Alumina is what stops a molten glaze running off the pot, and alumina trihydrate is one of the ways to add it directly rather than through a clay. It is also a principal ingredient of kiln wash, where its refractoriness is doing the opposite job: refusing to melt at all.
Stiffening a melt
A glaze is glass former, flux and stiffener, and alumina is the stiffener. Without enough of it a melt is fluid and runs onto the shelf; with too much the glaze is dry and underfired-looking. Most recipes get their alumina from china clay, and a direct alumina addition is the tool for adjusting a recipe without also adding the clay's silica.
When to add it directly
When a glaze runs and adding china clay would change the silica balance too, or when a recipe needs alumina without additional plasticity in the raw glaze. It is a corrective addition rather than a foundation material, and small percentages make a noticeable difference.
In kiln wash
Kiln wash is painted on shelves so that a run of glaze can be chipped off rather than welded to them, and alumina is a principal ingredient because it is highly refractory. A well-washed shelf is the cheapest insurance in the studio, and reapplying it is a routine job rather than an occasional one.
Handling and buying
A fine powder with the usual precautions: ventilation, a fitted respirator, wet cleaning. It is inexpensive and a small quantity lasts a long time in glaze use, while kiln wash consumes more. Suppliers list it among the raw materials rather than with kiln furniture, which is where people often look for it first.
Questions people ask about alumina trihydrate
What does alumina do in a glaze?
It stiffens the melt so the glaze stays on the pot instead of running onto the shelf. Too much makes a glaze dry and underfired-looking.
Should I add alumina or china clay?
China clay is the usual source and brings silica with it. Direct alumina is the tool when you need to stiffen without changing the silica balance.
What is kiln wash made of?
Refractory materials including alumina, painted on shelves so a glaze run can be chipped off rather than welded to them.