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I recently had a crack in a piece about 4 days after slumping. I presume this was an annealing problem. The piece varies in thickness from about 1/4" to 5/8". It's BE glass with high-temp wire through it. The crack is over one of the wires. The wire was included during the 1st firing using a "Patty Gray" casting method so the wire varies from being near the surface to deep within the piece. It's a rough fan-shape. There were subsequent firings to achieve a thinner, feathery edge on the top edge of the fan.

During the "casting" & shaping firings, I used an BE's annealing schedule for 3/4" glass. I slumped over firebrick and fiberpaper. The "points" of the fan never touched anything. I baffled these edged during the slump. I used the BE schedule for 1" thick glass to anneal the slump The crack started to form in the part of the glass over the firebrick.

Obviously I wasn't conservative enough, but I'm not sure how much more converative I need to be. I'm also unsure of the physics of the wire and the firebrick to understand how the internal temp of the glass is being affected. I guess I just realized, that I presumed that the stress occurred during the last (slump) firing. Could it have been from the early firings?

Jackie in Washington DC wrote:I recently had a crack in a piece about 4 days after slumping. I presume this was an annealing problem. The piece varies in thickness from about 1/4" to 5/8". It's BE glass with high-temp wire through it. The crack is over one of the wires. The wire was included during the 1st firing using a "Patty Gray" casting method so the wire varies from being near the surface to deep within the piece. It's a rough fan-shape. There were subsequent firings to achieve a thinner, feathery edge on the top edge of the fan.

During the "casting" & shaping firings, I used an BE's annealing schedule for 3/4" glass. I slumped over firebrick and fiberpaper. The "points" of the fan never touched anything. I baffled these edged during the slump. I used the BE schedule for 1" thick glass to anneal the slump The crack started to form in the part of the glass over the firebrick.

Obviously I wasn't conservative enough, but I'm not sure how much more converative I need to be. I'm also unsure of the physics of the wire and the firebrick to understand how the internal temp of the glass is being affected. I guess I just realized, that I presumed that the stress occurred during the last (slump) firing. Could it have been from the early firings?

A new casting method? Tell us about it please. Brock

Did the crack occur where the glass was thinner? And how close to the surface was the wire at that point.

Tony, The crack occurred at the thickest part of the piece -- also the approximate center of the piece. At that point, the wire was about 1/4" from the surface (about 3/8" from the bottom). There were several other places where the wire was much closer to or on the surface. Those places were fine.

Brock, I was referring to the method Patty Gray teaches where you mound lots of small glass pieces and glass components within a glass box or dam. I probably used the term "casting" incorrectly.

If you could post an image, that would be helpful in trying to figure this out. I have not used hi temp as an inclusion so I can't speak from experience, but the crack being centered on the wire may be a clue. Of course the firebrick may have a role in this too, but that would have related to an uneven cool down, not an annealing issue. I wonder if there was a small crack when you took it out, and it went unnoticed, and over the course of a few days, and some temperature cycling, it propagated.

Tony -- sorry I've already flattened the piece (resulted in the score running to the edges) and I'm re-fusing as we speak.

After talking with others about the high-temp wire, I decided the culprit was more likely the slump over firebrick/fiberpaper where the center (thickest) part of the glass was being insulated by the firebrick and kept warmer than the edges which were not in contact with anything.

In my next slump, I'm going to use a slow, controlled ramp from 700 to 400 instead of letting the kiln cool naturally.

Thanks for your help.

Sounds like a good idea Jackie. When I do 1" pieces I cool them over a two day period, and wait until they're below 100F before taking them out. In the winter when the studio is colder I may wait for them to be at 80 before removing.