Within the same alumina level, refractories made using different sources of alumina vary widely in their densification and refractory properties. Refractories based on the more volume stable synthetic mullite and sillimanite grains are dense and more creep resisting than the continuously bloating easily deformed bauxite ones. The mineralogical composition of the grog grain used and the matrix developed in a fired brick appears to be the most important factor determining its resistance at high temperatures