Kiln question

[Brenda Riggs]

I have a brand new scutt kiln, 818. I know the reasons that cause a piece to explode, I thought. It is moisture in the clay or a air bubble or cooling too fast. My piece was untacked at 300 degrees in the cool down, so between then and finished.  Can it explode that late? Do I need to set the kiln to a slower cool down. Please let me know if there are any other reasons why it would explode?

Brenda

[neilestrick]

Cooling too fast does not cause explosions. It can cause cracking (what we call dunting), but not explosions. Dunting typically occurs in glaze firings, not bisque. Air bubbles do not cause explosions, either. Air does not expand enough as it heats to blow the clay apart. Only moisture causes explosions. When water turns to steam, it expands about 1700 times, blowing the clay apart. It only happens at the beginning of a firing. When you put a piece in the kiln, it is not totally dry, no matter how long you let it dry. There is always a little bit of remaining water. That water must evaporate off before turning to steam. If your pot is thick, it needs more time for the water to evaporate, so you need to go slowly at the beginning, or do a preheat to drive off the moisture.

[MFP]

I have to admit that looking at these new firing schedules gives me the willies. I used to pre-heat overnight!  The kiln would be at about 010 when I got up in the morning....but it was a 9 inch soft brick kiln with four burners. I would only have two on low for the pre-heat. 

[neilestrick]

5 hours ago, MFP said:

I have to admit that looking at these new firing schedules gives me the willies. I used to pre-heat overnight!  The kiln would be at about 010 when I got up in the morning....but it was a 9 inch soft brick kiln with four burners. I would only have two on low for the pre-heat. 

Cone 010 is not a preheat, that's a slow ramp. In these electric kiln bisque schedules we're trying to keep the temp below 200F, to get everything dry before ramping up. Gas kilns don't usually hold at low temps the way electric kilns do. I used to preheat my gas kiln overnight with just the pilots, which would hold at about 250F. With the burners on as low as possible I'd hit about 1000F

[MFP]

Usually took 12 hours to get there.....I am not an early riser.  If I had to fire overnight for some reason....I had "stops" where I would set a concrete block that I would sit on and go to sleep...and when the temperature got to a certain temp....it would wake me up....I would check on where things were....do whatever was needed and then place the block at the next "stop" and go back to sleep    You guys are just so high tech now.  There is a manual crystal procedure in one of the archives on here for an electric  kiln like mine.