Bisque firing help please

[Bev77]

Hello, I acquired an old used Duncan kiln w/kiln sitter. I never used a kiln before and my first bisque fire went great. My second not so much. I had quite a few pots explode. I did some research and thought I found out this happened because the pots were not dry enough. So today I was going to fire low for an extra two hours and within 30 minutes I heard a pot explode. Everything that was in the kiln sat drying from 1 to 3 weeks and were not "cool" to the touch. Can someone please tell me what I am doing wrong before I ruin anything else.

[liambesaw]

Either the pots were thick and had moisture trapped, or they got too hot too fast, or what is more likely is both.  Even heating is important, especially if there are thick or damp pieces inside, so maybe a piece next to the elements was thicker and blew.

[hitchmss]

Agree with Liam; pots exploding are almost exclusively caused by moisture converting into steam, expanding rapidly, and blowing the pot up. Water converts to steam at 212* F, and expands 1600 times its volume, that's a LOT of expansion! Even when my pots are nice and dry, and I dont have super thick to thin areas which could hold moisture differently, I will generally hold the kiln at 210* for about 30 mins or so, to make sure all the physical moisture has been driven off, before I could potentially convert it to steam.

With manual kilns (switches and kiln sitter, not control panel and thermocouple) when you turn a switch to a heat setting, it will provide a constant amount of heat at that setting. That amount of heat MAY be too much given some circumstances. Its common, with manual kilns, to leave the lid propped 1-2", and only turn the bottom switch on LOW for a few hours, or even overnight. Heat rises, and the bottom elements will warm the work up top too, the lid being open allows heat to escape, and not get above that 212* point. The lid can then be closed, give the kiln/work some time to adjust to the lid being closed (1-2 hours) and then turn all elements to low, 2 hours later all to MED, 2 hours later, all to HIGH until done.

1-3 weeks may be your issue; unless I am running a dehumidifier, or sitting pots in front of a heat source, even my very thin walled pots are rarely ready to fire 1 week after making(Ohio, dampish climate). 90% of the moisture may be gone, but they are not ready. Use the back of your hand, or hold a pot to your cheek (both skin surfaces which are more adept to telling temp differences than the rough insides of your hands) to check for "coldness". If you have a heat source in your studio, you can always stick pots in front of that, before firing, to allow them to dry out further. It takes time and practice to learn when pots are "good to go". If they are varying colors of "dryness" then they definitely are NOT ready to fire.

It COULD be a technical issue with your kiln (im thinking something silly, like maybe the dial for a switch is improperly aligned (showing low, but it actually means HIGH)), but with manual kilns there's not a whole lot to fail to cause rapid heating issues. Id do one switch at low for a while with lid cracked, then all switches to low at a later point. This should fix your issues.

[Bev77]

Thank you so much for your answers. I will definitely take your advice and try this method. 

[Mark C.]

Use a soft vacuum brush to clean out the small blown up bisque ware from the elemnts -they need to be clean or a crevice tool but do not touch the element with that hard plastic tool end. The element damage very easily . But they need to be clean of exploded pieces to be fired again. 

dry your work more and go slower with heat.

[neilestrick]

How thick are your pieces? You can fire thick work, but you have to fire a lot slower. There will always be a little bit of moisture in the clay, even if it sits for months. That last little bit has to evaporate before it turns to steam. In thick pots, that can take several hours at temps under 212F. Lots of folks keep the kiln on overnight with the bottom switch on Low and the lid cracked an inch, to make sure everything is good and dry before starting the firing.