Above 2050°C, pure alumina (Al2O3) melts forming a covalent and nonconducting liquid that
upon cooling yields a solidified mass of corundum. Corundum is also called in the ceramics
and refractory industries white fused alumina. White fused alumina exhibits a fine-grained
microstructure with euhedral crystals. most of the large tonnage production uses a tilting
electric-arc furnace with three electrodes operating in an AC mode. Once molten and homogeneous, the alumina melt is poured into molds and allowed to cool slowly until demolding.
Beta-alumina represents the major impurity observed in white fused alumina due to the concentration of sodium occurring in certain regions. However, the volatilization of the sodium
occurs at 2100°C and creates pores that are beneficial. To improve is mechanical strength,
usually 2 wt.% of chromia (Cr2O3) is added to the melt. Actually, trivalent chromium substitutes isomorphically for the Al3+ increasing the toughness of white fused alumina.
