Melting temperature (Tm in units of °C or K) and melting range were discussed previously. The former will be the higher of the two and represents the temperature at which the phase pure oxide melts. As has been discussed, melting temperature data for selected oxides are included in Tables 2-5. A review of the literature also yields melting temperatures for many thousands of oxides that would not be classified as refractory.
During application of refractory oxides, melting range is typically more important than melting temperature. Softening point is defined as the temperature at which a
material begins to deform under its own load. With phase pure oxide systems, melting temperature and softening point are equivalent; however, in most practical systems impurities are inherent. These impurities lead to low melting point eutectic formation that can lower the maximum use temperature of the oxide.
Phase equilibria diagrams yield an estimate of the softening point for a refractory oxide. Considering the binary phase diagram for the oxide and the predominant impurity, the invariant temperature (eutectic, peritectic, or monotectic) or the invariant that is closest to the refractory oxide composition indicates the lowest temperature that will result in liquid formation and, therefore, the lowest possible softening point. Although an appropriate ternary phase diagram is required, the situation is only slightly more complex when two impurities are present in significant concentrations. In that situation, initial liquid formation is defined by the invariant point for the Alkemade triangle between the refractory oxide and the two impurities. For example, pure SiO2 has an equilibrium melting temperature of 1713°C. The addition of Na2O reduces the eutectic temperature to ~800°C at the SiO2-rich end of the diagram. Adding a third oxide, K2O, reduces the eutectic temperature to ~540°C at the SiO2- rich end of the diagram.