Like most things in clay, there are numerous techniques for adding sodium to hot kilns during soda firings. Below are several methods that work well for various artists.
So what’s the best way to get the soda into the kiln? Although just dumping it in—an approach I’ve often taken—may be the quickest and easiest method, it isn’t always the best option. Just throwing powdered soda into the kiln has mostly resulted in piles of soda on the floor of the kiln, instead of on the sides of pots. Some soda does migrate to the surface of the pottery, but it is minimal and concentrated near the soda ports. Of course, there are other ways to get soda into the kiln.
Fuming, not so dissimilar from just stoking soda chunks or powder, can work to subtly enhance specific sections of the kiln. Fill a small slab or bowl with chunks of baking soda, salt, or small amounts of oxides, and place them in strategic locations while loading the kiln. The material will fume in small sections of the kiln during the firing. This can be done in conjunction with other soda introduction methods mentioned below.
Filling a bucket with water and dissolving 1–2 pounds of baking soda, soda ash, or salt, then dipping pots into the bucket can be an option for preparing them for a firing. Bede Clarke uses one cup of salt/soda to one gallon of water to create a brine. “I often bathe bisque-fired pots in a salt brine as the primary means of introducing soda/salt,” says Clarke. “I will also embed thin clay slabs with rock salt and mask areas of the pots with these. The upper temperature range for this technique is probably cone 08.”
The consensus among those working most frequently with soda firing is to mix baking soda or soda ash with warm or hot water, and spray it into the kiln using a garden sprayer with a metal tip. A typical mixture is one pound of soda ash to one gallon of hot or boiling water.
Each artist I spoke with has experimented with modifying this recipe to an extent. For example, I often use a mixture of baking soda, soda ash, and borax in hot water.
A Personal Preference
Some artists I spoke with have used only baking soda or only soda ash. When asked about her soda-spraying process, Teresa Pietsch says: “I spray soda over two sessions using a solution of one pound of soda ash to one gallon of water. I spray half of my solution in the kiln when cone 04 is soft in the coldest part of the kiln. Then I lightly reduce the kiln to give the clay a darker color and variation. I spray the other half of the solution at cone 01–1 and continue firing for another 30 minutes to an hour. This allows the soda to thoroughly melt on the pots. I brush on a borax wash [prior to the firing] to have some added flux in the areas that I know won’t get hit by much soda, for example on the inside of a lid. I also allow the kiln to cool slowly to help encourage the iron crystals in the clay.”
Sodium is the chemical element of atomic number 11. It is part of the alkali metals column of the periodic table, which it shares with lithium, potassium, and several other metals. Although much of my work employs soda firing using sodium carbonate (soda ash) or sodium bicarbonate (baking soda), I’ve experimented with a variety of alkali metals at lower temperature ranges as well.
Of course, the more soda solution you use, the more prominent the effects will be. Additionally, as your kiln is used more frequently, it will season as residual soda builds up on the walls. This results in less added soda required to see the same fired results.
Having experimented with a variety of alkali metals also at high-fire temperatures, it seems that there is translation and similarities in color and effect across the temperature ranges. Of course, at lower temperatures, the soda-vapor glaze is more
subtle, and there is less time to build up layers of glaze as you might in high-
My research has led me to the following conclusions:
- Soda (sodium carbonate or sodium bicarbonate)—I use a mixture of baking soda and soda ash. This solution tends to result in more subtle gray/blue glaze hues, subtle flashing marks, more directional effects, and softer and flatter glaze texture on the surface. It seems to be necessary to introduce the soda suspended in water.
- Salt (table salt)—I use table salt. This approach tends to result in more subtle yellow/brown glaze hues, more prominent flashing marks, and harsher and more orange-peel-like glaze texture on the surface, but less directional effects. Using salt is an opportunity to introduce the sodium as a solid. I’ve added salt by pouring it onto a piece of angle iron and dumping the salt into the kiln in a series of charges, or additions.
- Potassium (potassium chloride)—I use crushed water softener pellets. This method tends to result in a look that is very similar to soda, but the added flux is introduced as a solid in pellet form, similar to table salt charges. It’s
difficult to get a directional flashing or glaze surface with this application method because the sodium is not being sprayed. However, some orange-peel effects seem possible, and further research is needed to confirm the exciting possibilities
that potassium chloride might provide.
Excerpted from Low-Fire Soda by Justin Rothshank, published by the American Ceramic Society and available in the Ceramic Arts Network Shop: https://mycan.ceramicartsnetwork.org/s/product-details?id=a1B3u000009udqwEAA.