ALHY

Alumina hydrate is aluminum hydroxide — alumina in its uncalcined form, with its water still present. It is the alumina in most kiln wash and wadding recipes, the layer that keeps ware and kiln furniture from fusing to the shelf.

The difference between this and calcined alumina is water. Alumina hydrate still holds its chemically combined water, and that water leaves early in the firing — about a third of the weight of the bag goes with it. What remains is alumina, refractory far beyond any temperature a pottery kiln reaches, and above stoneware temperature it is the same alumina as the calcined form. That is what a kiln wash depends on: it won't fuse to the ware or to the shelf. It is also the usual choice for protecting kiln furniture in salt and soda work. Most wash and wadding recipes specify this form rather than the calcined one, for reasons of handling rather than firing: being fine and hydrated, it stays in suspension and adheres better. Flaking in a wash is caused by the kaolin in it shrinking, or by the wash being applied too thickly, not by the alumina. In a glaze it is usually a small deliberate addition, stiffening a melt or reducing gloss, since clay and feldspar are the normal sources of alumina. It dissolves into a melt more readily than the calcined form.