Air Bubble in large panel when slumping.

[Richard Carder]

Hi, when slumping a large panel 8" x 15" on a wave mould I always get a large air bubble in the middle of the piece. I have air holes in the mould. The only thing I can think that is causing it is that panel is slight thickness differences in the panel.

[bob proulx]

Is your mould raised up off of your kiln shelf, if not that could be the problem.
Bob

[Richard Carder]

Yes, I have raised it off the kiln floor to let the air escape through the air holes.

[Bert Weiss]

You could try fiber paper on the mold, or lowering your slump temp. The first solution allows air to escape through the porous paper. The second keeps the glass too stiff for the bubble to be able to expand the glass. I am assuming the bubble is coming from under the glass.

[Mary Lou]

What is your schedule and how thick is the piece?

[Richard Carder]

Hi, the firing schedual is in cent, 85,538,20,85,633,5,465,716,12,Full,488,60,20,445,10,215,150

[Richard Carder]

Oh yes, the thickness is 3mm with fused frit on top.

[Bert Weiss]

Oh yes, the thickness is 3mm with fused frit on top.

Do not fire at full speed up to your top temp. 3mm glass picks up heat very quickly. Your fast firing up to top temp is making the glass get too hot and soft. Working with glass thinner than 6mm requires quite a bit of sensitivity.

I'd think about trying 85 633, 170 716 5 Your soaks are a waste of time unless you know something I don't, like glass broke without them. The 5 minute soak can be shortened or extended, as required, to get the job done. If that schedule results in a bubble, either slow down or lower temperature.

[Laurie Saukko]

I had been getting a pesty and persistent air bubble on one piece and I slowed my slump right down and it finally came out brilliant!

[Richard Carder]

Thanks everyone.

[Laurie Spray]

Recently i have changed my standard slumping schedule to
300 / 1100. No hold (Slower for thick or tack fused etc)
50 / 1250 then to anneal
Everything has been coming out more evenly slumped and perfect.
I think it all about slowing down at the top.

[rosanna gusler]

and is the bubble a 'bubble' or a lump resulting from the glass being so soft that is sliding/stretching down into the mold? r.

[Richard Carder]

Just open kiln and my panel is perfect. Thanks everyone for advice.