Calcined alumina powder does not work well in glazes or enamels as a source of Al2O3, it just does not dissolve into the melt unless exceedingly fine and in low percentages. However, the hydrated form can be effective to matte a glaze if (it has a very fine particle size). If possible, kaolin or feldspar (and nepheline syenite) are the best sources of Al2O3 for glass building. Kaolin especially is ideal as a source because it is so important to other physical slurry properties (i.e. suspension, adhesion, and shrinkage control). If glaze batches are being calculated from a source formula, it is normal to supply all possible alumina from feldspar until the alkali targets are met, then topped up with kaolin. If there are any additional Al2O3 requirements Bayer process alumina hydrate can be employed (but this is very rarely needed). Sometimes Bayer alumina is added in preference to kaolin where exceptional freedom from iron is needed.