Perplexed.....need help (pics)

[Tony Smith]

It's not completely a lose/lose Bert since you get the heat back from the bricks that you put into them on the way down, thereby reducing the amount of heat you need to generate on the way down. The real downside is all the heat that you can't lose fast enough under 400°

Tony

[Kevin Midgley]

I might look at several areas that could be causing the problem. #1 was all the glass from the same stock sheets or were they mixed and thus have some subtle variation in compatibility. This is probably not the problem. #2 shift in COE from repeated firings. Probably not the problem. #3 sometimes kiln shelves have markings or manufacturing differences. That could mean the shelf is thinner in one area or less dense in one area. Probably not the problem #4 air leak into or out of the kiln. Probably the problem. Hinges can shift, lids can warp, kiln posts propped under lids to cool kilns faster can allow micro air leaks that most of the time cause no problem. The crack is in a corner of the piece. Was that in a corner of the kiln? Bert and I hate thermal mass in a kiln but lacking it does sometimes cause problems. In a fibre kiln the thermocouple readings are way off in the cooling cycle. My kilns read 100F and the glass is probably at least 2 or three times that. You have to wait the day until the kiln is room temperature when working with thick glass. Kevin

[Lisa Allen]

Now ..I'm perplxed. Your initial firing schedule that you first mentined on this thread is different than the one you just gave with your response to me. Did you fire more than once? Initial firing then again for matte firing?..after sandblasting?

Three firings in all. One to fire the slab, one to fuse the slices, and one to get the matte finish after cleaning it up on the tile saw and sandblasting.

Lisa

[Lisa Allen]

I might look at several areas that could be causing the problem. #1 was all the glass from the same stock sheets or were they mixed and thus have some subtle variation in compatibility. This is probably not the problem. #2 shift in COE from repeated firings. Probably not the problem. #3 sometimes kiln shelves have markings or manufacturing differences. That could mean the shelf is thinner in one area or less dense in one area. Probably not the problem #4 air leak into or out of the kiln. Probably the problem. Hinges can shift, lids can warp, kiln posts propped under lids to cool kilns faster can allow micro air leaks that most of the time cause no problem. The crack is in a corner of the piece. Was that in a corner of the kiln? Bert and I hate thermal mass in a kiln but lacking it does sometimes cause problems. In a fibre kiln the thermocouple readings are way off in the cooling cycle. My kilns read 100F and the glass is probably at least 2 or three times that. You have to wait the day until the kiln is room temperature when working with thick glass. Kevin

Thanks for the trouble shooting, Kevin. I do think that you (and Tony) are on to something with the fiber kiln. It is new and I guess this is my learning curve..... For future firings, I am going to program in a segment between 700 and 400, as well as leaving it closed until the temp reads at room temp for a good while. This is going to make for long firings but since it is so big and I am able to fire so many pieces together, there is really no rush to get it open and reloaded.

Patience is currently not one of my virtues.......now, I am being forced to work on it.

Lisa

[Cynthia]

Now ..I'm perplxed. Your initial firing schedule that you first mentined on this thread is different than the one you just gave with your response to me. Did you fire more than once? Initial firing then again for matte firing?..after sandblasting?

I shouldn't answer for Lisa, but here I go anyway.

Doctac, I believe she is posting two schedules for two different firings. The first is the schedule she used for firing up the finished piece with already fired and sliced up pattern bars as inclusions. The latter is a schecule she used to create the pattern bar brick to use as her inclusion glass.

Lisa, If your kiln heats and cools unevenly (hot or cool spots), you can remedy it by extending your anneling process, but I lean more toward ramping down through the annealing zone more slowly rather than adding holds. You can introduce stress by extending your anneal and the concept than you can't over anneal is not accurate. Search for Lani's post about this in the old achives. Because my Denver has hot and cool spots, I only hold at the upper strain point, then ramp slowly through the zone to just below. Add a ramp from below 700 to 400 to control your descent there too and that might help. Having said that...

I kinda doubt that improper annealing is what is going on here. I lean toward a coe shift. If after this firing, you still have small cracks and they are running through the same spaces, then my bet would be that your glass has shifted it's coe. Hopefully my take is wrong, and you can salvage the piece with a different firing schedule. But if it is a coe shift, you can cool down your pattern bar schedule as Brock is considering and just let time do the work with a cooler schedule.

By the by...your colors choices are bold and spectacular. The transition from one to the next is delightful, purposeful and very successful. I love your work.

[Bob]

Barbara Muth wrote...."Lisa, I think Bob Leatherbarrow used to do anneal soaks at both temps and has stopped doing that since having conversations at Bullseye (if I remember correctly). Ask him about it. Bob???"

Yes I used to use a multiple step annealing program. It resulted from my misinterpretation of a Bullseye chart. I now use dthe Bullseye recommended annealing schedule and have had complete success with it. Dang, I hate having to admit when I'm wrong (heh heh).


I think that Lisa's problem is an incompatibility one.

Cheers,

Bob

[Paul Tarlow]

The point of the initial anneal soak is to achieve evenity before the slow decent. The point of the slow decent is to go slowly enough that the innermost and outermost temps in the glass never have a detrimental difference in temps -- where part of the glass would "set" while adjacent glass is still "soft" -- thus creating stress between the two areas of glass.

Given this, it seems you should always be okay doing your anneal soak too high -- so long as you go slowly enough on the way down.

That's why I don't buy averaging the anneal soak temps of opaque and transparent glass -- I always soak at the higher temp and, if needed, slow down the ramp.

If the crack is caused by stress then there there is a good chance the crack didn't alleviate all of it -- and it may also exist in other areas of the clear. I'd put it up to a stressometer and see what that says.

In any case I'll be saying an extra prayer to the kiln gods for you. I can only imagine your frustration given the amount of work that clearly went into that piece.

Btw, how did you finish off the wonderful sculptural piece you had at WGW?

- Paul

How bout this, since my pattern bars typically have both transparent and opaque, maybe a possible solution is to anneal with this in mind. The annealing point for opaque is 935 and for transparent is 990, so maybe a slow descent and hold at 990, 960 and 935 would solve the problem? I am going to stick this piecee back in and take it up to 1450 for 1hr to hopefully heal the fissure, then try out a new annealing schedule.

150dph to 1000 hold 10min
400dph to 1450 hold 1hr
9999 to 990 hold 1hr
50dph to 960 hold 1hr
50dph to 935 hold 1hr
50dph to 700 no hold

I'll post the results in 3 days when it is through firing........ha

Lisa

[Tony Smith]

...I think that Lisa's problem is an incompatibility one.

Cheers,

Bob

If it was incompatibility due to a COE shift from multiple firings, wouldn't you expect to see the cracks surround the incompatible color in more than one location and surround the color rather than shoot out in a single radiating crack????

Tony

[Paul Tarlow]

...I think that Lisa's problem is an incompatibility one.

Cheers,

Bob

If it was incompatibility due to a COE shift from multiple firings, wouldn't you expect to see the cracks surround the incompatible color in more than one location and surround the color rather than shoot out in a single radiating crack????

Tony

Is it possible that the pattern bars have shifted compatibilty -- but still stayed largely compatible with themselves -- and the clear & black circle having been fired less (I'm guessing) is not fully compatible with the surrounding glass?

In that case a crack in the clear piece might make sense (for example if it is being 'squeezed') -- and the interface between black and clear makes as much sense as any place else in the circle to start a crack.

Just thinking out load.

- Paul

[Bob]

Tony,

I was thinking back to Lisa's original photographs. I thought they looked like they were near the sharp angles in the bends of the glass. I had heard that the sharp bends could focus the stress differences. Just a guess.

Cheers,

Bob

[Tony Smith]

If I understand correctly, there are only some colors of BE that can shift compatibility and only after multiple firings. I don't believe three is considered multiple, because I thought BE tested all of their glasses for compatiblity up to three firings.

Tony

[Lisa Allen]

...I think that Lisa's problem is an incompatibility one.

Cheers,

Bob

If it was incompatibility due to a COE shift from multiple firings, wouldn't you expect to see the cracks surround the incompatible color in more than one location and surround the color rather than shoot out in a single radiating crack????

Tony

Is it possible that the pattern bars have shifted compatibilty -- but still stayed largely compatible with themselves -- and the clear & black circle having been fired less (I'm guessing) is not fully compatible with the surrounding glass?

In that case a crack in the clear piece might make sense (for example if it is being 'squeezed') -- and the interface between black and clear makes as much sense as any place else in the circle to start a crack.

Just thinking out load.

- Paul

I have a hard time believeing that it is incompatibility, especially since the first tiny cracks and the second larger one occurred in the clear. I assume that clear would be the most forgiving of all??? The frit cast circle has been fired the same amout of times as the pattern bars. Matter of fact, it was in the same load as the slab, in together for the fuse and for the last matte firing.

it is still in the kiln cooling, so I can know for sure, but my theory is that there was too much transparent glass in this piece not to start the anneal higher at the transparent anneal point, which is 990. I should know something by tomorrow. If it has one thing wrong with it, I'm pitching it....far too much grief for one piece!

[Bert Weiss]

Lisa

I think that you might have the understanding to figure out what happened. Look closely at where the cracks start and stop and where they go. This should give you a better clue as to why they started. The photo is helpful but maybe not as informative as a close visual examination.

The only trouble I have had with air leaks around the layer of bricks I use, is warped sheets. Once I closed up the bricks and refired, the warpage disappeared.

I still believe that the brick kiln is a lose lose. You have to pay a lot more for it to heat up and cool down than you do for a fiber kiln. The cost to run the controller on the way down is laughable relative to the extra power needed to get the bricks up to temp. Once you hit 500 all that heat is still there in a brick kiln so you paid to slow yourself unnecessarily.

That said, I always program a segment from 700 - 300 and somewhere during that cycle the power goes to zero. I can see that not doing that could definitely cause problems.

This is a classic reason why one firing schedule works for some and not for others.

[Tony Smith]

Just to add to the discussion, I went back and looked at Lani's post from over a year ago and came up with theis quote:

Lani <lanimcgregor@bullseye-glass.com> -- Wednesday, 10 July 2002 at 03:00

...Cad/sel glasses can shift COE on multiple firings., but the opals are much less likely to shift than the transparents, so I wonder if this is really the problem. The only way you’ll know is to test. The fact that the problem has occurred in the panels with yellow and not in the panels without, certainly looks suspicious. But there are other possibilities.

I'm not sure if the turquoise is a cad/selenium glass, but again, I would expect to see the cracks in more than one place if it was a compatibility shift. The clear glass should be unaffected by multiple firings.

Tony

[Lisa Allen]

Lisa

I think that you might have the understanding to figure out what happened. Look closely at where the cracks start and stop and where they go. This should give you a better clue as to why they started. The photo is helpful but maybe not as informative as a close visual examination.

The only trouble I have had with air leaks around the layer of bricks I use, is warped sheets. Once I closed up the bricks and refired, the warpage disappeared.

I still believe that the brick kiln is a lose lose. You have to pay a lot more for it to heat up and cool down than you do for a fiber kiln. The cost to run the controller on the way down is laughable relative to the extra power needed to get the bricks up to temp. Once you hit 500 all that heat is still there in a brick kiln so you paid to slow yourself unnecessarily.

That said, I always program a segment from 700 - 300 and somewhere during that cycle the power goes to zero. I can see that not doing that could definitely cause problems.

This is a classic reason why one firing schedule works for some and not for others.

Bert
Lose lose or not, I have a brick kiln and I can't imagine not using it. I have always just turned that one off at 700, no added cooling segment. I now have the fiber clam shell too and it is proving to be a way different firing experience.....a learning curve that I was not adequately prepared for to say the least. But, I think I am getting it by trial and error and of course suggestions from the board. From now on the fiber kiln will have an added cooling segment between 700 and 400. At least so far I've only had to potentially lose one piece to learn that.

Lisa

[Brock]

Lisa, I can't really tell from your photo, does the crack start at the interface of the black and clear, or in the black? Brock

[Lisa Allen]

Lisa, I can't really tell from your photo, does the crack start at the interface of the black and clear, or in the black? Brock

Brock
It was hard to tell, it looked in both incidences of cracking that the cracks started at the interface of the opaque pink and the clear then at the interface of the black and the clear. But since the cracks didn't go all the way through to the top and bottom of the piece they weren't visible in the opaque. Doesn't mean that they weren't there, just not visible. Even in the larger crack that extended through the clear and into the colored areas, you could only see the crack in the transparent portions of the span......weird.

Lisa

[Brock]

I don't know how or why but my money's on incompatability. Brock

[Lisa Allen]

I don't know how or why but my money's on incompatability. Brock

I hope that you are wrong. One of the reasons I am hesitant to believe that it is incompatibility is that the clear in the first cracks was from sheet glass and the clear in the second was crystal clear frit. It would at least make more sense to me if the clear was the problem, that it all came from the same source.

If this thing does turn out, I may stomp on it anyway, just to teach it a lesson. Nothing shoud be frought with this much hassle.

Lisa

[Brock]

I don't know how or why but my money's on incompatability. Brock

I hope that you are wrong. One of the reasons I am hesitant to believe that it is incompatibility is that the clear in the first cracks was from sheet glass and the clear in the second was crystal clear frit. It would at least make more sense to me if the clear was the problem, that it all came from the same source.

If this thing does turn out, I may stomp on it anyway, just to teach it a lesson. Nothing shoud be frought with this much hassle.

Lisa

You have a good point there. Bullseye's clear is their constant, and theoretically, (as I understand it) any compatible glass they ever made should fit it. But if individual colours are off just a leetle bit . . . .