Designing Pottery: Firing the Kiln

There are two ways to fire pottery, one by burning a fuel (wood, propane, sawdust, old tires, whatever produces heat), the other by running electricity through a metal coil (like a hot plate). There is a fundamental difference in these two processes. In a fire, oxygen is necessary to keep the fire burning; with electricity, oxygen is irrelevant. Many of the ingredients in clay and glazes are in the form of oxides. In a fuel burning situation, it possible to hinder the flow of oxygen by blocking portholes in the kiln and force the flames to strip oxygen from the clay and glazes, changing the look of the ware. This is called reduction firing because the chemically-bonded oxygen is reduced from the pots. There are localized ways to achieve this in an electric kiln (oxidation firing), but for the most part, the oxygen in pots fired in an electric kiln is there to stay. There is slightly different look to oxidation versus reduction pots but neither way to fire is intrinsically better or worse than the other. The bottom line is do you like the way the pot looks or not? Each type of firing has pros and cons but generally speaking, electric firing is more likely to produce consistent results.

When I spend days aiming towards a particular result, I want to eliminate the element of chance as much as possible so we use electric kilns. I have used a lot of brands of kilns over the years and I currently use Skutt kilns. We have two 1027s and one 818, a smaller kiln. The bottom of the kiln is usually slightly cooler than the rest of the kiln but other than that the kilns fire relatively evenly. Still, every time you fire the kiln, there is a potential for problems. A coil could burn out, a kiln sitter (the mechanism that helps control when the kiln goes off) could jam, the electric company could be sending less power (brown-outs) and the length of the firing could change, or some other glitch could occur. Never trust a kiln! A kiln always must be checked during the firing and at the time the firing should end. Every firing has the potential for surprises. There is a reason for the long tradition of potters putting kiln gods (little clay figures) in each firing for good luck!

We fire at cone 6 (mid-range stoneware temperatures) at Mudville. I have fired at cone 6 for all but the first few years I potted. Lately, the cones we have been getting from Orton are firing slightly hotter than those in past years meaning more melt and loss of sharpness in detail. I tried a few cone 5 firings but they have been too cool. I found an old box of bar cones from Bell Research which have been working well. When that box runs out, I'll have to come up with another solution to get the temperature I want.

One of the most difficult steps in making pottery is glaze application. Unfortunately, it's the last thing you do and the most important, so no matter how good the pot is up to this point, how well you glaze determines the success of the pot. Some glazes do best applied thickly; some need to be thin. Sometimes the glaze in the buckets is too thick due to water evaporation and sometimes someone has thinned a glaze too much. Less glaze will adhere to very thin pots (they saturate quickly) and a thick pot has the potential to absorb more glaze than you might want. It's difficult to get glaze on evenly on large pots. Some glazes have a very narrow temperature range and will be glossy in a hot kiln and matte in a cool one. The list of variables goes on and on. There is no resemblance to the way the unfired glaze looks to the final outcome so you can't see what you're doing. To some extent, the same is true for application of underglazes - too thick? too thin? - and this adds to the complications. I don't know of any other craft where what ultimately happens to your work is so out of your hands. Until the pot is finally out of the kiln, anything can happen. As they say, the kiln keeps the potter humble.

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