Kiln Atmosphere

Last quarter I offered the students more options than usual for firing temperature and atmosphere. Each quarters we fire our work in a bisque kiln to prepare it for glazing. Most work is then glazed for cone 10 reduction and fired in the gas kiln. A handful of pieces might be raku fired or smoke fired during our one quarterly raku/smoke firing day.

a student's horse hair raku pice fired in the fall

On the occasions when I have a hand-building class, I generally allow those students to fire glazed work to a low temperature. Since their sculpture is not meant to be food safe or water-tight it does not require a mature clay body and the glaze does not need to be food safe. The low-fire work can be glazed with underglazes, stains, slips and low temperature glazes. The work is then fired in the same electric kilns that are used for the bisque firings.

unfired work loaded into an electric kiln for bisque firing

Last quarter I tried to offer a high-fire oxidation firing in addition the the usual high-fire reduction firing. I say "tried," because despite my best efforts, the "oxidation firing" produced some very reduced results. Reduction or oxidation refers to the amount of oxygen and fuel available inside the kiln. Electric kilns generally produce oxidation (or neutral) atmospheres because no fuel is added and plenty of oxygen is available inside the kiln. Gas kilns can be manipulated to produce oxidation or reduction atmospheres by either introducing more air or gas in the burner or by allowing or preventing air from entering the kiln from other openings.

the studio gas kiln about to be unloaded

The most efficient firing is a neutral atmosphere, though I believe many potters tend to blur the line between neutral and oxidizing atmospheres, referring to both as oxidizing because the glaze results and clay body color are similar. As with most areas of life, there isn't a pure distinction between oxidizing, neutral and reducing. Instead there is a gradual shift from one end of the spectrum to another so that a kiln can be very reduced or only slightly reduced, etc. Reduction levels can also vary over the course of the firing.

clay bodies (without glaze) containing iron turn darker or speckled in reduction; porcelain turns white

Some functional glazes call for a reduction atmosphere in the kiln in order to create the desired color or texture on the work. In a reduction firing, the kiln has more fuel than air, causing the fuel to look for any available oxygen with which to react. In a reduction firing the fuel can react with the iron oxide in the clay, causing the clay body to turn brownish or speckled. The fuel can also react with oxides in the glazes, such as copper oxide, pulling out the oxide and leaving the metal. This process causes green copper oxide to turn into red copper in a reduction fired glaze.

copper glaze reduced in what I thought would be an oxidation firing

Until recently most of what I knew about atmospheres and glaze reactions came from what I was taught in school and what I had read, particularly as pertained to glaze recipes and specific effects. However, this weekend I read Nils Lou's "The Art of Firing," which explains this process much more thoroughly. The book also talked about when during the firing process atmosphere affects the materials in clay and glazes. I knew that the clay body reduction happens earlier than glaze reduction, but this weekend was the first I'd ever heard that copper reduction happens during cool down.

Copper reds don't flash until they are cooling!

Copper reduction happens during cool down. This one short line at the bottom of the page blew my mind. For those of my readers not amazed by this bit of missing chemistry information, this means the copper oxide in the glaze is not affected by the lack of oxygen during the firing. Instead, the copper reacts after the firing, during the cool down time. 

Usually when I finish firing, I close the damper and the other kiln openings up tight to prevent the work from cooling too rapidly. Though I am not adding any fuel, I am also not allowing any air into the kiln. According to the Lou, the oxygen deprivation around 1500 to 1100 degrees is what causes my reduced copper glazes. This bit of information blew my mind because it suddenly solved a mystery for me from last quarter. When I tried to run an oxidation firing for my students, I tried to allow more air during the firing but closed the kiln up normally at the end of the firing. Students were disappointed in the resulting bright red coppers, since I had promised them greens. At the time I thought this was an error in my firing, now I suspect it was an error in my cooling.

Now I wish I had finished reading Lou's book years ago when I started. I stopped because I found the writing dry and the information too simple and too specific. The first chapter deals with kiln construction and the second with the basics of atmosphere. I'm not building a kiln, I want to improve my firing. I understand the basic concepts, I wanted some insight into how I might adjust my firing protocol on a good kiln to create better results.

When I started reading again, I was still frustrated because the book's initial solution to recognizing kiln atmospheres is to use an oxyprobe. That's great if you've got one, but we don't. (On a side note, I plan to order one ASAP.) An oxyprobe is a much more accurate way of gauging the reduction happening in the firing. The other methods are watching the flame color and how big the flame is in the chimney or coming out the spy hole. I was hoping the book would give some more subtle suggestions, but besides the oxyprobe it was stuff I already knew.

reducing flame after I opened the spy hole

I learned to fire a variety of kilns in college and graduate school, but I hadn't fired high temperature gas firings all that often. My most extensive practice has been in the YVCC studio. When I first came to YVCC the kiln was an old updraft kiln, which means it had an opening at the top for air to exit. The air inside the kiln could be controlled with a damper across the top opening. The damper consisted of a couple of shelves with handles to push them across the center opening. In my experience this kiln was difficult to reduce, which may have been a result of the kiln's age or the kiln's structure. The opening at the top and the cracks at various places allowed air out. I rarely got bright reds in that kiln and the clay body was generally pretty light. Knowing what I do now, I would guess that the kiln was hard to close up after the firing, resulting in plenty of air in the kiln and therefore oxidized (green) copper glazes.

oxidized copper on the interior of a raku fired piece

In any kiln, one can adjust the mix of air and gas at the burner or one can also adjust some of the openings in the kiln, mainly the damper in the chimney or at the exit of the kiln. These adjustments can cause the atmosphere during any stage of firing (or cooling) to be oxidizing or reducing. On the current YVCC gas kiln we have a chimney which opens into the kiln near the floor. The air exit at the bottom makes our kiln a downdraft. It also tends to make for a more reducing and more even firing. In the old updraft kiln there was a significant temperature difference from the top to the bottom. The new kiln fires pretty evenly as long as the firing is controlled well.

Though I have fired the kiln many times and can comfortably get repeatable results with little variation caused by weather or the type of work in the kiln, the oxidation firing results shook my confidence and made me want to figure out what was wrong. It is easy to get into a comfortable habit of firing and a bit intimidating to try something else in case it doesn't work as well. Last quarter a bisque firing for tall work (too tall for our electric kilns) didn't work out perfectly and that also bothered me. There may have been some thickness issues in the work, but what I have now read makes me wonder if the kiln was reducing during the cool down, resulting in some of the cracks that showed up in the work.

tall bisqued work in the gas kiln

Though I thought it was helpful, I was also bothered by the tone of "The Art of Firing." The book sorta hinted that anyone who was firing kilns and didn't know this stuff wasn't using common sense. On the other hand, the book was written--for whom? Having learned firing basics in college and then bits and pieces of kiln loading and firing from at least ten different people, I am surprised that this was the first I had heard of things like copper flashing in cool down. There were other parts of the reading that were more of a reminder. In that way it was like talking with someone who knows about firing. In my current position I am the clay, firing and glaze expert so there are few people nearby with whom I can discuss firing adjustments and errors. Reaching out to an expert or colleague in another town seems like asking for a solution rather than engaging in a discussion or brainstorming to come up with an adjustment.

reduction fired load ready to come out of the kiln

"The Art of Firing" had some useful information, as I said, but I was surprised that some of the information contradicts the hand-me-down knowledge I had hitherto learned in graduate school. This quarter I hope to find time to test some of Lou's recommendations. I threw some plates bowls and mugs last week after classes in hopes that I can fill most of a kiln with my work for a second test oxidation firing.