slump bubble

[brerearl]

I slumped a double thickness of Bullseye glass, 14.5" diameter into a shallow dish mold. During the slump a bubble the size of an egg formed halfway up one side. I welcome any comments as to why the bubble formed and how I can get remedy the problem.

[Brad Walker]

Did the mold have a hole in the lowest part of it to allow air to escape?

If not, drill two small holes in the lowest part of the mold (1/8" or smaller).

If the mold did have a hole, please tell us more about the problem. Firing schedule, where the hole was, etc. This could also have been caused by over-firing or firing too high.

[brerearl]

It was a standard Bullyseye mold with clear vent holes. The firinG schedule was:
RA1 500
F1 750
HD 0

RA2 1000
F2 1275
HD 10 MINUTES

RA3 FULL (AFAP)
F13 960
HD 30 MINUTES

RA4 150
F4 750
HD 0

RA5 IDLE TO ROOMTEMP


The bubble was halfway up one side of the plate, I am not certain but I think it was towards a corner of the kiln. The bowl was not centered on the kiln shelf. There is no apparent thinning of the glass.

[Lynne Chappell]

Well, I think your second ramp is way too fast. It probably didn't give the glass enough time to settle nicely into the mold. I don't know if "bubble" is really the right word since the glass hasn't thinned (and bubbles don't usually form at 1250). It just didn't settle down against the form in that spot. Go slower on the second ramp (400-600 per hour).

[brerearl]

Do you think the "lump" would go away if I re-slumped the piece at a very slow rate? perhaps at a slightly lower temp as well, to alow the lump to relax without having the rest of the bowl puddle?

[tom suter]

I can't say if the bubble will fire out from just trying to reslump it, but you possible can refire it flat and start the slump all over. From my own experience slumping at 400 deg. is still too fast. Slower is almost always better allowing gravity to play a bigger role than a higher fire rate. I usually don't fire over 1150 on a slump and just give it a much longer time to soak. If I am not satisified with the results I will increase the temp 25 deg. and allow it to soak once again. There is just no use in being in a hurry fusing, slumping and annealing. Actually once I get at 1000 degrees I slow down to 100 dph to finish the slump. This has one heck of a learning curve. Bslow Bsafe Bhappy and spare the kiln gods an offering. Tom

[Lynne Chappell]

Reslumping will only work if the reason for the lump is that the slump didn't really finish. I would expect that air would be trapped behind the lump and prevent it from settling against the mold. Put it in the mold and see if it is contacting solidly except for the lump. If so, forget it. You could reflatten and slump again.

[Rob Morey]

If the bubble is not inside the glass but just pushing up from the back then re-slump. What have you got to loose?
I think you went way too fast, especially with a piece that is two layers thick and 14" I use BE so I would use this schedule


350 to 1000, hold 15 min
250 to 1200, hold 15 min
ASAP 960, hold 45 min
250 to 920, hold 45 min
500 to 700 hold 20 min
cool to room temp

Rob

[brerearl]

Thanks to all for the valuable suggestions. Here is what I did. I cut two 1/8" strips of thinfire. I put the plate back in the mold with the thinfire strips creating a small airway from the rim into the lump. I then refired the bowl at:
RA1 350
F1 1000
HD 0

RA2 1000
F2 1250
HD 10 minutes

RA3 Full
F3 960
HD 60 minutes

RA4 150
F4 750

RA5 0

There was a slight distortion from the lump and two very small paper prints from the thinfire. This piece will not take first in a competion but is still a very saleable piece and not a Christmas gift for a cousin.

Thanks again for everyone's help and I hope I can return the favor someday.