We just tried to fire a newly bought glaze that required 1160 degrees on stoneware that is labelled 1100 - 1280 degrees.
The kiln was opened at around 250 which we have done many times in the past. However, this time we heard several pinging sounds from the largest stoneware piece, so we immediately closed it until it got under 200. Then we propped open the lid with a brick until it got down to 100. All the time we heard the pinging sounds. At around 70 we removed all pieces and they were fine, except the largest one that kept "pinging" away. So we wrapped it up in a large piece of cloth and kept it like that overnight.
Today, almost 24 hours after it has gotten down to room temperature, it still "pings"!
Some bone china pieces did spontaneously shatter with a glaze from the same manufacturer, but that happened within minutes of them becoming room temp. This large piece has lasted way longer now. On another forum someone said we should tap it and see if it rings out like a bell or if it only produces a dull noise, which means it's internally cracked, but it does ring out like a bell still. We've heard that the glaze apparently isn't compatible with the stoneware, but does anyone know what we can expect? Will it spontaneously shatter a week from now, a month, a year?
EDIT: WRONG FORUM! My mistake. Can a moderator please move this to the "in the studio" forum? Thank you!