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Has anyone had a fusing mold sag? I have an 8" plate mold which I have used before with no problem. Recently, I slumped a two layer piece at 1400 degrees for 20 minutes, combining the fusing and slumping. There was a bubble in the middle, because the two vent holes had become plugged by the glass. I drilled a hole in the center of the mold and sagged it again at a maximum temperature of 1380 degrees until the bubble collapsed. When I took the plate out, the bottom was not flat, and when I laid a straight edge in the mold, it was no longer flat either. I don't understand how this happened, as the mold should work at a much higher temperature.

Any ideas?

Most people do not use molds the way you do. Instead, they fuse in one firing and slump in a second. This gives you much more control over the slump and the look of the piece, as well as allow the slump to take place at a much lower temperature, well below 1300F. Slumping molds are designed to work at that temperature and bisqueware that is sold for slumping isn't really designed to be used for slumping at all.

That said, what kind of mold was it? How deep? What kind of glass were you using?