In the Plainsman lab we have standardized on a simple firing curve for test specimens that provides a soak for final drying and then proceeds to 100 degrees below the final temperature at 400 degrees per hour, then slows to 100 degrees per hour for the final stage. The cool is freefall. Of course, depending on the kiln used, the rate-of-rise might be slower, especially in the later stages.
|1||100°C/hr to 100C||180°F/hr to 212F||60min||2:02|
|2||150°C/hr to 960C||270°F/hr to 1760F||0||7:46|
|3||100°C/hr to 1060C||180°F/hr to 1940F||5min||8:51|
|4||Freefall°C/hr to 30C||0°F/hr to 86F||8:51|
Here is an example of our lab firing schedule for cone 10 oxidation (which the cone-fire mode does not do correctly). To actually go to cone 10 we need to manually create a program that fires higher than the built in cone-fire one. Determining how high to go is a matter repeated firings verified using a self supporting cone (regular cones are not accurate). In our lab we keep notes in the schedule record in our account at insight-live.com. And we have a chart on the wall showing the latest temperature for each of the cones we fire to. What about cone 6? Controllers fire it to 2235, we put down a cone at 2200!
Four o'clock. These are self-supporting cones, use these. I was consistently getting the cone on the left using a custom-programmed firing schedule to 2204F (for cone 6 with ten minute hold). However Orton recommends that the tip of the self supporting cone should be even with the top of the base, not the bottom. So I adjusted the program to finish at 2200F and got the cone on the right. But note: This applies to that kiln with that pyrometer, our other test kiln puts cone 6 at 4 o'clock at 2195F. Of course, if you want the kiln to hold at cone 6 for longer the cone will bend further, so the top temperature would need to be reduced to compensate for that. If you are using the automatic programs (e.g. cone 6 schedules go to around 2230!) your kiln is almost certainly over firing.
G2853B - Cone 04 Clear Ravenscrag School Glaze
A Ravenscrag Slip base made by simply mixing it 50:50 with a frit
Plainsman Electric Bisque Firing Schedule
Three-step to 1832F