Viscosity and liquidus temperature of quaternary glasses close to an eutectic composition in the CaO–MgO–Al2O3–SiO2 system

The thermal properties of the quaternary glass-forming CaO–MgO–Al2O3–SiO2 system were evaluated. Different glass compositions with an expected liquidus temperature—the lowest temperature at which both the crystal phase and the melt are thermodynamically stable—below 1300 °C were melted from pure raw materials. Each oxide concentration did not deviate by more than ±3 mol% from that of the centre glass composition of 61.2 SiO2, 26.3 CaO, 8.6 Al2O3 and 3.8 MgO. Viscosity data and the liquidus temperatures were determined, as well as the exact compositions of the respective glasses by X-ray fluorescence. It was of special interest for this study, whether small compositional changes have a stronger influence on T Liq or T 3. The T 3-values, which are the temperatures attributed to a viscosity of 103 dPa s—showed a much larger variation and are in the range from 1263 to 1363 °C. An exchange of the network formers Al2O3 or SiO2 by network modifiers CaO and MgO resulted in a decrease in viscosity. The effect of MgO was also shown by a comparison with glasses of similar compositions but without any MgO. The exchange between Al2O3 and SiO2 did not show a significant difference in viscosity whereas the substitution of CaO by MgO increased the viscosity. The liquidus temperature varied between 1137 and 1192 °C—less than expected for a composition close to an eutectic system.