The soda ash that the ceramic industry knows is a refined fine granular white material. It is very pure chemically and has been calcined (often referred to as dense soda ash). 

The common ceramic use of soda ash is as a soluble deflocculant in ceramic slips and glazes. It works well in combination with sodium silicate to produce body slips that do not gel too quickly and whose rheology can be adjusted for changes in the hardness of the water. Higher soda ash proportions (vs sodium silicate) will produce a slip that gives a softer cast (stays wet longer). Sodium carbonate is also the preferred deflocculant for thinning glaze slurries.

Soda ash is not normally used as a source of Na2O in glazes because it is soluble. However it is used as a key source of sodium in frits and glass. Its solubility makes it an ideal flux for Egyptian paste glazes. Although soda ash is a pure source a soda, a powerful flux in ceramics, by itself the powder of the dense version only begins caking at 1500F.