cracking/breaking in glass days after finished fusing and slumping

[marV]

My 12″ round system 96 white opaque background plate with black opaque and transparent blue overlaid design came thru the full fused phase just fine. I cold worked the edges a little and then slumped into a shallow bowl form. AgaIn looked good.  However several days later the piece cracked across down the middle. I left it in the mold on a shelf. Now about 10 days later it is cracked into several more pieces. Was it because of the difference between white background and opaque black having different slumping qualities or was it not annealed sufficiently?

Fired at full fuse at 300F /hr to 1220F soak 30 min. Then 600F hr to 1465 hold 10 m. Full to 960F hold 40 min then 100F/hr to 800 and left in kiln till room temp. Slump at 300F/hr to 1250F hold 15 min full to 960F hold 1 hr 150F/hr to 800 left in kiln till room temp.

What caused this? How to avoid problem in future?  Can I break it up and use the glass pieces in another piece on a different overall background?  I did that with another piece that had original white background that I was unhappy with, broke it up into smaller pieces and fused and slumped it on a clear square and it seems to be fine after at least a month or more.

[wordana]

By crack, is that hairline cracks or cracked in pieces?

In my experience, complete breaks are generally due to too fast heating or cooling. Smaller hairline cracks within the piece are due to thermal shock after annealing. It sounds like it was a fairly thick piece — perhaps it wasn’t completely cool when you removed it after slumping??

Also, I typically hold at a lower temp in the first segment — 1000 degrees rather than 1220. Seems to work for me, but as always YMMV.

Dana W.

[marV]

First crack was break thru entire piece after about one week. The other breaks were also all the way thru – those didn’t happen until about two weeks later. The entire fused and slumped piece was about 1/4′ thick when done. Begain with one piece opaque white and another layer of opaque pieces along with transparent blue and a few black stringers. I can only think that it was caused by improper anealing or cooling. The piece was at room temperature when I took it out of the kiln the next day. I did take it out of the mold and left it on a shelf in the room. It remained then for about one week when I noticed the first crack. I then put it into the mold to see if there would be any way for it to be fit together if I refired it. The two pieces would not have easily fit together so I just left it in the mold.

[marV]

I’ve left this in the mold, sitting on a shelf at room temperature and it continues to crack thru into smaller and smaller pieces for at least 3 or 4 weeks now.  Kinda interesting to check on. I do wish i knew why tho so I wouldn’t repeat the process.

[wordana]

I would suspect annealing, but it’s always difficult to tell. This info may help you better diagnose the problem:

http://www.corvallisglassfusion.com/?q=taxonomy/term/7

Dana W.

 

[Georgeann]

I suspect the fast drop to 960…. try dropping to afap to 1200 (hold) then 100 to 900

[Author]

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