Problem slumping into large round molds

[LAVA Lady]

Hi there,
I am hoping someone can offer some advice with this and how I might be able to fix it. It is a problem I have had a couple of times and this time I am beside myself 
I am using spectrum 96 glass, I have slumped a piece into a large 500mm round mold.
The piece has a full fused base, which is opaque. There is a tack fused design put onto it.
I have then slumped it into the mold and it has a large blister In it. for the want of a better expression.
The firing schedule was:
1. 150 c to 535c hold 20 mins
2. 300c to 675c hold 20 mins
3. Full to 515 hold 60 mins
4. 200c to 150
5. Cool to room temp

The mold was elevated and the hole in the mold was clear.
Any advice is appreciated.
Thank you
LAVA Lady

[Brad Walker]

If by "blister" you mean a large "air bubble" between the glass and the mold, then I suspect the problem is that you're firing too hot.

For a large mold I'd try firing to only 620C/1150F, rather than as high as 675C/1250F. The problem is that the extra heat is giving the piece time to slide slightly down the edges of the mold, causing the bulge in the glass.