Glass thinning on project

Jenb · Post by **Jenb** » Wed Mar 17, 2004 11:33 pm

I have a sporadic problem with an area of glass getting thin and uneven on different areas on different projects. Sometimes the underside of the glass where the thin area is has a very shiny firepolished look, and it is slightly indented. Sometimes the underside is flat like it should be.
This has occured with different size projects, different firing schedules (some pretty slow), on both Spectrum and Bullseye glasses, different thicknesses and different primers (Hi-fire & Primo). I have also lowered the firing temp to 1450 instead of my normal 1475 (bullseye).
For a while I was thinking that I wasn't letting the shelves cool off enough after curing the primer, but that does not seem to be the problem.
My guess is that some (or just 1) of my shelves are uneven and just every once in a while I seem to pick the spot to avoid (which would explain why it doesn't happen all of the time). I've tried to use a ruler to pinpoint an area, but I cannot see anything, might it be just very slight and hard to see?
Has this happened to anyone else? I really appreciate any help or thoughts you might have on this problem. I am so tired of ruining things and having no idea why.

Thanks in advance, Jen b

Ron Coleman · Post by **Ron Coleman** » Thu Mar 18, 2004 12:22 am

It sounds to me that you may be seeing the results of shelf bubbles in your glass. A shelf bubble is caused by air being trapped between your kiln shelf and the underside of your glass.

The shiny firepolished look to the underside of the glass indicates it wasn't touching the shelf at high temperature. Thin areas also indicate a bubble formed and stretched the glass.

For whatever reason, the bubbles are going away but the damage still evident.

One method used to eliminate shelf bubbles is a long soak at about 1225 on the way up. Maybe about 20-30 min and then a slow rise to fuse aver about 30-45 min. This will help your glass conform to the shelf before it is soft enough to form a bubble.

Hope this helps

Barbara Muth · Post by **Barbara Muth** » Thu Mar 18, 2004 9:27 am

and make sure the kiln wash is bone dry before you put your piece on the shelf. I find that this type of bubbling is most likely to happen when I fire on an undried shelf (yes I do it at times!) or when the piece being fired is very large.

If you can't find uneven spots on your shelf that would definitely trap the air, then make sure the shelves are well dried before you fire.

Finally, if you do have uneven spots on your shelf, there are two solutions.

The first is to replace the shelf. Buy one in person and test it all over with a straightedge.

The second is to use fiberpaper or thinfire between shelf and glass. (It allows the air to escape).

Good luck!

Barbara

Tim Swann · Post by **Tim Swann** » Thu Mar 18, 2004 9:38 am

Jen,

I agree with Ron. You have a shelf bubble onder the glass. If it always happening on the same shelf, it may becaused by the shelf not being flat. Small low spots in the shelf will give you shelf bubbles, as Barbara stated.

Tim

Pat Loboda · Post by **Pat Loboda** » Thu Mar 18, 2004 9:54 am

I kept having a similar problem - but I was getting rather large bubbles sometime. I examined my shelves for an uneven surface, but didn't see any irregularities. However, thinfire solved my problem. Since I never figured out which of my shelves was the culprit, I always use thinfire under my projects which cover the whole shelf. Solved the problem.

Pat Loboda

Pat Loboda · Post by **Pat Loboda** » Thu Mar 18, 2004 9:55 am

Well, actually, I use thinfire for smaller projects, too, because some of them will come out with the little indents that you describe if I don't.

Pat L.

charlie · Post by **charlie** » Thu Mar 18, 2004 1:09 pm

the shelf could be warping only when it's hot, and flat when it's cool.