I just took a glass piece out of the kiln after slumping it into the large organic mold. It's kind of 3 layers in that one piece is a flat coral design, the next layer is frit lace and the last layer is an irridized clear circle. All Bullseye glass. It slumped nicely using the following schedule:
250 - 1000 -15
100 -1175 - 23
AFAP - 900- 40
100 - 800 - 0
200 - 700 - 0
400 - 100 - off
I felt like this was a very conservative schedule so am very surprised to hear pinging. The coral piece was 11" in diameter and the lace and clear circle were approx. 10" dia.
I'm wondering if I can put it back in the kiln and refire up to 1000 F and then do a longer hold at 900F, perhaps for 60 min? That is provided it doesn't break before I get it in there again.
What do you think??
That schedule probably needed a longer anneal hold, at least an hour.
If the ping caused a visible break, then refiring to re-anneal won't heal the crack, you'd have to go higher. If there's no crack, re-firing and re-annealing should work.
Yes, anneal hold at least 60 minutes, 75 wouldn't hurt. And I'd probably be extra cautious and go up a bit slower, say 200 dph (may be overkill, but why take chances?).
the big thing everybody overlooks is uneven thickness. You always have to allow more time than the thickest part. When there is a differential between the thicknesses you can thermal shock it.