Soda ash is sodium carbonate, a strong and very soluble flux. In a glaze it lowers the melting point sharply; in a casting slip it is half of the deflocculant system that lets slip pour at low water content. Its solubility is both what makes it useful and what makes it awkward.
As a flux
Sodium is one of the most powerful fluxes available, so soda ash brings a melt down in temperature faster than almost anything else per unit weight. Glazes relying on it tend to be glossy and bright, and sodium also gives high thermal expansion, which means soda-rich glazes craze readily. That is a fault on tableware and the mechanism behind deliberate crackle glazes.
As a deflocculant in casting slip
Adding a small amount of soda ash, usually with sodium silicate, makes clay particles repel each other, so a slip flows freely with much less water in it. Less water means less shrinkage in the mould and fewer cracked casts. The quantities are tiny, fractions of a percent, and overdosing gels the slip into something unusable, so it is weighed rather than estimated.
Why solubility complicates a glaze
Soda ash dissolves in the water of a glaze bucket rather than sitting as a suspended powder, so it migrates: into the porous bisque as the glaze dries, and to the surface as water evaporates. That changes the glaze's composition where it matters and is why most recipes use a frit, where the sodium is already melted into an insoluble glass, instead.
Handling and storage
Soda ash is alkaline and dries and irritates skin, so gloves are sensible, and it is a fine powder, so the usual respirator and ventilation apply. It also absorbs moisture from the air and cakes, which changes what a scoop weighs; keep the bag sealed and weigh rather than measure by volume.
Questions people ask about soda ash
What is soda ash?
Sodium carbonate. In pottery it is a strong flux in glazes and part of the deflocculant system in casting slip.
Why use a frit instead of soda ash?
Frit has the sodium already melted into an insoluble glass, so it does not dissolve in the bucket and migrate as the glaze dries.
How much soda ash goes in a casting slip?
A fraction of a percent, weighed rather than estimated. Too much gels the slip and makes it unusable.