What is soda firing?
This post is part of a handout that I give to my soda firing students at Lillstreet Art Center. It is an attempt to explain a little bit about the history of soda firing, and tries to help answer the question, "what is soda firing?". It is not the full story, but I hope that you find it helpful.
Soda firing is an atmospheric firing technique where “soda” is introduced into the kiln near top temperature (2350°, ∆10). The soda that we use is: sodium bi-carbonate, also know as baking soda (the Arm and Hammer™ kind), and sodium carbonate, which is also known as soda ash.
“Soda ash is the trade name for sodium carbonate, a chemical refined from the mineral trona or sodium-carbonate-bearing brines (both referred to as "natural soda ash") or manufactured from one of several chemical processes (referred to as "synthetic soda ash"). It is an essential raw material in glass, chemicals, detergents, and other important industrial products.”
The soda vaporizes and is carried on the flame throughout the kiln. The soda vapors create a glaze when it lands on a piece (or a kiln post, or the wall of the kiln). Wherever the flame travels- so does the soda. When placing the pieces in the kiln during loading, you have to think carefully about when and where you want a piece to get lots of soda, or when and where you want a piece to be more protected. The kiln must be evenly loaded because the flame will travel on the path of least resistance (and therefore the soda will also be traveling on the path of least resistance). You also have to think about whether or not the piece is glazed. The soda is basically a glaze, and when two glazes mix, they can react chemically with one another and run down the side of the piece. It’s beautiful when you can control the run- but can be disastrous when it gets away from you!
What is the history of soda? Where did it come from?
The predecessor of the modern day soda firing, is salt firing. It is believed that salt firing began in Germany in the 13th century. As many things go, it was most likely come upon by accident. Perhaps some salt soaked wood (from pickling barrels?) was tossed into the kiln for the wood fuel. The salt vaporized and glazed the pieces inside the kiln. It was a great time saving measure. No need to glaze the pieces before they went into the kiln. Old German jugs were salt glazed, along with tankards and sewer pipes. The pieces that we think of as early American traditional ceramics from the southwest corner of the US were also salt glazed. Can you picture a big whiskey jug with cobalt blue decoration on it? Those were salt fired. The process that I’m talking about is wood firing with salt thrown in. The salt easily glosses up a piece and helps the wood ash flux out. Salt vaporizes at a fairly low temperature and can work its way into all sorts of nooks and crannies. In a salt firing, the salt vaporizes and the sodium chloride splits into sodium and chlorine gas. When the chlorine is exposed to moisture, it forms hydrochloric acid. The acid goes into the kiln atmosphere and is released from the chimney. The remaining sodium combines with alumina and silica in the clay to forming a glaze on the surface of the piece.
Although the previous paragraph doesn’t help sort out what soda firing is, it does give some important background information. Salt firing continued to be a technique used by potters up through the 1970’s (and is still is used as a firing method today). In the 70’s as people became more aware of the environment, they realized that the black smoke and hydrochloric acid wasn’t such a great idea. A couple of graduate students from Alfred University, NY studied sodium alternatives to salt firings, hoping to find something that was more environmentally friendly, and maybe even something that could happen in an urban environment.
The results were soda ash and baking soda. They produce carbon dioxide instead of hydrochloric acid. The soda doesn’t get into all the nooks and crannies like the salt does, but it does produce brighter and more vivid colors. Pots are usually glazed with an interior or “liner glaze” because the soda vapors won’t work their way into those hard to reach places. You can achieve a rich glossy surface that is heavy with soda, or a pebbled surface that is also referred to as an “orange peel” texture. This is often juxtaposed with a “drier” area of the clay that wasn’t hit directly with the soda. It’s all of these varied surfaces together that make up the rich look and feel of a soda fired pot. 30 years ago, when soda firing first began, most ceramicists were just trying to mimic the effects of salt firing. In the last 5 years that has changed. The true characteristics of soda firing are unique and are something to explore and achieve.
The soda vapors aren’t actually colored, but they are reacting with the alumina, silica and iron in the clay (and slips) to create the various colors of flashing, and associated textures. The resulting colors can be a range of oranges with yellow and red tones, to rich browns, golds and tans. If there is some copper in the kiln, there can be pink blushing. Or a cobalt glaze on a piece can cause a blue twinge to the soda. Sometimes the carbon from the firing can add a gray hue that can look like shark skin on porcelain.
So perhaps now you know a little bit more about soda firing.
I've included the recipe that I use for my soda firings below.
My soda recipe: (a variation on Gail Nichol's process)
2 lbs. soda ash
3.5 lbs. sodium bi-carb
5.5 lbs. whiting (calcium carbonate)
Mixed with ½ of a 5 gal. bucket of wood chips, and water
*mix the dry stuff with the wood chips, and then add COLD water. Just enough so it sticks together. It should have a consistancy similar to oatmeal cookie dough or tunafish salad.
-Add soda into kiln when ∆9 is soft. Add 1 ½ angle irons full of soda mixture through each port. Wait 15 minutes between additions. Usually takes 3 turns to add in all of the soda.