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Continuing cracks- large glass! Why?

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Bonnie Rubinstein
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Continuing cracks- large glass! Why?

Postby Bonnie Rubinstein » Sun Sep 29, 2013 6:38 pm

Please help me do the detective work..as I am bewildered..I wish I could say I have solved my saga of poorly annealed larger pieces.. but my latest piece to replace the cracked ones, cracked also!

It is 37” x 26” x 3/8” thick. Because my kiln runs about 15 deg. too hot.. I set the following schedule for System 96…very conservative anneal. Spectrum advised me on it: (remember- these are 15 deg. lower due to compensating for thermocouple error).

100 1050 0.30
225 1250 0.30
250 1390 0.04
full 930 3.00
100 775 0.30
100 600 0.20
100 100 0.00

Four days after taking out of the kiln, my NEW piece cracked right up the middle. My 4th piece in a row.

Factors that might have contributed:
· My shelf has been resting in the kiln floor, with no heat circulation around it (I just cut my shelf down, so now this will change)
· There may be uneven heating the kiln, so I will do a test for this.
· My thermocouple was off , but I thought I compensated for this in the schedule (I ordered a new one, which just came).
· I had replaced the insulation in the front of the top of the lid ..so it is slightly more ‘pooffy’ than the other insulation. perhaps this absorbs more heat (?)
· Could my controller be at fault? The relays malfunctioning? If so, how do I tell?
* · Is the anneal too high?

I really need advice from my fellow fusers.. so much thanks.
Bonnie Rubinstein

Morganica
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Re: Continuing cracks- large glass! Why?

Postby Morganica » Sun Sep 29, 2013 7:26 pm

Need photos. When you say up the middle, did it split lengthwise directly in the center? What colors are you using? Is this a full, flat fuse? Since it is 9mm thick, how are you damming it? How close are you to the sides of the kiln? Where does the piece sit in relationship to the elements?

If you are firing in the kiln floor, or on a shelf flat on the floor, yes, you are super insulating the bottom of your piece so that it lags behind the rest of the glass. That could indeed be the problem. But there are at least two other (possibly supplemental) possibilities I can think of, depending on what that crack looks like.
Cynthia Morgan
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Bonnie Rubinstein
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Re: Continuing cracks- large glass! Why?

Postby Bonnie Rubinstein » Sun Sep 29, 2013 8:17 pm

The glass is 26", and the kiln is 28 1/2" deep, so it is close to the insulation in the width direction. I dammed the 26" sides. I used all cathedral glass, areas of only clear layers across the bottom and top of the artpiece and the middle has a lot of blues and aquas. A 3-D fuse.. not completely flat. The crack goes straight up the middle, the 26” width. I will get a photo tomorrow AM.

The heating elements are on top, 10” above the glass.

Thanks, Morganica.
Bonnie Rubinstein

Bert Weiss
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Re: Continuing cracks- large glass! Why?

Postby Bert Weiss » Sun Sep 29, 2013 8:57 pm

Do you mean 28.5" wide? If so, glass an inch from the walls coupled with a too long anneal cycle could cause this problem. I think 3 hour soak is too long.

I routinely anneal on the floor of my kiln My floor is 5" thick brick covered with several layers of boards and blankets. In general area has little to do with anneal schedules, thickness does. However evenivity is the key concept. During an anneal cycle, brick kiln walls would be hotter than the air, and giving off heat. This will make the glass edges hotter than the middle. The longer you soak the glass, the more it will reflect this difference. If this difference is greater than 5ºC you can not get a good anneal. I think 3 hours is too long. for 9mm. If you were not so close to the walls, the 3 hour anneal would present less of a problem.

If you are having thermocouple issues, it won't help, but unevenivity is likely the culprit.
Bert

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Bonnie Rubinstein
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Re: Continuing cracks- large glass! Why?

Postby Bonnie Rubinstein » Sun Sep 29, 2013 10:32 pm

Yes, the glass was about 1" from the insulation. ( I need to make the glass 26".) the walls of the kiln are blanket insulation. So, the walls of the kiln are not giving off the heat of brick. Do you think I need to shorten the anneal? Spectrum did advise the 3 hour anneal, but I can shorten it.

I installed the new thermocouple and tested the temp..it is working well.
Bonnie Rubinstein

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Re: Continuing cracks- large glass! Why?

Postby JestersBaubles » Sun Sep 29, 2013 10:57 pm

Bonnie Rubinstein wrote:100 775 0.30
100 600 0.20


You could possibly be dropping too fast in the drop from 930 to 600. I would try 50 down to 700, and then cool however you normally do (my firings end at 700 and I let the kiln cool off by itself, but my kilns are firebrick and cool very slowly).

Dana W.

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Re: Continuing cracks- large glass! Why?

Postby Bert Weiss » Mon Sep 30, 2013 10:04 am

Bonnie Rubinstein wrote:Yes, the glass was about 1" from the insulation. ( I need to make the glass 26".) the walls of the kiln are blanket insulation. So, the walls of the kiln are not giving off the heat of brick. Do you think I need to shorten the anneal? Spectrum did advise the 3 hour anneal, but I can shorten it.

I installed the new thermocouple and tested the temp..it is working well.
Bonnie, Fiber is certainly different from brick. Do the Bullseye know your kiln test (technotes 1) to see what the heatwork looks like an inch from the walls verses the center. Another test is to take identical stacks of concentrically smaller squares, place them around the kiln, and fire to somewhere below full flat fuse. You will see if the center is hotter than the edges. Or spend the big bucks and do the technotes 7 tests. The first tests show you heatwork which isn't exactly temperature, but they will give you a clue about what is going on at different spots in the kiln.

My guess is if you make a piece 24" long and put it through the same anneal cycle, it might live. The tricky thing is that a shorter anneal cycle could also do the trick, but it is really hard to assess. I've never tried this, but I wonder what a totally clear piece of glass looks like in a polarizer. You might be able to determine how stressed out the edges are. Note, after the glass breaks, stresses are totally redistributed.

So to reiterate, a long anneal soak time is just fine in an evenly heated environment. In an unevenly heated environment, too long a soak will get the glass to reflect the environment it is in. If the temperature difference is greater than 5ºC, the glass can not relieve enough stresses. Sometimes a quicker anneal cycle can accomplish getting the glass within the range at a more even place. As the controller enters the anneal range, the glass is hotter than the air. As time goes by, they begin to equalize. The air near the walls is different than the air in the center.
Bert



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Bonnie Rubinstein
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Re: Continuing cracks- large glass! Why?

Postby Bonnie Rubinstein » Mon Sep 30, 2013 10:57 am

yes, I plan to an even heating test today on my newly raised-up-on-feet shelf, and new thermocouple. so, if it is uneven, what do fusers do, if they can't easily replace elements? (besides work on a anneal that works).
Bonnie Rubinstein

Bert Weiss
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Re: Continuing cracks- large glass! Why?

Postby Bert Weiss » Mon Sep 30, 2013 11:05 am

Bonnie Rubinstein wrote:yes, I plan to an even heating test today on my newly raised-up-on-feet shelf, and new thermocouple. so, if it is uneven, what do fusers do, if they can't easily replace elements? (besides work on a anneal that works).
You might have to restrict size, or maybe tweak anneal cycle. Another possibility is create convection inside the kiln. I tried this once and failed, but I think there are ways to get it to work. You need some sort of fan or blower to push the air around. One person told me he does it with a water pump. The fan can be outside the kiln. I'd love to hear what people have done to move air inside a kiln. Rosanna uses a kilnvent on her side fired kiln.
Bert



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Laurie Spray
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Re: Continuing cracks- large glass! Why?

Postby Laurie Spray » Mon Sep 30, 2013 11:41 am

I think you need to slow down after anneal also.....I think I would go

anneal
100 hr to 800 and hold 60 min
100 hr to 700 no hold

since we have the same kiln ........so sorry you are still having these issues......
I do occasionally fire large that come to 1" of the fiber walls.
Laurie Spray

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Bonnie Rubinstein
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Re: Continuing cracks- large glass! Why?

Postby Bonnie Rubinstein » Mon Sep 30, 2013 12:10 pm

Hi Laurie, yep, it' s a bummer... after we spoke, I still lost another piece. I don't think it is my relays, though.. they seem to be okay. It has to be the anneal schedule and heating issues withing the kiln.

Also, I was going 100 dph after the anneal. maybe I should go slower, as Dana suggested.
Bonnie Rubinstein

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Re: Continuing cracks- large glass! Why?

Postby Morganica » Mon Sep 30, 2013 1:51 pm

I'm going to reserve judgement until photos, but when you say "3D fuse" just how 3D are we talking? Is there any correspondence between the crack and the components? (IOW, does the crack appear to outline any particular component(s)?)

Unevenivity is meaningless. "Thermal equilibrium," or lack of same, is probably what Bert means. And while area may not affect annealing schedules, shape can. I don't think that's a problem here--I THINK you've got an ineffective annealing schedule, a problem with the position of the piece in the kiln and potentially a problem with components causing additional stress--but whenever you make pieces with aspect ratios greater than 2:1 you definitely want to take shape into account in the schedule.

The whole "too long a soak will get the glass to reflect the environment it is in" bit doesn't take air movement into account. Unless the kiln is hermetically sealed, airflow will move the heat around in the kiln (even more if you vent). That helps distribute heat and slows down the "reflect environment" tendency. In fact, you can eliminate the need for kiln "zones," i.e., the ability to fire banks of elements at different rates to heat varying masses more evenly, by introducing a little more air current into the kiln.

A three-day soak might cause problems, but a three-hour soak probably isn't going to be an issue. If it is, shortening the anneal soak by an hour won't help you much.
Cynthia Morgan
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Bonnie Rubinstein
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Re: Continuing cracks- large glass! Why?

Postby Bonnie Rubinstein » Mon Sep 30, 2013 3:11 pm

[attachment=0]Cracked glass 003.jpg[/attachment]

here is the pic. The crack just broke the piece, unaffected by design or shapes. The crack also has another crack about 4" from the bottom, making that triangle -( the visual "line" going off to the right is a just line of layered glass, not a crack).

What aspect ration are you referring to?
I was just about to do the even heating test (I have been cutting bricks for an hour to create the set up to do this!) I figure it can't hurt to do.
I can only vent my kiln on the top 2 'windows' and by lifting the lid. (bottom) Is there a way to safely create air flow with these?

I agree after reading tons of material, that it is the annealing. So, I am working on it.. but I sure do hear differing opinions from fusers, as well as Spectrum.
Thanks..!
Attachments
Cracked glass 003.jpg
Bonnie Rubinstein

Lauri Levanto
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Re: Continuing cracks- large glass! Why?

Postby Lauri Levanto » Mon Sep 30, 2013 3:13 pm

can't proviede cues, but need one.
You said your thermocouple was 15 F off.
How to measure thermocouple calibration?

-lauri

Bonnie Rubinstein
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Re: Continuing cracks- large glass! Why?

Postby Bonnie Rubinstein » Mon Sep 30, 2013 3:21 pm

I took a thermocouple out of another kiln, inserted it and tested.

Also in talking the the manuf. of the kiln, I believe it was time for a new one.
Bonnie Rubinstein

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Re: Continuing cracks- large glass! Why?

Postby Morganica » Mon Sep 30, 2013 7:06 pm

Bonnie Rubinstein wrote:
Cracked glass 003.jpg


here is the pic. The crack just broke the piece, unaffected by design or shapes. The crack also has another crack about 4" from the bottom, making that triangle -( the visual "line" going off to the right is a just line of layered glass, not a crack).

What aspect ration are you referring to?
I was just about to do the even heating test (I have been cutting bricks for an hour to create the set up to do this!) I figure it can't hurt to do.
I can only vent my kiln on the top 2 'windows' and by lifting the lid. (bottom) Is there a way to safely create air flow with these?

I agree after reading tons of material, that it is the annealing. So, I am working on it.. but I sure do hear differing opinions from fusers, as well as Spectrum.
Thanks..!

AHA!!!! (BTW, this isn't a lengthwise split (in my book, anyway)...I'd call this a crosswise split. More on that in a minute.)

When I look at this pic I see a bunch of coarse frit filling in the spaces between decorative components. I'm assuming they're attached to the base glass and also to each other and possibly the components on either side...right? Kinda like this, but different configuration:
Image
http://www.morganica.com/bloggery/2007/glasswork/fusingtack-fusing/lotusbowl/
That bowl gave me fits because of the nice fritball/coarse frit decoration in the center. Each one of those components adheres to the glass when hot, then shrinks and tries to pull in from its fellows during cooldown. The potential for stress creation is tremendous, and I wound up giving LotusBowl one of the longer schedules I've ever given a fused piece--I think the schedule I wound up with was for 4.5X the actual thickness. The final bowl was extremely strong (until my niece's baseball hit it, anyway), but it took a lot of experimenting to get there. If I take the fritballs out of the center and just make this with flat glass, the schedule is MUCH shorter.

So...if I'm right, you've got a tack-fuse, a particularly complicated tack fuse because you have a LOT of pieces that need to stick together. The anneal isn't conservative enough. I'm thinking you START with a schedule intended for about 3X what you'd expect for a 3/8 inch piece, i.e., anneal for something closer to 1 or even 1 1/8 inches (27mm). That means you've got a 4-hour anneal soak, your next segment goes down at 27dph, then 54 dph, then finally 100dph.

I think it's being made worse by annealing on the floor of the kiln and if you elevate at least an inch it'll be easier to maintain thermal equilibrium. And since it cracked across the middle of the short length and is so close to the elements I'd also wonder if you don't have an issue with uneven contraction between upper and lower surfaces. If the glass is cooling a lot faster on the top center surface, it's also contracting faster. There's a chance that you're getting some additional stress in the center because it's contracting faster than the edges or the bottom, introducing more stress.

Normally I've only seen that break the piece when there's a much bigger distance in length vs. width, i.e., an aspect ratio of about 3:1, but I suppose it could be contributing additional stress.

So...either melt the glass into the surface more, so it's all one piece (even if it's "contoured"), or really extend your annealing schedule. If you've got access to a polarizing filter, try looking at a successful piece with it--I'll bet you'll see some stress.
Cynthia Morgan
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Bonnie Rubinstein
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Re: Continuing cracks- large glass! Why?

Postby Bonnie Rubinstein » Mon Sep 30, 2013 8:11 pm

I will carefully read this and respond, just want too quickly say that I have several other pieces that cracked similarly, and were more fused, with much less tack fusing.

I just did a even heat test, (with glass strips as per Bullseye) and the far right side and bottom get less heat, but not significantly so. And as thought, the center gets the most.

How that impacts my schedule, who knows.

Bert, I know you feel that a shorter anneal is called for; Cynthia longer. Both of you are fine fusers.. There is no simple solution. I could keep trying, and I could loose more pieces.

Annealing. bah humbug.
Bonnie Rubinstein

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Re: Continuing cracks- large glass! Why?

Postby Morganica » Mon Sep 30, 2013 8:59 pm

Well, I don't think there is a single reason for all the cracking, which is probably where some of the confusion is coming from. But basically, get some air circulating under the glass, move it away from the edges or baffle against the close elements, make your schedule consistent all the way down, and anneal for the individual piece, not everything 3/8 inch.

On this type of piece, you need a longer anneal. Cutting the soak and cooldown isn't going to help much.
Cynthia Morgan
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Bonnie Rubinstein
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Re: Continuing cracks- large glass! Why?

Postby Bonnie Rubinstein » Mon Sep 30, 2013 9:27 pm

( My 27 hour fuse last time I would have thought was long enough!)

to add to the confusion..I have made large pieces in the past..no problems and shorter anneal times. same kiln, same shelf.

So, along with the new thermocouple, newly raised shelf, I will also bring it to a fuller fuse.

...And meditate on the anneal time. Maybe I will reach enlightenment. (anybody up there listening?)
Bonnie Rubinstein

Bonnie Rubinstein
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Re: Continuing cracks- large glass! Why?

Postby Bonnie Rubinstein » Mon Sep 30, 2013 11:45 pm

I read this on Bullseye website... in answer to a question posted re: breakage..

" it could be (most probably) that your kiln isn't even in heat across the entire span, making annealing of wide projects impossible.

if this is true, then longer anneal times actually work against you, as they freeze in the temp differences across the span, and only parts of it get annealed, leading to stress. shorter times may possibly work in this case."

so, there is the other viewpoint.
Bonnie Rubinstein


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