7' bubble in bowl during slump

[Carol Ann]

I slumped a beautiful (mostly iridized) 20 inch bowl in a 22 inch pottery mold. It developed a 7 inch diameter bubble near the center. I need a schedule to re slump or to flatten it and re-slump it in order to save it.

Here is the flat firing schedule.
Ramp 300-1100 degrees, 30 min hold,
Ramp 250 -1300 degrees, no hold
Full to 1480 degrees 10 min hold
Full to 900 degrees, 1 hour 30 min hold
Ramp down 100 dph to 700 degrees, no hold
End

Slump schedule:
Ramp 250 – 1230 degrees, hold 15 min
Full to 900, hold 1.5 hours
Ramp 100 – 700 degrees
End

Help is appreciated
[attachment=0]BOWL 2.jpg[/attachment]

[Carol Ann]

standing by for reply. anyone around?
carol ann

namaste@sundancer.com

[Brad Walker]

It's hard to tell exactly what happened from the photo, but it looks like either you had a plugged airhole at the bottom of the mold or else you fired too high, which caused the piece to slide down the edge and create the bubble.

Either way, it's possible to flatten the piece again, but it may be distorted or be a bit weird where the bubble is. I would fire something like 250 to 1100 hold 10, then 250 to 1400 hold until flat (look in the kiln), then anneal as normal. For the slump, don't fire as high as you did the first time. Try only going to 1150 and holding longer.

[Carol Ann]

Thank you Brad for the suggestions. I will try your firing schedule and go from there. Thank you for your advice.
Carol Ann

[Buttercup]

Hi Carol Ann, Just doing a response when I saw Brad had already replied.

Don't overlook his response concerning the possibility of a plugged, or no, hole in the bottom of the mold.

Good luck with the refire. Let us know how it goes.

[Carol Ann]

It seems the mold hole was not fully clear but I have cleared it out. My husband suggested just slumping again. Thoughts? Schedule?

[Brad Walker]

If there's a bulge in the bottom then you'll need to flatten first.

[Carol Ann]

Brad,
I refired the bowl to flat and then slumped all at your recommended
schedule and it worked perfect. Thank you for your help.
Carol Ann

[Dick]

also 1250 is a little high for a regular slump. I usually slump at 1150

[Carol Ann]

Ok, thanks and good to know the lower temperature.

[Tom Stout]

Dear Carol,
I had the same thing happen to me. The pin holes at the bottom of mold have already been addressed. How about a different slant on the same question? Is your kiln a top firing or a side firing kiln? I have both (Paragons). What I have discovered is that a side firing kiln needs a longer firing rate to allow the center of the glass to achieve a uniform temp. If the slump temp is hotter on the edges than the center, the edges with slump first, and the center will slump last. On a top firing kiln, you don't have that problem.

Tom Stout

[Tom Stout]

Dear Carol,
A couple of other thoughts: I have found the book "Joy of Fusing" by the Wardells to be extremely helpful. On slumping, they suggest two segments before you go to the annealing stage. Segment one: 250 rate. temp 1000 degrees, hold for 10 minutes; Segment two: 300 rate, 1260 degrees, hold for 10 minutes. Peek in to see.

Re-firing to achieve a flat starting point? Yes, however, I might paint the glass to avoid a devitrification on the re-firing.