Bubbled glass during slump

Dee Newbery · Post by **Dee Newbery** » Thu May 01, 2003 6:20 pm

I've been fusing about 1 year with relative success until this last week I fused a 15" round, blk, dbl rolled piece with just a few geometric designs (opaque, dbl rolled) and some stringer. The fusing went fine, but when I slumped it, it bubbled where the bottom should have been. When I say bubbled, it was as big as my palm. It not only bubbled in the bottom but in several other places with sizes ranging from a half dollar to a nickle. I was using mold #8651 from Bullseye. I kiln washed it four coats and kiln fired it to 500 with a 1 hour hold. I slumped it as follows
315 dph to 1000, 10 min. hold
500 dph to 1285 with 25 min hold
9999 to 1000 with 2 hr. hold
80 dph to 700 with no hold
9999 to room
I have read the posting about kiln wash, but have used this wash since I bought my kiln - it came with the kiln (Paragon). There are 2 vent holes in the mold and they seemed clear of any kiln wash.
I have bought new kiln wash with the intention of redoing my shelf and all my molds. I bought a cone 10 wash from my local ceramics dealer. After reading other posting about kiln washes, I am wondering if I should buy Bullseye wash instead.
Any help/suggestions would be appreciated.
Dee

Jo Holt · Post by **Jo Holt** » Thu May 01, 2003 8:00 pm

Dee,

Although the vent holes were clear of kiln wash, was the mold elevated off the shelf to allow the air to escape between the mold and the shelf? That could be a solution if it's a mulite shelf and not fiber. I usually elevate slumping molds with 1" posts; then place the glass blank on and check with a level. When in doubt, give the air an escape route

As for kiln wash, even though I don't think it would be why your glass bubbled when slumping, I definitely prefer Bullseye kiln wash over those made for ceramics; especially on the shelf used when fusing.

Hope this helped!

Jo

Ron Coleman · Post by **Ron Coleman** » Thu May 01, 2003 9:33 pm

Looking at the 8651 mold and its size I think you are firing a little high.

Black glass softens much lower than most other glass and the soft glass is probably sealing any path for the air to excape. I think you're getting a shelf bubble inside the mold.

For a piece this big (15 inch) you shouldn't have to go over 1200 f to get a nice slump. Try slowing the firing rate too, maybe 350 dph from 1000 to 1200 and see if that helps. By slumping at a lower temperature you'll get a better finish on the back of the bowl too.

Remember that lower fusing temperatures may take longer too, but 30 min should be adaquate for this size bowl.

Ron

Go for the BE kiln wash, it's hard to beat. My rule of thumb for drying shelves is, if the wet color is gone from the shelf it's time to cook glass.

Dee Newbery · Post by **Dee Newbery** » Thu May 01, 2003 10:29 pm

Jo, I did wonder about elevating the mold. I didn't, but will next time.
Ron, thanks for your advice on the lower temp. I can't wait to try again and this time I'll use the Bullseye wash.
Thanks to much. I live in an area that is not only hard to get supplies easily, but also to find others who are fusing glass. I really appreciate this web site and all the expertise that is available!
Dee

Jerry Barnett · Post by **Jerry Barnett** » Fri May 02, 2003 2:20 am

Excellent suggestions from both Jo and Ron, but also avoid having a two layer part of the piece above the air holes. The concern is that the double weight would cause that part of the piece to hit the mold earlier than it would otherwise. If you cannot do that, try a two layer base with the design as the third layer.

Assuming that you are using Bullseye glass, you might find the firing schedule from their web site helpful. They would suggest a different soak temperature and time than what you are using.

Jerry

Dee Newbery · Post by **Dee Newbery** » Fri May 02, 2003 8:38 am

Thanks so much for the additional information. If I understand you correctly the glass, in effect, sealed off the lower area hence a bubble where the steam couldn't escape. Thanks also regarding a different temp.
D

Ron Coleman · Post by **Ron Coleman** » Fri May 02, 2003 11:05 am

Dee Newbery wrote:Thanks so much for the additional information. If I understand you correctly the glass, in effect, sealed off the lower area hence a bubble where the steam couldn't escape. Thanks also regarding a different temp.
D

At fusing temps there won't be any water to generate steam, but trapped air will expand and cause a bubble if there is no path for the air to excape. The trick is to find a balance between using enough heat to get the glass to slump into the desired shape and getting it too soft that bubbles will grow.

Ron