂ as products of dissociation, but consist of a mixture of metasilicate and disilicate when the Na₂O exceeds 34% and of disilicate and silica when less than 34% Na₂O is present. Confirmatory evidence of this interpretation was derived from the temperature coefficients of volatilisation of the seven glasses and from the results of the study of the relationship 10³/t log(A−2x)/A=K set out in the paper.

E. Preston & W. E. S. Turner