Refractoriness is an indicator of the ability a material have to retain its physical shape and chemical identity when subjected to high temperatures. Refractoriness is not a physical constant of a material, but a technical indicator of the quality of being refractory. 

The refractoriness is not identical with the melting point of the pure raw material. The melting point is the temperature at which the liquid phase and the crystalline phase of the pure material are in balance. However, refractory materials are always multiphase mixtures composed of a variety of minerals, so they do not have a uniform melting point, but begin to soften at a certain temperature and are fully melt into liquid phase at another temperature. Between the two temperatures, the solid phase and the liquid phase coexist. Therefore, refractoriness is used to indicate the softening and melting characteristics of the refractory materials. 

 

Refractoriness is a not an absolute physical quantity of a material, but a relative technical indicator of a material under a certain experimental conditions. The material is made into a truncated triangular pyramid in a predetermined size. When heated at a heating rate, the material will be gradually bent due to gravity. The temperature at which the top of the pyramid touches the underpan is called refractoriness. 

 

Refractoriness is up to the properties of the material, such as chemical composition, dispersion, and the proportion and viscosity of liquid phase. Impurities can severely reduce the refractoriness. Uneven distribution of the chemical composition can reduce the refractoriness too. 

 

The refractoriness is not equal to the service temperature of the material, since besides of the high temperature, the materials are also subjected to the influence of the load and fluxes. The actual service temperature of the material is much lower than the refractoriness. But after all, refractoriness is still an important indicator for the selection of refractory materials.