>I had a piece blow up that had already been fired and was in for the glaze
>firing...it blew and ruined other pieces.
>in consistent kiln temp?
>. . .
1. This sometimes happens when re-firing a piece that was previously glaze fired because the first glaze seals the surface. To be safe, pre-dry it with heat before trying to re-fire a re-glazed piece.
2. Once in a while a student glazes a dry piece of greenware by mistake and puts it with pieces to be glaze fired. This can be trouble if you fire it like a pre-fired ware.
3. I suppose a thick bisque piece that is still full of glazing water could explode if fired fast before it has a chance to dry. This is very rare.
I think Jan Hillmer's system of preheating on the prior day is a good idea. However, firing with it propped open until 1000 degrees may be a bit extreme for today's kilns. If the kiln has variable switches, it is more energy efficient to keep it closed, but set the switch(s) to a low enough setting to get the same slow heating time as was used by propping open an old kiln that just had on-off switches. The most convenient is to use the new computer controlled kilns that can be programmed with a long drying phase so the lid can be closed the whole time. If your kiln has only on-off switches, you may want to see if your school electrician could install one "variable" switch for one of the elements. This allows firing with the lid closed, would save energy (paying for switch), and be the "right thing to do" for the environment.
If your kiln needs better venting (if you smell anything while firing), check the ads in the latest Ceramics Monthly for the new spring-loaded bottom vents. See if your vent can be upgraded by next fall.