The color of brown aluminum oxide after calcining is a reproducible quality indicator for both consumers and manufacturers. When calcined in an oxidizing atmosphere, the brown particles turn blue. This blue color is caused by dissolved Ti2O3.
Ti2O3 is the only oxide of titanium that can be dissolved in α-alumina grains. Furthermore, TiO2 is the most thermodynamically stable oxide of titanium. Above 1000°C, oxygen diffuses into the α-alumina grains, oxidizing the Ti2O3 to the more stable TiO2. This is then encapsulated within the α-alumina grains.
TiO2 nuclei continue to coalesce and grow at a temperature-dependent rate. Once the titanium dioxide grains reach a size of 0.01 to 0.1μm, they begin to scatter light similar to a colloidal suspension. This selective scattering of shorter wavelengths of blue light gives the α-alumina grains their blue color. This selective scattering of light by small particles is known as the "Tyndall effect." If the TiO₂ nuclei are allowed to grow to approximately 0.1 μm, they no longer scatter only blue light but all wavelengths, resulting in a light gray coloration of heat-treated brown aluminum oxide.
Only brown aluminum oxide containing dissolved titanium oxide (Ti₂O₃) exhibits a calcined blue color; those with low Ti₂O₃ contents do not exhibit a blue coloration. Like many properties of particulate materials, the blue coloration after calcination is the average color of all particles in the sample. In one calcined brown aluminum oxide sample, the proportion of blue particles falls within the high to medium range of the typical titanium dioxide (TiO₂) content specification of 2.4% to 3.0%, while the proportion of non-blue particles falls below the lower limit of the specification. It is possible that a brown aluminum oxide with a chemical composition within the typical specification range does not exhibit a blue coloration after calcination because it is primarily a mixture of low-TiO₂ particles with some high-TiO₂ content.
