Your best bet is to go very

[Anonymous]

Your best bet is to go very slowly (50 degrees an hour or so) starting at about 1050°F and creep up to about 1225°F.

You should also make sure your kiln/shelf/mold are level.

Even when you do everything right, deep slumps often drop catawampus.  Very often the piece can be repositioned in the mold and slumped a second time.

One more option is to make your bowl blank bigger than needed and put a drop ring above the mold — then cut the rim after slumping.  That’s how I did the bowl in this forum thread here.

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Paul
FusedGlass.Org
Helios Kiln Glass Studio
PaulTarlow.com