I would do two tests using a different cycle.
In this new cycle, lower the process temperature to 1450 or 1475°F.
1) Set up a 4 x 4" tile with two layers of the same gray glass, place it on a simple but similar shelf paper layered arrangement. See if the glass flowed enough and stuck less.
2) Set up a 4 x 4" tile with two layers of the same gray glass, place it on a simple but similar shelf paper layered arrangement--but this one is coated with shelf primer (allowed to dry).
Fire both in the same firing.
If tile #1 flowed enough and stuck less, you can use that cycle for your next piece.
Tile #2 is for comparison, to see if your idea about using shelf primer is a better solution.
Another completely different solution to test: pre-fire your two pieces of gray glass into a single tile in a separate firing. Then, because bubble trapping will no longer be a potential problem, adjust your kilncasting cycle by eliminating the soak at 1200°F.
Let us know if you try any of these!
Mary Kay Nitchie
Bullseye Glass Co.
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