Plainsman Clays | ALHY
Alumina Hydrate
Alumina hydrate is aluminum hydroxide, the uncalcined form of alumina, and it still contains chemically bonded water. It is the alumina in most kiln wash and wadding recipes, the layer that stops ware and kiln furniture fusing to the shelf.
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The only difference between this and calcined alumina is water. Alumina hydrate still contains chemically bonded water, and about a third of the weight of the bag leaves as steam early in the firing. What is left is the same refractory alumina as the calcined form, and it will not fuse to a pot or a shelf at any pottery temperature on its own. Most wash and wadding recipes call for the hydrate because it is fine and stays in suspension, so the wash brushes on evenly and sticks to the shelf better. It is also the usual choice for protecting kiln furniture in salt and soda firing. If a wash flakes, the cause can be the kaolin in it shrinking or the wash being put on too thick, not the alumina. In a glaze, it is a small addition used to stiffen the melt or cut gloss, and it dissolves into the melt more readily than the calcined form.
- Aluminum hydroxide, Al(OH)3
- The alumina in most kiln wash and wadding recipes
- About a third of its weight leaves as water in the firing
- Stays in suspension and sticks better than calcined alumina
- Will not fuse to a pot or a shelf
- Dissolves into a glaze melt more readily than the calcined form
