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slump failure help?

Posted: Sun Aug 31, 2003 1:38 am
by doc
Just slumped my first large 17 inch deep BE bowl. The first fuse came out gr8....beautiful red transparent with stringers etc....then I put it into a mold to slump.....bowl shrunk down from original diamenter of 17 in to 14 and got one big bubble in the center. Didnt pop and kinda looks interesting because of the design but I know it should not be there. Any explanations why this happened. I fired very slow and long. Also color changed from fire red to a burgendy....Can I refire? Is there a way to get rid of it? :roll: Also fired on thin fib paper and shelf was level.

Posted: Mon Sep 01, 2003 3:57 am
by Melodie
The bubble could be caused by the vent holes in your mold being clogged with kiln wash. After applying the wash be sure that the tiny holes are not filled in, if they are then use a thin wire or tip of a toothpick to reopen the holes. The holes are needed for the air to escape out of the bottom since your glass is hugging the sides while slumping.

Did the finished piece sort of slide down the sides and that is why it is so much smaller? Hard to tell without a picture but I would venture a guess that you may have fired a bit too hot. The glass will actually slide towards the bottom and sort of ripple up if you take it too high. This happened to a larger bowl I was once working on … I was firing higher then I should have and decided to give it just two more minutes. That two minutes (and high temp) cost me a 16â€

Posted: Mon Sep 01, 2003 10:10 am
by Tom White
rj, just for fun use a flexible tape to measure the inside of your bowl mold. Hold it at the top of the rim of the mold and make it conform to the inside of the mold down one side, across the bottom and up the far side at its widest point (diameter measurement). The measurement obtained would be the diameter of the glass blank needed to produce a bowl the full size of your mold. Next, place the 8 1/2" mark of the tape at the center of the bottom of the mold and conform it to the shape of the mold. The end of the tape should be at about the location where the rim of your slumped bowl fits. Our glass does not stick to the rim of the mold and stretch to fit the mold. Instead the whole blank softens and conforms to the mold shape, sliding down away from the rim of the mold as it settles into the mold shape. The only way I know to fill a bowl mold completely is to cut the glass to the diameter determined by the measurement described above, fuse it, slump that into an intermediate mold that will leave the diameter of the slumped glass the same as the final mold (hard to find) then slump into the final mold. Another alternative to produce a 17" diameter bowl would be to locate another larger mold of the same height that would accept the diameter blank determined above and slump it in one firing, allowing for the blank to slide down into the mold and end up 17" in diameter. With a bowl mold for slumping the deeper the mold in relation to its diameter the shallower and narrower the slumped bowl from a flat blank will be. Shallow bowl molds produce slumped bowls much nearer the diameter of the mold than deep bowl molds.

Tne bubble issue has been addressed. About the color issue, was the glass used BE tested compatable or just plain BE glass? Red glasses are subject to striking or changing color from repeated or prolonged firing. Tested compatable BE glass should not change in just a fuse firing and one slump firing. If you provide more exact information about the glass used perhaps Lani could offer some thoughts on that.

Best wishes,
Tom in Texas