Alumina Hydrate

Since it’s decomposition occurs about 220C as a glaze material it poses no bubble clouding issues in the melt. That being said, considerable amounts of water are generated and these could affect the density of the glaze lay-down or its adherence to the body or another glaze (or underglaze) layer.

Hydrated aluminas are fine granular white powders that have good flow properties. As with any form of alumina, this material has a very high melting temperature. Notwithstanding this, it disassociates enough in many glaze types to be useful as a source of Al₂O₃ to the melt (if the particle size the is really fine). The hydrated version of alumina stays in suspension better in glaze slurries and has better adhesive qualities than the calcine. Also, using hydrated alumina in glazes and glasses can promote a fining by coalescing finely dispersed gas bubbles. Small additions of fine alumina hydrate added to a glaze can also enhance the color of Cr-Al pinks. Larger additions of fine material can impart matteness if the glaze is able to take it into solution (sourcing alumina from kaolin, feldspar and frits is obviously more practical since these decompose readily in glaze melts).

Alumina hydrate promotes opacity in enamels and glazes by generating gas bubbles in the glaze melt.