fused glass leaking under support ?

[LisaDC]

Hello!
Thanks in advance for your answers!

We're fusing / casting BE glass into a self-made mold 23 x 90 cm (9 x 35 in) for eventual use in a lamp. A single piece of white BE was placed in each form, and then pieces of colored and transparent glass were artfully strewed over it. The side supports are a sort of ceraboard material that were held in place by a single layer of brick surrounding the outside. Two lamp pieces were placed in the oven at one time, with a piece of ceraboard material, approx. 1 inch thick, separated the two.

We leveled the oven and used a program that fired the glass at 830 C (1526 F) for 35 min - we were attempting to get an even layer of 7 mm throughout. That didn't quite happen, one side was 8-ish mm and the other side 6-7 mm, which is still oke.

But! The glass pushed the end support about 1/2 in out and seeped slightly under the support. We weren't expecting this kind of muscle from the glass -- any suggestions on how to prevent this on the next go-round? Some sort of sealant around the mold, material other than bricks?

Also - any thoughts on the flinging open of the oven once the 830 C / 1526 F step has been completed?

cheers, kind regards,
Lisa

[Brad Walker]

Sounds like the board you used was fairly light, at least in comparison to cut up kiln shelves (mullite/cordierite strips) that many people use. If the product was light enough, it could get pushed back by the weight of the glass being fired, especially in a thick piece. You can minimize the likelihood by either backing with more bricks, using something heavier than the board you used, or both.

Regarding your question about "flinging open ... the oven" at 830C, if you're asking about looking to see if things are ok for a quick peek then that's no problem. If you're doing it to cool the piece quickly down to the annealing temperature, then that can work but be aware that opening the kiln at high temperatures can cause small cracks in the kiln brick.

[Warren Weiss]

In addition to what Brad mentioned, you may want to line the inside of your frame with 1/8" fiber paper.

Warren

[Morganica]

If you were using the firebrick, the porous, light yellow brick that's almost like pumice, it was probably too lightweight to hold the glass. Ceramic/mullite brick is much heavier, and usually the better choice for dams. You can also stack mullite bricks on the dams/mold to hold it down and keep things in place.

Or, if you have access to stainless steel, it makes exceptionally good kiln weights in the same type of application. I had a series of stainless steel pipe molds made that I also use to weight down dams (or in press molds).