Why Pots Break in the Kiln
So often potters talk about pots blowing up in the kiln because they have an air bubble in them but this is actually not completely true. Yes, expanding air can make a pot crack or break but that usually happens in the drying process. The clay gets smaller as it dries and squeezes the air which causes a crack or break. Sometimes the cracks are so small they aren't seen until the kiln is fired and expansion and contraction have done the damage. So it is a good idea to avoid air pockets without some means of egress for the air. But we usually make air pockets when we load the kiln. In a bisque kiln we often stack pieces which create an air pocket. Any piece with a foot wil create an air pocket just by sitting on the kiln shelf.
Jar with Stuck Lid
When I made this lidded jar I forgot to put paper or cloth between the jar and the lid while the clay dried. The result is that the lid stuck to the rim of the jar. Since I had not done anything to make it a strong connection- like slipping and scoring, I figured the lid might come free in the firing where a poor connection will usually separate. Of course, it'll only come apart if you DON'T want it to.
If air pockets explode in the kiln this would have blown up, but it didn't. Actually, I've been unable to get it to separate at all. So sad.
So what makes pots blow up? Moisture. As the water expands from the heat of the kiln it needs room and breaks apart the clay. It's the worst enemy of a pot in the kiln. If you make sure the clay is completely dry before you get the kiln over 212 degrees Fahrenheit you should not have any problems.
I have yet to have a pot blow up but I've had lots of rim cracks and even one S crack on a slab bottom, I have read that there is also chemical moisture that fires out at 500 to 600 F I think, not sure if that would make a pot blow up. I set my kiln to hold at 200 for ten minutes since I once set it at 212 but It went slightly above, so I reasoned just below would keep the water from boiling.
ReplyDeleteYes, setting the temp at 200 is probably better than 212 but I wonder how much good holding at only 10 minutes is doing. Most people I know candle for a few hours. Of course it’s working for you so keep doing what you’re doing.
DeleteThanks Cheri, it’s no big deal, if I cried over broken pots I’d be in tears every week. It’s all part of being a potter.
ReplyDelete