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Good Morning.
So sad to find this happened to my platter. On first fuse, without the black square with letter on it, it developed a huge bubble in the middle, which I broke, filled in with fruit, and covered with the black piece and Japanese letter. But the air may have been trapped, or schedule too fast for the black glass.

Glass is 1/4", and I fired; Broke on the way up:
150 to 400
250 to 1050
150 to 1250
then slowly cooled, but was already broken underneath the black square with letter.

So, I have now filled the crack BENEATH the square with frit, but if i cap with a black rectangle to cover the crack across the width, i will trap air again. Can't figure this one out. I may have to fill the crack with frit without a 'cap' , then fire AGAIN, with a cap to save the visual quality of the design. But that would be 4 firings total.

HELP! Thank you!

Picture(s)?

Sorry- I thought I had posted it.. here it is. System 96.

The glass actually separated., as you can see...and because it is a double thick clear base, with another single black layer, my ramp may have been too fast.

Can't figure it out.

Bonnie Rubinstein wrote:Sorry- I thought I had posted it.. here it is. System 96.

The glass actually separated., as you can see...and because it is a double thick clear base, with another single black layer, my ramp may have been too fast.

previous post: Glass is 1/4", and I fired; Broke on the way up:
150 to 400
250 to 1050
150 to 1250
then slowly cooled, but was already broken underneath the black square with letter.

Can't figure it out.

OK, I'm going to speculate here and then see what the experts say. The ramp according to Stone using greater thickness schedules to be conservative may be OK for 1/2" but is too fast for 5/8"; this is probably significant in that the top layer of black glass (layer 3) absorbed heat not allowing the portion of shielded black glass below it in layer 2 to heat up as quickly as the exposed black glass in that same layer (layer 2) thus causing the break on the way up due to uneven heating. This is a double Uneven Heating/Thermal Mass issue because of the partial third layer and the black glass absorbing heat faster than other glass.

An issue to consider on the redo is that Spectrum suggests 1250F as a slumping temperature and uses 1350F as the lowest tack fuse temperature in their 'Forming Chart.' My experience is that a less than complete tack fuse introduces stress at the sporatic contact points produced by the low tack fuse temp and will also cause cracking.

Thanks for responding, Jerry.

I will go up much more conservatively next time, for sure. But the thickness at the most, at any time was 3/8" I believe. The glass, after the first fusing, was 1/4", and then with the added 3rd black rectangle layer to tack on, was another 1/8". When I peeked in at 1250, the glass had already separated, before the complete tack fuse had happened, so not sure about the 2nd hypothesis.

So, your first theory may be closer to what the issue was.

On to buy more black glass, it appears!

Seems to me that you are correct in thinking it would have been better to fire after filling the bubble hole with frit and then do a separate firing to add the decorative element on top. That would have helped with the trapped air and the volume control issue.

yes, Vonon. That would have been wise! next time that happens..I shall do this. Hopefully won't need to.

I started over from scratch and carefully and slowly fired the piece.. (no Elmers glue this time. I believe a blob of that in the middle of the center black piece on the first firing caused the original bubble!)

This time I fired it all at once..no layers. It turned out beautifully.

Thanks for the advice!