|a student's horse hair raku pice fired in the fall|
|unfired work loaded into an electric kiln for bisque firing|
|the studio gas kiln about to be unloaded|
|clay bodies (without glaze) containing iron turn darker or speckled in reduction; porcelain turns white|
|copper glaze reduced in what I thought would be an oxidation firing|
|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.
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|
|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|
|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.