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Now I am totally perplexed! Here are pics of the piece that had the tiny fissures near the pink glass. I put it in last night for a matte finish on my new kiln shelf. Schedule was
150dph to 1170 hold 20 min
9999 to 960 hold 2 hrs 30 min
50dph to 700 no hold

The cracks in the clear around the pink are invisible now, gone. But a NEW crack has appeared!!! In a totally different area at the bottom of the piece in the clear and blue/green area. What am I doing wrong? The kiln shelf was up on posts. This was fired in a new fiber kiln. Is it cooling too fast from 700 to room temp? Am I over annealing? Please help.

Thanks Lisa

This must be very frustrating for you. I am amazed that the cracks you originally had disappeared at what I consider to be a low temp. of 1170.

I doubt very much that your new crack is from thermal shock, but I don't know what it is from. I think your schedule is fine, again. Sorry, no help.
Brock

Lisa

Your crack in the picture seems to eminate from the black cross on white background. Black and white can certainly be a difficult combo. First the black is a soft glass and white a hard one. Then there are the opposite ways in which they absorb and radiate heat. I'm not clear on which of these properties caused the crack, but it was likely one of them.

Just because 2 glasses have similar COE, does not necessarily make them compatible.

Bert Weiss wrote:Lisa

Your crack in the picture seems to eminate from the black cross on white background. Black and white can certainly be a difficult combo. First the black is a soft glass and white a hard one. Then there are the opposite ways in which they absorb and radiate heat. I'm not clear on which of these properties caused the crack, but it was likely one of them.

Just because 2 glasses have similar COE, does not necessarily make them compatible.

I think it's black and clear and they are fairly forgiving together. Brock

It is black and clear, no white in the piece. I am so stumped by this piece and am reluctant to fire the rest oof the 1/2" thick pieces that are ready to go back in for their final firing. I am also questioning my all opaque pieces.....do they have internal fissures that aren't visible.......oh, this is a bad day for me.

All I can think to do is dam it and refuse in the brick kiln and just see what happens. the only different thing about this piece is the cross hair circle is frit cast with crystal clear frit and black sheet. But they are all supposedly tested compatible.....

Dazed and confused, Lisa

If it's any consolation, and it isn't, I've had some problems with pattern bars also. Supposedly compatible glasses seem to turn incompatible when cooked at 1500 for a while. Again, no answer yet. I'm going to be trying longer soaks at lower temps in the future. My current practice is nuclear to 1500, hold for 30 - 45 minutes, and anneal, sort of. Because I know I'm going to cut them up, refuse and anneal the final product, I sometimes cheat on annealing the bars themselves. May rethink that! Brock

Nice piece Lisa, and sorry glass deities are demanding some kind of penance from you. Your schedule looks fine, and you can't over anneal, so that's not an issue. Like Brock, I'm puzzled that the first cracks healed at such a low temp. Not that I think this is related to the crack, but are you confident of your controller and thermocouple? Hard to see from image, but where does crack go after it exits clear circle? Before going further, I'd make up a few test pieces of the same thickness just using the black and clear cross pattern in your image. You could even lay it up square. Should be fast and easy to assemble three or four. Then I'd fire them to the same schedules you used for the first piece. If they all survived then at least you know it's not a black and clear interaction issue. One more thought, did you look at the crack area with polarizing filters?

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

Lisa Allen wrote: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

Dam it? Nothing can go wrong, go wrong, go wron . . . . .

Tony Serviente wrote:Nice piece Lisa, and sorry glass deities are demanding some kind of penance from you. Your schedule looks fine, and you can't over anneal, so that's not an issue. Like Brock, I'm puzzled that the first cracks healed at such a low temp. Not that I think this is related to the crack, but are you confident of your controller and thermocouple? Hard to see from image, but where does crack go after it exits clear circle? Before going further, I'd make up a few test pieces of the same thickness just using the black and clear cross pattern in your image. You could even lay it up square. Should be fast and easy to assemble three or four. Then I'd fire them to the same schedules you used for the first piece. If they all survived then at least you know it's not a black and clear interaction issue. One more thought, did you look at the crack area with polarizing filters?

Thanks, tony. I must have bad kiln karma or something!

Controller and thermocouple are fine. Everything tests ok. The crack extends from the clear about 1 and 1/2" into the green/blue/purple area. I did samples and all were fine........this should have worked! My only idea now is annealing more specifically for the mix of glasses. the cracks both times went through clear, so I am hoping that by starting the anneal sooner at 990 and going really slow down to 700 with holds at 900, 960 and 935 I can cover all the bases and get this thing fixed. If it works, it will be a good lesson to have learned. If it doesn't, I will cry.

Lisa

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???

My heartfelt condeolences. I hate it when I can't figure out what is going wrong. Unfortunately I have no clues for you.

Barbara

But...........the bars are breathtaking...wonderful colors.Would love an explanation how you set up your bars to get these shapes and graduated colors....How big to you make the bars. Read your tutorial but it doesnt seem to me that you are making them in the same manner....Areas are much larger in consistance of pattern. Care to give a tutorial here?

Hi Lisa,
Any chance there was a tiny bubble in the clear frit/black cross area?
Might that have caused enough pressure to crack?
I've been following this thread, and your earlier post, because I've been doing pattern bars too. (After a Brock and Avery infusion!) The first block I made cracked half way thru while cutting it on the tile saw, (thermal shock--I rushed the final cooling to room temp.) I'm curious what size block you’re making initially, and what that firing schedule is?
Beautiful work! Your web site is very inspiring!
TamiC

Lisa,

I know you said your Denver holds the heat really well, but that is usually true for fiber kilns in the last 200 to 300 degrees. I would be more concerned with the temperature drop after your last hold at 700°. Controlling the temp drop to 450° at 100 dph would reduce any potential for thermal shocking on the way down.

Tony

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???

My heartfelt condeolences. I hate it when I can't figure out what is going wrong. Unfortunately I have no clues for you.

Barbara

Thanks, barbara. I am trying the multiple holds as a last resort to try to save this piece. I have never had to consider it before, but with such a mix of trans and opaque, it is all my overworked brain could come up with. Its in now with holds at all 990, 960 and 935. I'll be sure to post with the results.

doctac and Tami, thanks for the compliment! The slabs that I start out with are about 10x12x1.5" after firing. I just dam that area in my kiln and load it up with about 120 strips of glass per slab. the gradation comes with how the slab is set up. The tutorial at http://www.ebsqart.com/artMagazine/za_184.htm is basically the same priciple, just on a smaller scale. You are still cutting cross sections of the slab, on the tutorial brick, you cut it like a loaf of bread and on the bigger slabs you are cutting long slices like bacon. You can see that I like food analogies.......

My slab schedule is:

300dph to 1000 hold 10min
400dph to 1500 hold 30min
9999 to 960 hold 2hrs 30min
50dph to 700 no hold

However, this might be changing to accomodate the mix if my experiment works with slowing annealing wy down. I'll let ya'll know.

Lisa

Tony Smith wrote:Lisa,

I know you said your Denver holds the heat really well, but that is usually true for fiber kilns in the last 200 to 300 degrees. I would be more concerned with the temperature drop after your last hold at 700°. Controlling the temp drop to 450° at 100 dph would reduce any potential for thermal shocking on the way down.

Tony

Hi Tony. That's actually something that I wondered about and silly me, I haven't been monitoring the descent from 700, because it is usually when I am sound asleep. I will program in a 100dph drop to 400. Right now it is in the brick kiln, so I think it will be fine to turn off at 700.

Lisa

One of the advantages/disadvantages of the brick kiln is that it does heat up during the firing, so during cooling it has heat to give back which slows the cooling process.

Good luck

Tony

Tony Smith wrote:One of the advantages/disadvantages of the brick kiln is that it does heat up during the firing, so during cooling it has heat to give back which slows the cooling process.

Good luck

Tony

I see that as a pure disadvantage. You pay to heat the bricks up. Controllers are made to control the heatup and cooldown. It is quite a bit better in my estimation to control another segment of the ramp down verses having to pay for the bricks to heat up and then wait for them to cool down. That is a lose lose.

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?