[Clayart] Kiln schedule question
claywork at flying-snail.com
Mon Feb 22 15:10:53 UTC 2021
> On Feb 21, 2021, at 10:36 PM, David Woof <woofpots at hotmail.com> wrote:
> Firing faster but of longer duration is a dubious way to "save" What?
Personally, I find most pre-programmed firing schedules to be overly conservative in their ramp rates, but deficient in their candling and soak/hold, which I always add on with extra time..
My own kilns are all manual, but I have spent a lot of time with the electronic kilns where I teach, comparing schedules and outcomes. Our principal constraint (back in the ‘Before Times’) was the every-other-day class schedule. As students there have no out-of-class access to the facility, it’s important for them to get their work back in a timely fashion, to apply what they learn from their results. Bisque firings in particular were an issue, as the standard ’slow’ schedule with a decent candling phase left the kiln too hot to readily unload by the next class session. I experimented with variations, and concluded that a ‘fast bisque’ schedule on a Bartlett controller, or a ‘medium' on a Skutt controller worked just fine if the candling phase was extended and a soak added. It still completed the firing sooner, and was cool enough to unload in the following class session. (Our class clay and glaze combo was prone to pinholing from inadequate bisque, but this does not happen with the current schedule.) The same is true of glaze firings…a faster firing ramp with a longer final soak gives better results in less time. (I fire my manual kilns on a much faster schedule than any preprogrammed schedule anyway, and the recommended schedule posted for most manual kilns is also much faster!)
Based solely on observation of outcome, I have concluded two things, and they are closely linked: 1: There is little reason to go slowly through the middle ranges of any pottery firing. I seldom use the ‘medium’ setting on my manual kilns except for controlled cooling. Although we are often told that release of molecular water, carbon and sulfur burnout, and other critical stages require a more gradual heat increase, this is not confirmed in my experience. 2: No standard electric kiln is really capable of firing 'too fast' anyway. Even on full power they are pretty slow, and firing outcomes are good for both bisque and glaze firings, as long as a sufficient candle phase is added to bisque firings and a sufficient soak(hold) is added to glaze firings. Gas kilns often have excess heat capacity and should usually not be fired at full speed, but a typical top-loading studio electric kiln is a Subaru, not a Masarati.
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