Expert: Sam Kelly - 6/30/2009

Question
I was wondering if cones expire, or do not function properly after a certain amount of time? I work in a community art studio space which relies on donations to survive- including donated cones (which could be 15-20 years old) and kilns. Right now I'm trying to assess if the troubles we are having with firing are coming from the kiln or the cones themselves. We've overfired our cone 6 glazes twice now after making adjustments to the automatic shut-off mechanism. But now it seems that the cones themselves are not slumping at cone 6 temperature.

Answer
Hi Ashley, I have used and am still using cones that are many years old with no problems. An easy test is to buy a single new cone and place it in the kiln with your old one, fire and note the results by comparison of each.
When the cones did not slump was the glaze underfired.
Sam