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The problem occurred with standard Plainsman M340, M390, M350, M370 and P300. The stonewares have porosities of 2.5%, the M370 1% and the P300 0.5%. Thus, all of these have comfortable margins for over firing. The G2926B glaze, used on all of them, does seal the surface pretty early so it will contribute to over-fired ware bloating sooner than typical. The problem here is the cone-fire modes on kiln controllers. For this kiln, the cone 6 program goes to 2236F. That's cone 7. Or more. Most people do not want to manually program their controllers. This is unfortunate since they do not benefit from the drop-and-soak PLC6DS and slow-cool C6DHSC firing schedules. And they end up over-firing their ware. Here is what many do: They cone-fire to cone 5. That produces cone 6!
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.
I document programs in my account at insight-live.com, then print them out and enter them into the controller. This controller can hold six, it calls them Users. The one I last edited is the one that runs when I press "Start". When I press the "Enter Program" button it asks which User: I key in "2" (for my cone 6 lab tests). It asks how many segments: I press Enter to accept the 3 (remember, I am editing the program). After that it asks questions about each step (rows 2, 3, 4): the Ramp "rA" (degrees F/hr), the Temperature to go to (°F) to and the Hold time in minutes (HLdx). In this program I am heating at 300F/hr to 240F and holding 60 minutes, then 400/hr to 2095 and holding zero minutes, then at 108/hr to 2195 and holding 10 minutes. The last step is to set a temperature where an alarm should start sounding (I set 9999 so it will never sound). When complete it reads "Idle". Then I press the "Start" button to begin. If I want to change it I press the "Stop" button. Those ten other buttons? Don't use them, automatic firing is not accurate. One more thing: If it is not responding to "Enter Program" press the Stop button first.
Put the pots in, select a cone, press start. It is time to rethink that approach! Seriously. The Bartlett Genesis kiln controller is standard equipment on hobby and production electric kilns now. It is not meant to be run like a toaster! Good glazes are about much more than recipes, they are about firing schedules. None of the built-in "toaster schedules" have hold times on any segments, drop-and-hold sequences or controlled cools. Or even fire-to-cone accuracy. Yet such are a must for defect-free glazes, enhancing the effects of reactive glazes that must develop crystallization or variegation or firing accurately. It is easy to program: Tap the blue edit button to edit a program, tap a column of any segment to edit its value. Tap a segment number to delete or duplicate it. Google "bartlett genesis controller" for videos on creating and editing a schedule.
Designing a good kiln firing schedule for your ware is a very important, and often overlooked factor for obtained successful firings.
When clay materials and bodies bubble as they melt or over fire. This normally happens in raw materials that contain particulates that produce gases during firing.
Bloating in clay bodies occurs when the firing goes high enough to seal the surface and prevent the passage of gases releasing inside.