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I am having trouble full fusing black backed iridized rainbow glass. If I use any pieces bigger than 2 inches square I get bumps. I assumed that meant I needed a slower schedule so I switched to one that was supposed to be a bubble squeeze. Still getting bumps. I'm also getting bumps in streakies. No problems with regular opaques or transparents. I just started fusing in my own kiln. First used the presets. Then went to Slumpy's schedules. Now scouring the web...

What schedule are you using? What brand of glass?

This is bullseye glass. Here's the latest schedule I've tried:
400 1000 20
400 1150 15
50 1250 20
850 1450 15
Afap 950 60
400 100 0

Another schedule used:
300 1150 30
200 1370 20
400 1480 20
Afap 950 60
150 800 10
300 100 0

Interestingly i took a small piece off another bumpy plate and full fused it again. Most bumps disappeared. Then when I slumped it all the bumps disappeared. 3 inches square. Large piece with 2 inch squares from this same sheet fused fine.

NinaMiller wrote:This is bullseye glass. Here's the latest schedule I've tried:
400 1000 20
400 1150 15
50 1250 20
850 1450 15
Afap 950 60
400 100 0

Another schedule used:
300 1150 30
200 1370 20
400 1480 20
Afap 950 60
150 800 10
300 100 0

Interestingly i took a small piece off another bumpy plate and full fused it again. Most bumps disappeared. Then when I slumped it all the bumps disappeared. 3 inches square. Large piece with 2 inch squares from this same sheet fused fine.

Depending upon the piece and the kiln, 20 minutes may not be a long enough bubble squeeze at 1250. Try 60 minutes -- seems to work well in my Skutt GM1414 (side and top elements).

Other thoughts:
You said you are using Bullseye Glass, but the anneal at 950 is a typical temp for System 96. Bullseye is 920 deg F
If you are fusing large pieces, or pieces that have been previously fired, a ramp of 400/hr on the way up to 1150 may be too quick. You could end up with thermal shock.

Dana W.