Iridized Bumps or Bubbles

[NinaMiller]

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...

[Brad Walker]

What schedule are you using? What brand of glass?

[NinaMiller]

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.

[JestersBaubles]

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.