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The copper is .004" thick. There is only one hole in the center of the mold.

Brad, Thanks for your thoughts. I know that the air hole wasn't blocked because just before it was fired someone noted that the hole was plugged and we fixed that. I don't have the firing schedule since these were done at a local glass school. (long story) I've included a couple of pics (at least I ...

So, I slumped into a new circular plate mold for the first time and ended up with what looks like voids on the bottom. Someone suggested that what might have happened is that the middle of the glass slumped first, and blocked off the air hole, the edges sealed and the remaining air got pushed to the...