It has nothing to do with

[Anonymous]

It has nothing to do with the glass being 96 COE.

It sounds like the glass rim is being pulled into the mold instead of staying in place while the sides stretching.  Slowing down your firing so that the glass softens more evenly may help – but it is unlikely to fully correct the problem which is typical for square molds.

Take a look at this Bullseye mold:

http://shop.bullseyeglass.com/tools-supplies/molds/retro-party-bowl-17-375-x-17-375-x-4-in-441-x-441-x-102-mm-slumping-mold.htm

If you click the middle thumbnail, you’ll see that even the sample piece has edges that curve inward.  It is the nature of the beast.

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Paul
FusedGlass.Org
Helios Kiln Glass Studio
PaulTarlow.com