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In my research, I wanted to maintain the integrity of salt-glazing effects and surfaces without the polluting characteristics of salt glazing. Sodium carbonate (soda ash) and sodium bicarbonate (baking soda) seemed to be the best substitutes for salt (sodium chloride) while still giving comparable results and remaining physically and economically suitable for the studio potter. The byproducts of soda ash and baking soda are carbon dioxide and water vapor, which are safer and less damaging than the hydrochloric acid and sodium chloride fumes that are given off during the traditional salt-glaze firing.
To insure an accurate observation of the effects of sodium carbonate, I decided to build a new sprung-arch 40 cu/ft kiln. A standard, high-quality type firebrick was used as the main refractory for the hot face, and insulation soft brick was used as the cold face outer layer of the kiln. It was fired by four natural draft, inspirating burners using natural gas. The firing chamber was coated with a wash of 85% calcined alumina, and 15% E.P.K. clay to protect the bricks from the corrosive effects of sodium vapor glazing.
Soda ash was loaded on an angle iron and fed through two salting ports located above each firebox. A total of 35 lbs of soda ash was introduced in this procedure. After a series of firings, I noted that the best effects were achieved when the vapors were completely contained within the kiln by closing the damper. Because soda ash volatizes at a higher temperature with less vapor pressure than salt, it is necessary to introduce the soda ash in small amounts high above each firebox to insure more efficient volatilization. After subsequent tests, it was determined that due to the low vapor pressure of sodium carbonate, two ports of entry (one on each side of the kiln) distributed the material more efficiently for vaporization.
In soda firings, I have been able to duplicate the orange peel texture and various other characteristics of traditional salt firing. Glaze and slip colors were generally brighter in the soda firing, resulting in yellows and copper reds that were more easily developed. The brighter glaze effects may be due to the absence of sodium chloride and hydrochloric acid vapors in the kiln atmosphere during the firing.
Sodium carbonate vapor glazing requires a somewhat more advanced working knowledge of the material and its characteristics than the traditional salt glazing, but the results are at least as good as salt and may offer an even broader range of color possibilities. Sodium carbonate vapor glazing is more than just a substitute for salt glazing. In addition to its favorable effect on glaze colors, it is less corrosive to metals and is nonpolluting in the atmosphere. Further tests were documented in my book, What Every Potter Should Know.