unuseal spontaneous crack

Leslie Ihde · Post by **Leslie Ihde** » Fri Apr 11, 2003 9:05 pm

I checked the archives before posting this.

I had a 20" fused circle, 1/4" thick spontaneously crack after I had been cold working it a small grinder, and then with diamond sponges. I had put it down, still partly damp, and some 15 minutes later, I heard the crack. It had been lying around, before cold working for a couple of weeks. The crack is curious, cutting near, but not along a color change.

Why did it crack? I believe my annealing was ok, but maybe not.

Can I refuse it? The area that's cracked is cobalt blue. I was very dissapponted, I really loved this piece and had planned to slump it into a shallow bowl.

Thanks in advance, again,
Leslie

Lisa Allen · Post by **Lisa Allen** » Fri Apr 11, 2003 11:54 pm

What was your firing schedule?

Leslie Ihde · Post by **Leslie Ihde** » Sat Apr 12, 2003 6:53 am

270/hr to 700 soak 15
AFAP to 1230 soak 15
AFAP to 1400 soak 15
300/hr to 1450 soak 30
AFAP to 960 soak 45
100/hr to 700 soak 1 min
200/hr to 200 and off

Jerry · Post by **Jerry** » Sat Apr 12, 2003 7:56 am

Given the way you described it, it does sound like an annealing fracture. Your sked looks okay but you might try doubling the time spent at 960. You didn't say what kind of glass you are using so I'm assuming BE or Spectrum and that's a good temp for both. The rule is that you anneal 1 hour for ever 1/4". You are a bit under that so that's where I'd go.

You asked if you could "refuse" it? Well, don't refuse it; REFIRE it

Jerry

Lisa Allen · Post by **Lisa Allen** » Sat Apr 12, 2003 11:40 am

Leslie Ihde wrote:270/hr to 700 soak 15
AFAP to 1230 soak 15
AFAP to 1400 soak 15
300/hr to 1450 soak 30
AFAP to 960 soak 45
100/hr to 700 soak 1 min
200/hr to 200 and off

I agree with Jerry that you could spend a bit more time at 960Âº. I would do at least an hour there. But in the past I have had trouble in the 960-700 segment. Now, I go no more than 60dph from 960Âº to 700Âº and that has solved all of my problems so far.......

At 700Âº, I just turn my kiln off to cool on its own.

Lisa

Jerry · Post by **Jerry** » Sat Apr 12, 2003 12:16 pm

That's good advice. Henry Halem's book advises to hold a annealing for some appropriate time and then to go from annealing to about 600 at 75 degrees per hour. The theory is that annealing will occur if you go through this range slowly enough. So I think you are onto something.

Thanks for reminding me
Jerry

Lani McGregor · Post by **Lani McGregor** » Sat Apr 12, 2003 12:27 pm

Leslie, have you tested your kiln for the evenness of heating/cooling?

If your elements arenâ€™t distributing heat evenly (any burned out? bunched up?, drafts in the firing chamber?), slowing down the annealing schedule can actually make it worse.

L

Barbara Muth · Post by **Barbara Muth** » Sat Apr 12, 2003 12:29 pm

Leslie, Bullseye recommends a 30 minute anneal soak at 960 and then cooling to 750 at 210 dph. If your glass was BE, then your schedule was fine. You could even cut your anneal soak by 15 minutes. That always works fine for me. If your piece is uneven then you should increase the anneal soak to correspond to the greatest thickness of the piece.

Doesn't sound like incompatibility to me. If it were the crack would have started right at the stress point where the incompatible glasses met.

Is it at all possible that you stressed your piece while coldworking? If it gets too hot in a spot while coldworking you introduce temporary stress that is either relieved when the piece cools down or, if the stress is too great, is relieved by cracking. If that is the problem, then re-fusing should be an easy solution. Just take it up real slow...

BTW, what was the shape of the crack? That can sometimes be a clue as to its cause.

good luck!
Barbara

S. Klein · Post by **S. Klein** » Sat Apr 12, 2003 2:31 pm

BE's suggested schedules are for pieces up to 12" in diam. I find tht when I get over 12 " many of the rules change. If you have side elements than the outside of the piece is heating more because of its proximity to the elements. This requires a much longer anneal or preferably baffles in the kiln. I arbitrarily add 50% to 100% to the suggested times when I'm over 12". Theadditional time includes slowing down on the way down. You can refire, however you probably will have some type of scar from the break. Refire anyways, as if you don't like the results you can cut the piece up for another composition............steve

Leslie Ihde · Post by **Leslie Ihde** » Sat Apr 12, 2003 2:46 pm

Most people say slow down, but Barbara Muth, I think you said speed up! I'll have to re-read the notes. Yes, it's BE, and no, I haven't tested my kiln for eveness, but it's round, a evenheat, with side elementsand top elements. The piece is almost the size of my shelf- just 1/2" from the edges of the shelf. Thanks to all the suggestions, I think I will refuse and cover the crack with a think layer of the same color glass hoping to avoid scars, and slow down the annealing. The crack is shaped something like a large "S" and follows the diameter of the piece, going from one end almost to the other

I will re read all notes a bit more carefully before I try again. I sure love glass, but it does seem touchy sometimes!

Leslie

Amy on Salt Spring · Post by **Amy on Salt Spring** » Sat Apr 12, 2003 3:55 pm

S curved breaks are almost always an annealing problem, since the piece did not break completely apart I would be extremely careful on the initial heating since there still may be unrelieved stress in the piece that heat might catalyze.
-A

Barbara Muth · Post by **Barbara Muth** » Sat Apr 12, 2003 6:28 pm

S. Klein wrote:BE's suggested schedules are for pieces up to 12" in diam. I find tht when I get over 12 " many of the rules change. If you have side elements than the outside of the piece is heating more because of its proximity to the elements. This requires a much longer anneal or preferably baffles in the kiln. I arbitrarily add 50% to 100% to the suggested times when I'm over 12". Theadditional time includes slowing down on the way down. You can refire, however you probably will have some type of scar from the break. Refire anyways, as if you don't like the results you can cut the piece up for another composition............steve

OOPS. I missed the 20 inch diameter. I agree with Steve on the time. Ignore my former comment on anneal soak time.

And yes it was an annealing problem. S crack is the big clue. If you add glass, you will probably need to dam the piece to make sure it doesn't flow off your shelf. Also with that size I would go up quite slowly. Increase the anneal soak and slow down between 960 and 700 not only to account for the past insufficient anneal but also for the additional glass. And in addition to increasing your anneal soak you may want to try out Brian's baffle 'n go technology. Being that close to your side elements may account for some temperature differentials across the piece that could be somewhat reducced if you shielded it some from the elements. I have never done that, can someone who has speak up?

Barbara

Barbara

Leslie Ihde · Post by **Leslie Ihde** » Sat Apr 12, 2003 8:35 pm

Thanks, Barbara, for your updated advice. I'm not sure what is meant by baffle, but I assume sheilding the glass from direct heat from the sides. However, the glass is still 2" from the sides, and I can compensate by slowing down the heat up phase, as suggested by Amy. As far as flowing off my shelves goes, I think I'll be ok because I was only going to lay a 1/2" wide piece of thin glass along the break so that the glass will heal and not widen. Anyway, that's my plan, and I think it would add only a miniscule amount to my volume, probably not more than what I coldworked off. If someone thinks I'm off the wall, don't hesitate to let me know!

Thanks to everyone for the help. I find this board invaluable, I only wish there were some questions I could answer from time to time. I guess there will be eventually.

Leslie

Leslie Ihde · Post by **Leslie Ihde** » Wed Apr 16, 2003 7:57 am

Just a final note on my re-fired 20" platter. I sucessfully refired it by going slower as advised and soaking for 2 hrs at 960 just to be safe. Hopefully it is well annealled. I did get some devit on the edges, and over the lighter glass- which is about 1/2 of the piece. I don't think the devit looks terrible, so it will be ok. There is a scar on the underside where the crack once was. I laid a 1/2 strip of the same color on 1/3 of the crack, and sprinkled frit on the other 2/3 s to a depth of about 3/16ths piled on the crack, and 3/16ths wide. I used frit because in that area the crack was so close to the color change, I didn't want a strip to alter the nice sharp line in my design. It worked with one problem. There was a line of whitish dust left along the area I pushed the frit. The frit was blue.

I might try to rub that out with a fine diamond pad and then slump. In anycase, a moderately sucessful experiement. I imagine I could have refired with out laying anything on the crack because the piece was slightly thicker than 1/4" Not sure though. Finally, it did spread out a little bit, but not over my shelf edges.
Thanks for all the help.

WarmGlass.com

unuseal spontaneous crack

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