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Pinpointing Kiln AtmosphereMost potters are aware of how important the ingredients of a particular glaze recipe are, but equal importance should be given to the development of a proper firing cycle. A firing cycle consists of a time vs. temperature curve, as well as the optimum atmosphere required to produce any given special glaze effect. Many potters determine the kiln atmosphere by visually evaluating the color of the flame exiting the flue or visible through the kiln's eye sight/view port. The problem with this method is that the properties of the flame being observed change continuously as the kiln temperature increases.
A more precise method of analyzing kiln atmosphere is through the use of a commercially available digital oxy-probe. The oxy-probe continuously samples the kiln's atmosphere and provides a much higher level of firing and atmosphere control. (A side benefit to using an oxy-probe is that it doubles as a highly accurate digital type R or S thermocouple.) Changes in damper position, gas or air supply are identified on the instrument's display screen within minutes of the change. This allows the kiln operator to see the magnitude of the change and make adjustments, if necessary, to save fuel and time.
This level of control can also help potters produce the "perfect" copper red glaze. To develop a copper red glaze, the source of copper must be changed to a state different from when it was introduced. For example, a Raku glaze with copper carbonate as a coloring agent would be green when fired in oxidation; after post-firing reduction, the glaze can reveal a variety of iridescent metallic colors, including a color reflecting a new copper penny. This example indicates that when copper is reduced to its base metal, it will tend to develop a red color rather than an oxidized green color, and that copper is highly susceptible to reduction at the cone 06 Raku temperature. By accurately pinpointing the kiln atmosphere with an oxy-probe, you can more easily control the effect of the finished glaze.
If most copper red glazes are fired to cone 10, what would be the optimum temperature to change the state of the copper used as a coloring agent in a glaze? Would it be easier to effect a change to the oxide in a fully mature and fired glaze, or when the glaze is more open-say, before the majority of ingredients begin to sinter?
The more porous a glaze, the greater the effect of the atmosphere on the oxides used. In a copper red glaze, copper oxide is most susceptible to change between 1450 and 1650°F. If we reduce our oxide in this temperature range and never allow the copper oxide to re-oxidize, we can make the most dramatic and uniform change to our glaze. The amount of reduction required depends on the kiln size and burner input. In my testing, I have found that moderate to heavy reduction for 30 to 40 minutes (according to my oxy-probe reduction chart) is best.
Most potters who fire reduction glazes tend to clear their kiln after a period of glaze reduction to minimize pinholes in the glaze. However, copper is a fairly volatile oxide, and the typical "glaze reduction" period condoned by potters at peak temperature, followed by "clearing" of the kiln atmosphere, tends to wash out colors by liberating the oxide closest to the glaze surface. If the idea is to reduce the copper from green to red, then why re-oxidize this same oxide in a clearing cycle? A firing that is adequately reduced at the proper temperature, and fired to peak temperature at the proper rate of climb while maintaining a neutral (not oxidizing, not reducing) atmosphere, should provide a consistent and repeatable firing cycle, with a clear, hard glaze surface and a uniform copper red throughout the kiln.
A Cool TipThe most overlooked area of any firing cycle is the time that a kiln takes to cool. Are you aware that the cooling cycle is actually a very long period of oxidation? Once the gas is turned off, the kiln atmosphere immediately begins to oxidize. When you close the exhaust damper at the theoretical end of the firing, you are actually prolonging a period of oxidation at an extreme temperature by minimizing the kiln's ability to cool. In effect, you are forcing the kiln to cool more slowly. Slow cooling promotes the development of crystalline growth, which can be recognized as a matting of the glaze surface. However, the faster you can initially cool a glaze, the brighter and glossier that glaze will be. Quick cooling of a glaze "freezes" the glass and allows less time for the development of any type of crystalline structures that could produce a dull finish or promote opacity.
When I fire my kiln, I monitor three zones of temperature, as well as the atmosphere via an oxy-probe. I record all of the temperatures, oxygen readings, fuel valve position, and exhaust damper changes every 15 to 30 minutes. These records enable me to repeat any given firing cycle, and also provide me with the ability to modify a firing in the hopes of improving on the last firing. After I unload my kiln, I make comments on the back of my firing schedule regarding loading patterns, losses and sweet spots in the kiln for future reference. Whether I'm producing copper reds or some other special effects glaze, this level of detail helps ensure that the end result is successful and repeatable.
The ingredients in a copper red glaze are important. But don't forget that your kiln is an equally important tool in the creative process.
Jerry Wagner began his wholesale pottery operation, Wagner Pottery, in September 2001. He currently divides his time between producing pottery and building kilns for potters and industrial ceramic manufacturers as a kiln engineer at SBL Kiln Services Inc., Bridgeville, Pa. If there is a specific a tip or technique that you would like to see covered in a future column, contact Jerry at (724) 438-6957 or firstname.lastname@example.org .