Expert: Sam Kelly - 9/4/2008

Question
I had a kiln full of ceramics made by children and adult students in an art  program.  It was a high fire stoneware clay fired at cone 6 using an elec kiln with a kiln setter.  I used the pyro bar.  Almost everything in the kiln was blown to bits, and I do mean blown to bits.  I wondered if I had put the pyro bar back to far in the setter, or if I had not heated the kiln up slowly enough.  I have fired alot of pottery in this kiln with those pyro bars and the same clay.  This has never happened to me and I wondered if you could shed some light on it for me.
Whatever caused it I don't want to make the same mistake again.  
Thanks so much!
Marsha

Answer
Hi Marsha, this usually happens for 2 reasons only, fired to quick at the lower temperature, if it fires to quickly up to 230degC they will explode. There is a conversion in the structure of the clay at around 200degC, its most important on the way up and the way down that at this temp things are taken slowly.
The other is moisture in the clay, if it's not dry enough it will blow up at this temp. in the kiln because of the water content.
There is still water in the molecular format in clay up until 600degC, it's not until this temp. is all moisture removed.
I also work at a college 2 days a week in the ceramics department firing 5 kilns for the students, most blow ups have been hand built fairly uneven in thickness to wet pots. Pieces blowing off but leaving the pot standing, eg breasts and bums, is because there is air trapped between the clay, and it doesn't have to be very much air either.
If the kiln overfired the pots would have just cooked past thier maximum temp and then melted.
A test to do if it was at bisque is to touch your tongue on the pot, it will give a sticking sensation indicating it has not gone past the "being able to suck up glaze stage.
If they have blown up in the final glaze firing then to me there is only one reason, fired to quickly, or someone has opened the door way to early, there is a quartz conversion at 600degC that is as important as 200DegC.
Fire again with a few pots only, using kiln bricks to substitute for the rest of the kiln load. You don't need a lot of bricks as they are solid and will absorb good heat to replicate pot content.

Sam