Combing cracked, can I refire to anneal?

[Sandy Pendleton]

I have been working on my first larger combings (18" x 18"). Now, a few weeks after taking them out of the kiln, I have one that is badly cracked starting out from the center. I have another that has just a tiny fracture beginning. I would like to re-fire the one with the little crack and hopefully anneal it properly.
I am using system 96 glass, the piece was in a stainless dam lined with fiber paper on a kiln washed shelf. My original cool down schedule has worked on non-combed 18" pieces before, but not this time. The schedule was:
9999 1650 h30
9999 1000 H10
300 950 h60
100 800 h10
300 300

In researching the problem, I noticed Patty Gray has a schedule for cooling an 18" combing on the Spectrum site that I interpreted as:
heat and comb at 1650, then cool as follows
9999 960 h 4 hours
9999 850 h 2 hours
cool naturally.

My questions are:
1. If I re-fire the combed piece to add a rim or fire polish, do I need the same very slow cool down?
2. If I try to recover the piece with the small fracture, can anyone suggest a heating rate to prevent it from exploding? (I really like the design).
3. Where do I find firing schedules for larger, but not necessarily thicker, pieces?
Thanks for your help,
Sandy

[Laurie Spray]

If you are going to take that back up to full fuse personally i would heat it up at 100 degrees an hour to 800 then 250 to full fuse. Follow Pattys anneal. She know lg 96. I would add one more cycle to cool till 600 slowly though.

[Stephen Richard]

I would prefer that cool from 960F to 850F to be at a rate of 45F/hr (achieves the same time) to ensure all of the glass is cooling evenly. I agree on a further cooling segment too, although I might go as low as 400F at only 90F/hr.

You need to consider the additional thickness that most combings have when scheduling the cooling part of the firing.

[Sandy Pendleton]

Thank you for the input.
I refired the piece with a very conservative schedule as follows. It seems to have annealed properly, but I can still see where the original fracture was. At least now I can cut it up for something else without it breaking into pieces.

100 300 H10
100 800 h10
100 1050 h10
200 1460 h10
9999 960 h 4 hours
9999 850 h 2 hours
100 700
300 300

I have now done additional 17" x 17" combings using the cool down part of the schedule with no cracking problems.
Sandy

[Morganica]

Wait a minute, though: Do you know what caused the cracks? It might have been thermal shock from poor annealing...or it could be a shift in compatibility of one or more of the glasses. The first is fixable and/or can be cut up for other things. The second, not so much.

[S.TImmerman]

Just because they haven't cracked yet, doesn't mean you're safe- I've heard of pieces cracking long after they were made. Good luck,

[Stephen Richard]

Do you have polarised filters to observe what stress remains in your pieces? That is one way of avoiding the wait to see if it will stand up over time.

[Tony Smith]

It's interesting that those instructions have been on her website for 10 years. In rereading the instructions, they aren't clear. Patty doesn't go AFAP from 960 to 850. I'll check with her and update her instructions, but I wouldn't go faster than 100 deg/hr. I would also take the glass down to 700F before turning off the kiln.

Tony