I fired the first pieces of a new design over the weekend. There were a total of five pieces (8" circles). Four were two pieces of BE with either black or clear irid on the bottom (irid down) and an opal on the top. The fifth piece was irid-down on irid-up. All five pieces had been previously fused in a different kiln and were sandblasted on the top (except the irid/irid) piece.
The kiln is a Jen-Ken oval 9. Schedule was:
300 to 1200 hold 20
100 to 1500 hold 20
9999 to 960 hold 20
200 to 560 hold 20
off normal cool down
overnight fireing
All but the irid/irid piece came out with a fairly perfect hole 3.25 inch in from the edge (1" to 1.25" in diameter) The irid/irid piece was fine. I have read all the big, bad bubble posts but these were at different spots on two shelves and the hole placement is almost identical! I know this kiln fires hotter than my Olympic but I have fired many 8" pieces with out problems (although never 5 at once)
The perfect symmetry on all four pieces makes me wonder if there is a heat/work volume issue going on that is common to all pieces. As an additional clue, the holes were nice and smooth. Actually I wish I could make this happen!
Any ideas will be appreciated. I plan to fire a single piece in the morning to see if I can diplicate it. Any ideas or suggestionbs would be appreciated folks!
I have also had problems with this controller. Bartlett sent me a new one with ferrite cores to suppress noise that was effecting the controller settings. Since it was over night???
I think you're going too fast after the 20 min soak at 1200. The rapid firing could be softening the glass enough to seal the edges down on the kiln shelf and trapping air. Above 1200 I can't imagine the glass warping with rapid heating but maybe your shelf is warping a little.