Problem with slumping

[SarahM.]

Hi!

I am having a slumping problem-perhaps someone can help

When I am slumping my fused glass piece into a rectangle sushi plate-and it comes out of the kiln-the plate is slumped fine-however, the edges are not straight-but kind of ruffled-indented. And they are straight to begin with.

Any help would be appreciated!
Thanks, Sarah
http://www.lampworkerslounge.com

[Stuart Clayman]

Sarah,
Is the size of the mold the same as the glass, ie that the only thing that it can do is slump down. Or is it rippling like a hangerchief vase and the mold is smaller and the glass ripples when it slumps?

Stuart

[Cynthia]

When I am slumping my fused glass piece into a rectangle sushi plate-and it comes out of the kiln-the plate is slumped fine-however, the edges are not straight-but kind of ruffled-indented. And they are straight to begin with.

Any help would be appreciated!
Thanks, Sarah

I would expect distortion like you describe out of a steep sided square mold or if draping, but a sushi mold? Is the mold a simple slight flare, or does it have steep sides like a tray?

If the latter, I think that you probably should use a slower schedule and allow the glass to fully lay down before ending the schedule. I ramp up at 100 dph from 1000 to about 1200 to get a full bend. If the mold is small and the sides are steep like a tray mold, then it will take longer for the piece to conform to the mold shape.

Often the edges will lift up as the center dips down into the mold. You need to allow the edges to lay down again, so perhaps you aren't holding long enough or using enough heat for this particular mold. If the mold is a simple, slightly flared, or curved mold, then I am stumped...but your best bet regardless is to watch the progress and make adjustments according to the piece and the mold.

Hope this is helpful.

[Bob]

Hi Sarah,

Good question. I have tried to figure this one out and have had no success. I believe the pull on the glass is greatest in the central part of each straight side of the mold. In the corners the strain is minimal because the glass is pulled from both sides (I bet that doesn't make a whole lot of sense).

I think the only two ways to prevent/fix this are to use a very wide lip on the mold (which could be a design problem) or to cold work the piece to straight sides after slumping.

Cheers,

Bob

[Stuart Clayman]

I would expect distortion like you describe out of a steep sided square mold
.

Is there anyway to not get the distortion with a steep sided square mold? Maybe going slower?

Stuart

[Cynthia]

I think I misunderstood the kind of distortion explained in the original post, but to this last question, the answer would be the same. Bend at the lowest temp possible and fire up slowly (100dph) from 1000 while checking frequently to see what results you are getting.

If you go slow, the glass will stretch and bend more easily at the shoulder of the rim, decreasing the amount of distortion. If you scream up to 1300 and hold, the glass will drop in the center rapidly, pulling the sides in with it.

You can also use a larger blank than the mold. One that perhaps bows out slightly along the edges to compensate for the fact that the sides will pull in a bit.

[SarahM.]

wow! Thanks everyone the quick replies! The sides of the mold are not very steep-I think I will fire more slowly-and see what happens. Thanks again- sarah

[SarahM.]

I just wanted to give an update on my problem.--thanks to your input I was able to solve my slumping issues.

So my sushi plates were coming out of the kiln (after slumping) with rippled/indented sides-not smooth and straight in other words.

Your advice was to fire up more slowly and that changed everything. I'm not at my shop, so I'm not looking at my kiln log, but just to give you an idea-I was going up 400 degrees and then 350 in the first 2 segments. I slowed this down to 275 and 250 and made a couple other changes in the other segments. Result:

Perfect plates! Thanks again, Sarah