Why do pieces explode in the kiln?
The culprit is too much moisture and hollow air pockets in the clay. Water turns into steam at 212°F degrees. The moisture turns into steam and creates pressure. With nowhere for the steam and pressure to escape – it explodes like popcorn.
Here are a few ways to reduce the chances of pieces blowing up in the kiln.
1. Air dry pieces for as long as possible. Higher humidity level as well as thicker clay walls will slow dry time.
Here are two quick tests for moisture:
ONE: Put the greenware up to your cheek. Is it colder than room temperature? It still has too much moisture in it. Let it dry for longer.
TWO: Place the greenware on a sheet of paper. Does the paper wrinkle? If so, there is still moisture trying to escape the piece. Keep on letting it dry.
2. Create a hole for steam to escape in hollow forms that need to be set upright. A hollow piece, set upright and without a hole can become a grenade in the kiln. While the clay is still leather hard, use a needle tool to make a small inconspicuous hole somewhere on the piece. Make sure the hole is large enough, so that when the clay naturally shrinks from moisture loss, the hole still remains big enough for steam to escape.
3. Unless absolutely necessary, build clay walls less than 1 inch thick. Less than 1 inch of clay thickness lowers the risk of having pockets of air and moisture deep within the piece. Clay pieces will dry faster and lighter after firing.
4. Fire the kiln using a long preheat. Use the preheat function on a programmable electric kiln. Most electric kilns can hold temperature at 180°-200°F for a programmable amount of time to allow moisture to evaporate from the greenware before the kiln heats up past 212°F. Consult your kiln’s manual for more details on how to program the pre-heat.
5. Fire slow. This works for both older cone firing kilns and newer programmable electronic kilns. When in doubt about the dryness of the piece, fire slow. On newer electronic kilns – consult your manual for the slowest programmable speed. For example, on a SKUTT kiln, this would be the “slow” button when using cone fire mode. On older kilns that use a cone to fire and a switch to control time, start with the kiln on low for 4 hours, medium for 3 and then switch to high until the cone melts.
With proper preparation, you can reduce your chances of losing your artwork to a kiln explosion.