Couple of big bubbles during slump

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Anea
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Couple of big bubbles during slump

Post by Anea »

Okay, I did a large tile platter (the ones where you stack up glass tiles, slowly getting smaller, ranging from 4-5 layers of tiling). It had a 20" round base of two pieces of clear and then the many tiles were on it. Made it through the fusing fine, decided to sandblast the front of the piece to get a smoother shinier finish. Here is where the problem happened.

I was extremely conservative on the slump, into a 20" round ball surface mold from bullseye. I took it 50 dph set 1100 hold 30 minutes (it was already fully slumped at this point), but I wanted to take it to 1325 for just a second to ensure I got a nice fire polish as well. Then I went 225 dph set 1325 no hold and went right back down. I check back on the piece when it was at about 12something and all was well. The next time I looked three large bubbles had formed, 2 were a bit smaller about 2-3inches and one was large about 4-5 inches. They are all sort of in the center, though none are around the one hole that is drilled in the mold.

I was devastated when I saw this. Thought maybe I could squeeze them out, so I held at 1130 for two hours but it didn't help. Just let it finish the firing and annealing schedule. Now we are up tp the present and I want to get rid of these bubbles by just firing it on a flat surface. I have never done this and wanted to know how to go about it. Do I need to do anything special, such as fire on fiber paper or thinfire to help the air get out from those three bubbles? I am just a little scared that they will get worse instead of better. I was thinking of following my conservative schedule of 50 dph set 1100 and hold it for maybe 3 hours or so.

Any and all advice welcome.

Thanks
Aimee
AKA Anea
Cynthia

Post by Cynthia »

Hi Aimee,

I have had this happen with the ball molds too...but mine don't have holes. Regardless, what I do is fire it flat with thinfire paper under it so the air can escape. I have a magnaform shelf, so the air should be able to escape anyway, but better safe than sorry, plus opals stick, so I have a serious paper habit.

Your 50 dph is really conservative, but you know your kiln. If the piece is 1/4" thick, I would fire at 300-350 dph (no side elements) to 1100, then ramp at 100 dph to 1400. I go slow here to help avoid distortion, but watch and stop once it is flat again. You can't do this in the mold, you have to fire it flat again, then try to bend again.

I've had pretty good success with this since the bend is less than 3 inches, it tends to lay down pretty well. Where the bubbles are the glass will have thinned a bit at the domes if they were pretty large (plum sized), dependent upon the glass and your design, that might not be an issue.
Marty
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Post by Marty »

drill more holes in the mold.
Peg
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Re: Couple of big bubbles during slump

Post by Peg »

three large bubbles had formed, 2 were a bit smaller about 2-3inches and one was large about 4-5 inches. They are all sort of in the center, though none are around the one hole that is drilled in the mold.
Drill holes in the mould under the bubbles and reslump - aligning the bubbles to the holes. Of course you may just end up with new bubbles in new places...

Or if the bubbles are so big that the glass has thinned too much, smash them out, slump flat, fire onto a fresh disc and start again...
Lisa Allen
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Re: Couple of big bubbles during slump

Post by Lisa Allen »

Anea wrote:The next time I looked three large bubbles had formed, 2 were a bit smaller about 2-3inches and one was large about 4-5 inches. They are all sort of in the center, though none are around the one hole that is drilled in the mold.
I would drill more holes and try reslumping, making sure that the mold is raised up off the floor a bit on kiln furniture so you don't negate the effect of the holes.

Lisa
Lisa Allen
http://www.lisa-allen.com
Today's mighty oak is just yesterday's nut that held its ground.
Marc Demian
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Post by Marc Demian »

I've had trouble with bubbles slumping into clay molds if I go over 1300. In future, you may try fire polishing at a slightly lower temp, holding a bit longer. I try to never go over 1300 while slumping after several sad experiments. If it were me, in future I would fire polish and then slump (not efficient, but safe). Sorry about your piece. I know how it feels. I've never had real luck trying to fix larger bubbles but it's worth a try. Marc
Anea
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Post by Anea »

Okay...I have good news. The piece is just now beginning to cool down and head for annealing, and it is completely flat!!! WOO-HOO!!! Thanks to everyone for all the advice, it worked and my piece is saved. We drilled the extra holes into the mold last night, so it should make it through the slump again, especially since I don't need to fire polish.

Thanks again,

Aimee
AKA
Anea =D>
lyndasglass
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Post by lyndasglass »

Aimee,
I have a question for you. Was this the first time you used that particular bullseye mold? I have found that on several of my Bullseye molds, the first time I slump into them I get bubbles in my glass. I doesn't seem to happen after that, just that first time. I thought that perhaps I needed to fire the molds without any glass the first time just to make sure they were completely free of moisture.

Has anyone else had this problem with the first time they fire with a Bullseye mold?

Just wondering,
Lynda
Anea
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Post by Anea »

Lynda-

This was the second or third firing in that mold actually. I had no problems with the previous firings either, just this one. Hope this info helps to solve your mystery.

aimee
aka
anea
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