Sodium carbonate, or soda ash, is a common glaze chemical for ceramics. Soda glaze produces a surface blush of color low firing, and becomes a unpredictable vapor at high temperatures. This unpredictability is valued by potters, since it produces a unique piece each time. Soda ash glazes provide an alternative to environmentally hazardous salt-firing, and produce only water vapor and carbon dioxide. Soda ash is an inexpensive glaze chemical that is safe to handle during all points in the glazing process.
At temperatures near cone ten (2350 degrees F), soda ash becomes a vapor. The vaporized soda ash is deposited on any solid surface it comes into contact with, including ceramic pieces, kiln walls and kiln posts. This results in a unique pattern on each piece, but can have unexpected results. Load the kiln carefully to prevent pieces from uneven coatings. Vaporized soda ash may also react chemically with other glazes.
Soda ash is a deflocculant, a substance that reduces viscosity. When mixed with a glaze or when soda ash vapor comes into contact with another glaze, it creates a thinning effect. This can be used to produce beautiful "drippy" glaze patterns. It can also ruin a carefully painted glaze, by causing it to run down the side of a piece and onto the kiln floor. Soda ash can be used to keep slips from thickening, as well.
Like salt glazes, soda ash glazes are primarily transparent, but not completely. Soda vapors react with aluminum, iron and silica in clay and slips to create colors ranging from yellow and red to pink, blue and gray. This coloration can be controlled to some degree by deliberately adding materials to the kiln, but is primarily random. Colored areas may be smooth and glassy, or pebbled "orange peel" surfaces, depending on how the soda is deposited.