Fundamental Studies of the Melting Processes of Glasses containing Lead Oxide. A Study of the Reactions between Silica and the Oxides of Lead and the rate of Formation of Glass from them.
JSGT 1941 V25 T136 T149
The rate of glass formation in the PbO-SiO2 system has been studied employing 0·5gm charges in the temperature range 1000-1375°, the compositions ranging from 60 to 90 per cent PbO. Outside this range glass formation was limited by the devitrification of compositions containing more than 90 per cent PbO, and by the formation of a scum of undissolved sand grains, which prolonged heating did not remove, in those containing more than 40 per cent SiO2. When, in the temperature range stated, glass formation was possible, solution of quartz was comparatively very rapid, the 80 per cent PbO mixture melting to a clear liquid in 4 mins.at 1375°, 8 mins at 1300°, 14 mins at 1200°, 18 mins at 1100°, and 30 mins at 1000°. The reaction between red lead and quartz, and litharge and quartz, was studied by microscopical and X-ray means. Pb3O4 was rapidly decomposed to PbO on heating, and the total loss in weight of the mixture comprised this oxygen loss, of approximately 2 per cent of the weight of Pb3O4, and a further slight loss due to volatilisation of PbO. In the litharge-quartz mixtures the only loss in weight was the small amount of PbO lost by volatilisation. As heating proceeded, the Pb3O4 (or PbO) lying between the sand grains progressively disappeared and was clearly transferred, by molecular attraction, and presumably in the vapour phase, to the surface of the quartz grain where reaction proceeded of a character dependent on the temperature. At temperatures above the melting point of PbO, a lead oxide-rich glass was formed on the surface of the sand grains, the PbO gradually penetrating them so that the amount of residual quartz gradually decreased with increased time and temperature of treatment. This was indicated microscopically and by the X-ray spectrograms, which also showed that no solid solution was formed. Immersion of the crushed product in bromoform showed also that the percentage floating on this liquid progressively decreased with longer times and higher temperatures of treatment. Below the eutectic temperature, 730°, between PbO,SiO2, and SiO2, a solid reaction appeared to proceed in the red lead-quartz and litharge-quartz mixtures, the X-ray spectrograms showing the formation of 2PbO,SiO2, though this could not be detected microscopically.
Eric Preston, Ph.D., D.Sc., F.I.C., F.Inst.P., and Professor W. E. S. Turner
