Page 1 of 2

Surprise crack

Posted: Thu Oct 25, 2012 10:56 pm
by Bonnie Rubinstein
I just finished fusing, slumping, and sandblasting a 25" bowl shape for a ceiling light. I have done this before, successfully. I had just drilled the holes for hanging, and saw, in the middle of the glass, this 3-way crack, about 3/4" long. I am very upset, and was to deliver this tomorrow.

First, what could have caused it? My schedule was very conservative. Is this a stress crack? I was applying pressure to drill the holes, as the bit was alittle dull, but I don't' think that would cause it..especially as it is no where near the holes. (?)

Second, will this continue to spread?

Thank you for any guidance on this..!

I have uploaded the photo of the crack.

Re: Surprise crack

Posted: Thu Oct 25, 2012 11:45 pm
by Morganica
If it's in the very bottom of the slump, could be that the glass was just a bit stiff there and not quite ready to slump, so it cracked slightly. I've had those little tricorner cracks happen when the slump was a bit too fast for the weight of the glass.

Re: Surprise crack

Posted: Thu Oct 25, 2012 11:55 pm
by Bonnie Rubinstein
hmm. maybe. it was 2" up from the bottom, though.

I ramped up at 120, to 1000, then 200 to 1145.. I thought this was conservative..but also, the glass was sitting up pretty high in the kiln..maybe this was too fast?

Re: Surprise crack

Posted: Fri Oct 26, 2012 12:00 am
by Bonnie Rubinstein
Cynthia,
do you think this crack is destined to continue? I know it is hard to say..just wondering what happened with the crack in your piece...

thanks

Re: Surprise crack

Posted: Fri Oct 26, 2012 1:20 am
by Bert Weiss
If the crack happened on the heatup, I don't think it will continue. If you got hot enough for a tack fuse, the new anneal should be in effect. I can't say I'm positive I'm right there. If, on the other hand, the crack happened after the anneal, it is live and could continue. I hate making these judgements from photos. If I were looking at it in person, I could tell you for sure when the crack happened. I think this was during the heatup, and the glass moved apart during the slump. I think the glass is technically safe, but aesthetically flawed.

The theory I understand about heat from the elements, is that, at some point between the height of an equilateral triangle and the distance between the elements, the heat evens out. So, if your glass was closer to the elements than their distance from one another, you probably heat shocked the glass because of this.

Re: Surprise crack

Posted: Fri Oct 26, 2012 9:16 am
by Bonnie Rubinstein
The crack did not show its ugly face until about 36 hours after the cooling. That is what is so upsetting and puzzeling. But I may have stressed it somehow while blasting or drilling. I was applying pressure, as the bit was dull, and the glass is a bowl shape, so the glass was held at an angle downward and I was pushing on it. That is the only thing that I can figure out. Does this sound like it could have caused it?

Re: Surprise crack

Posted: Fri Oct 26, 2012 9:22 am
by rosanna gusler
looks like you (or someone) dinged it on something. rosanna

Re: Surprise crack

Posted: Fri Oct 26, 2012 9:29 am
by Bonnie Rubinstein
That is what my husband (a non-glass artist) also believes. I just can't think of when that happened. As the glass is 3/8" thick, it is strong, and that would have to have been a good knock.

regardless, I am in an odd situation, as a gallery was going to hang it today. Don't know what to do! It may never spread, but I can't be sure and there are 2 weeks of hard work into this piece.

Re: Surprise crack

Posted: Fri Oct 26, 2012 11:00 am
by Valerie Adams
I certainly wouldn't risk hanging a cracked piece overhead!
Sounds scary and way too risky.

Re: Surprise crack

Posted: Fri Oct 26, 2012 11:11 am
by Mary Lou
Bonnie, I'd be concerned about submitting work to a gallery with an obvious, (serious?) flaw. Even though it will be hanging and may not be easily seen. Although hand made work is rarely flawless, a crack regardless if whether or not it will get worse, to me, speaks to the quality of work created by that artist. I know how you feel, been there, done that, but in the end, I did not submit the work. Consider your comfort level if you do submit the piece. I'd always be worried about it.

Re: Surprise crack

Posted: Fri Oct 26, 2012 11:18 am
by Bert Weiss
Bonnie Rubinstein wrote:The crack did not show its ugly face until about 36 hours after the cooling. That is what is so upsetting and puzzeling. But I may have stressed it somehow while blasting or drilling. I was applying pressure, as the bit was dull, and the glass is a bowl shape, so the glass was held at an angle downward and I was pushing on it. That is the only thing that I can figure out. Does this sound like it could have caused it?
If indeed it didn't crack for 36 hours, I have only one clue to consider. I have seen people claim that you could score a circle, and hit the glass with a steel ball exactly in the center of the circle and the score would open up. I have never actually seen this happen. If what they say is true, it might be possible to cause a crack in a different place than you were drilling. This is really a long shot though. The dull bit story is not a particularly good one, relative to the glass.

I agree with Valerie, I wouldn't risk hanging glass overhead. that has a crack that could open up and crash down.

Re: Surprise crack

Posted: Fri Oct 26, 2012 11:37 am
by Bonnie Rubinstein
oh, I agree..with the risk issue. What I intended to do was get it there, hang it, and if a customer orders it, I would make a new one. I just wanted to get it hung as per the deadline. You can't see the crack from the outside.

I hung the glass last night in the studio, to test it, and do not see the crack spreading yet. My gut says it will eventually.
So, I wish to deliver with the contingency that this is a "sample only".

Thanks for all your input, folks.

Re: Surprise crack

Posted: Fri Oct 26, 2012 12:12 pm
by Jeanne
Can you repair this like you would a car windshield (using their injectable products)?

Re: Surprise crack

Posted: Fri Oct 26, 2012 12:16 pm
by Mary Lou
What about Hxtal? Not for selling but for showing.

Re: Surprise crack

Posted: Fri Oct 26, 2012 12:21 pm
by Bonnie Rubinstein
There is no way to inject, as there is no 'hole'.

and I think a surface application does not protect an internal crack. (that would be nice, though!)

Re: Surprise crack

Posted: Fri Oct 26, 2012 1:14 pm
by Bert Weiss
hxtal does have the capability of seeping in to a crack by osmosis. It costs $60 (last I bought some) and takes a week to set up. When I tried this, I did not get it 100% in to the crack, which was in clear glass and easily visible. I did not get through the learning curve on curing a crack with hxtal. The windshield repair resins will yellow, hxtal will not.

Re: Surprise crack

Posted: Fri Oct 26, 2012 2:17 pm
by Morganica
Bonnie Rubinstein wrote:That is what my husband (a non-glass artist) also believes. I just can't think of when that happened. As the glass is 3/8" thick, it is strong, and that would have to have been a good knock.

regardless, I am in an odd situation, as a gallery was going to hang it today. Don't know what to do! It may never spread, but I can't be sure and there are 2 weeks of hard work into this piece.

Well, and then there's the other method of causing cracks: Whack it while coldworking. If there is some stress in the glass already it takes surprisingly little force to do it--set it down a little too hard, maybe. It's easy to do when you're coldworking a large piece, and heartbreaking.

If it was caused in the kiln by a too-fast slump it's actually pretty stable; I've had one like that in my shame box for probably ten years. It hasn't spread at all--the crack stabilized during the anneal. But if it was caused by a whack, yeah, it could spread, particularly if it's subject to the temperature changes inherent in light fixtures. Glass raining down on the customer's head is not good for sales. ;-)

Osmosis isn't magic; the crack has to provide a channel to the surface so the HXTAL can fully travel into it. If it doesn't, all the HXTAL will do is gum up the glass. If you can't feel the crack with a fingernail (or your tongue but I didn't say that), there's a good chance that all HXTAL will do is gum up the surface. You CAN extend the crack to the surface yourself by whacking it again and watching the crack travel. Even if the glass comes apart, HXTAL would glue it back relatively seamlessly.

Re: Surprise crack

Posted: Sat Oct 27, 2012 9:29 am
by Bonnie Rubinstein
to avoid all the issues, I told the gallery it was a temporary sample. I will make another, and than take the original and hang it over our kitchen island, and observe it, as a learning experience. (no one sits under it).

as for the crack, I believe it was during the cold working, as it did not appear sooner. I cannot feel the crack, it is beneath the surface.

But I can't put my finger on WHEN it happened, and that drievs me crazy, b/c that means it could happen again with another piece!

** Another, maybe important factor.. this crack appeared where the glass happens to be ST.. the majority of the art is double thick. Does that shed light on the 'why'?

Re: Surprise crack

Posted: Sat Oct 27, 2012 10:32 am
by Bert Weiss
Thickness has 2 factors. One is that a thinner spot is more difficult to anneal as it may have cooled off too much relative to the thicker section. The other is that thinner glass is simply easier to break. My advice would be to avoid working with dull drill bits. (Not sayin I haven't pushed a few beyond their proper useful lives). I am not fond of 3mm thick fused glass. It seems to cause a myriad of potential problems.

Re: Surprise crack

Posted: Sat Oct 27, 2012 10:43 am
by Bonnie Rubinstein
I started with the base being single thick, then added on layers, so it became DT..except in a few areas.

I have had luck with the DT to start with, and if it were not for the weight, I would have used DT for this as well. But then I would have added more glass, making it very thick, and for this fixture, I wanted it to be a little lighter. I think I should have added enough glass to make the whole piece DT, consistently.

.. and Bert, if you have a good source for hollow core bits that LAST, please inform..
thanks!