I recently built a 40 cu./ft. downdraft sprung arch, gas-fired, soda kiln. I introduce the soda ash on an angle iron through ports above the firebox. Some of my pots have rough areas on the unglazed parts of the clay body.

Soda firing has become popular over the years since my initial research but there are still some factors that have to be considered to achieve a satisfactory surface area on pots. Whether firing with soda ash (sodium carbonate, sodium bicarbonate) or salt (sodium chloride) it is important that only the vaporized sodium reacts with the alumina and silica present in the clay body forming a sodium, alumina, silicate glaze surface. Greater care has to be taken with sodium carbonate or sodium bicarbonate as it does not disperse broadly as vapor compared to sodium chloride (salt). For efficient vaporization to take place in ether salt or soda firing the material must be dispersed in small amounts over a wide surface area at the highest level above the firebox. In order for a traditional ‘orange peel” pebble-like surface or even a smooth glazed surface to form the clay body must be dense and vitreous. When material that is not vaporized lands on the exposed clay body it does not enter into an active melt causing rough pitted areas. Conversely, immature clay bodies can also be salt or soda fired which can result in flashing surfaces that are not vitrified. The degree of flashing and its location on the pottery depends on pots placement in the kiln in relation to the sodium source and the variables of kiln atmosphere.

There are several methods for a correction beginning with using less soda ash on the angle iron and slowly tipping it over away from the stacked pots in the kiln. Additionally, placing the pots away from the dropping soda ash will prevent direct contact with the material. A more complicated kiln design fix would be to place the soda entry ports at the highest possible level above the firebox allowing the soda ash the greatest distance to vaporize before hitting the firebox floor. This method will allow for the minimum amount of non-vaporized soda carbonate to pool at the bottom of the firebox. As in salt and soda kilns the bottom of the firebox will eventually collect molten pools of glass which upon cooling will contract drawing the surrounding firebox bricks out of alignment. High alumina content firebricks will prolong the life of the firebox and bag wall areas.

For information on soda firing see, “Sodium Vapor Firing” Alfred University, College of Ceramics, Alfred, NY; Part 1. June 1973; Part 2, April 1974 Jeff Zamek M.F.A. thesis
Additional information, “Alternative to Salt Glazing” Craft Horizons, June 1973, “What Every Potter Should Know” by Jeff Zamek, Chapter 19. Soda Vapor Firing.