Fundamental Studies of the Melting Processes of Glasses Containing Lead Oxide. II. The Effect of Alkali on the Rate of Melting and Devitrification Temperatures of Alkali-Lead Oxide-Silica Glasses.
JSGT 1941 V25 T231-T240
The rate of glass formation of three series of glasses, of percentage composition
(66 - x)SiO2,34PbO,xK2O; (66 - x)SiO2,34PbO,xNa2O; and 54SiO2,34PbO,(12 - x)K2O,xNa2O has been determined employing both 0.5gm and 450gm charges of batch at a melting temperature of 1375°. The melting rate varied rapidly with alkali content. Within the range 8-20% K2O, an increase of 1% K2O at the expense of SiO2 was sufficient to decrease the time needed to produce batch-free glass by 8-10 per cent, and a similar result was obtained with the soda containing glasses. Devitrification temperatures determined by the gradient method showed a continuous decrease with increasing potash content from 1125° for the 10% K2O glass to 695° for the 20% K2O glass, silica being the primary crystalline phase in every case. In the soda series the devitrification temperature was similarly reduced from 1105° for the 8% Na2O glass to 670° for the 20% Na2O glass, but in this case a new primary crystalline phase made its appearance beyond 18% Na2O, indicating a phase boundary between 16 and 18% Na2O in this series. The effect of loss of PbO by volatilisation on devitrification temperature was discussed. The substitution of soda for potash in the 54% SiO2,34PbO(K2O + Na20) glass resulted in accelerated melting and lower devitrification temperatures, both desirable features, the best glass being that of composition 54SiO2,34PbO,6K2O,6Na2O. The colourlessness and brilliance of the substituted glasses up to this stage were quite comparable with the full potash glass, but there was a noticeable deterioration when the Na2O content was increased beyond 8%.
Eric Preston, Ph.D., D.Sc., F.I.O., F.Inst.P., and Professor W. E. S. Turner
