SOAS

Soda ash is sodium carbonate, and in ceramics it goes mostly into a casting slip as a deflocculant. A very small amount thins a slip that would otherwise be too stiff to pour.

Soda ash is a deflocculant, and it works on the clay and on the water at once. The sodium exchanges onto the clay particles so they hold each other apart instead of clumping, while the carbonate takes calcium out of the water and drops it out of solution — which is why one dose behaves differently on hard water and on soft. A slip commonly carries a fraction of a part in a hundred of the dry clay weight, usually alongside a second deflocculant, and the effect reverses past its optimum: add more and the slip thickens again. It also draws moisture out of the air and cakes, and caked material carries less soda per gram than fresh does.