Water Explosions

As water gets to boiling point, it turns to steam. At atmospheric pressure, the volume of steam is about 1600 times that of water, and so this increased volume needs to escape - or the pressure builds up and the higher pressure either allows the steam to squeeze out through tighter holes, or the piece explodes.
If the piece has already been bisque fired, then the clay is porous, and the steam can escape (assuming that the glaze application is porous, which will be the case unless something like a very high proportion of CMC gum or similar has been added).
The problem is in the bisque firing, as unfired clay is not porous because all of the gaps between the clay particles are sealed with water, and the clay has not yet contracted enough in the firing to become porous. Thus the steam has nowhere to go, and with time the pressure builds up until it becomes too great, and the pot explodes.


When bisque firing, ensure that pieces are bone dry before firing, or do a candling firing first. A good way to see if they are bone dry is to put a piece against your cheek, and it will feel cool due to moisture evaporating off the surface. You can also monitor the weight of a piece and see if it is still going down (assuming it is in a warm, dry location and so is drying).

During the firing, if you open the top bung hole and hold a (cool) piece of mirror, glass or glazed ceramics over the hole, if there is significant moisture in the kiln it will condense out - but this won't tell you if the one thick walled piece has dried out and the low moisture is because all of the other thin walled pieces are dry.