Perplexed.....need help (pics)

[Kevin Midgley]

Rather than stomping on a panel, I tend to keep some of my stompable pieces around as examples to study and learn what not to do. I'm with Brock, possibly a leetle bit of incompatibility and an imperfect cooling to room temperature.

[Tony Smith]

The proof will be when she takes is out of her brick kiln after using her old schedule and having (hopefully) brought it up hot enough to re-fuse the crack. No crack - no incompatibility.

Tony

[Lisa Allen]

Well, the kiln is at 180º so I peeked and there are no visible cracks.........will know more later tonight when I can open it.

Lisa

[Bert Weiss]

Rather than stomping on a panel, I tend to keep some of my stompable pieces around as examples to study and learn what not to do. I'm with Brock, possibly a leetle bit of incompatibility and an imperfect cooling to room temperature.

I agree with Kevin here. I like the improper cooling theory. A little bit of incompatibility could rear its ugly head when cooled too fast.

Like Kevin said earlier, just because the thermocouple reads a particular temperature, the glass could still be quite a bit hotter.

BTW Lisa, I spend the same amount of time going from 900 - 700 as I do from 700 - 300. In other words the rate of time is twice as fast. That is for my float anneal which starts with a long soak at 1000 and the same amount of time going from 1000 - 900. For BE I would probably drop all temps about 80º.

[Kevin Midgley]

the kiln is at 180º so I peeked and there are no visible cracks

Air temperature! The glass had to be over 300 on its surface and probably 500 F or more internally for a thick piece. Haven't there been a long discussions on this board before about the 300F glass transition temperatures, the ones I believe Frederick Schuler talked about in his "glassforming" book?
Just because most of the time thin glass can be crash cooled from 700F if properly annealed in its upper range doesn't mean you can do the same with a thick fused piece. I can't remember all the thread but did you peek on the earlier firings that cracked? With a fibre kiln the only thermal mass you have in the kiln is the glass. You cannot peek. The peek can be the source of the equivalent of a tiny air leak in your kiln, but all at once. The firing isn't over until the kiln is at room temperature and the glass must not be removed from the kiln until it too is room temperature. Add the above to Brock's "leetle incompatibility" and you have a crack in some fine fused work. You have a bunch of time and materials invested in fused glass so peeking, although tempting just isn't worth it.
Hope it turns out. Kevin

[Lisa Allen]

the kiln is at 180º so I peeked and there are no visible cracks

Air temperature! The glass had to be over 300 on its surface and probably 500 F or more internally for a thick piece. Haven't there been a long discussions on this board before about the 300F glass transition temperatures, the ones I believe Frederick Schuler talked about in his "glassforming" book?
Just because most of the time thin glass can be crash cooled from 700F if properly annealed in its upper range doesn't mean you can do the same with a thick fused piece. I can't remember all the thread but did you peek on the earlier firings that cracked? With a fibre kiln the only thermal mass you have in the kiln is the glass. You cannot peek. The peek can be the source of the equivalent of a tiny air leak in your kiln, but all at once. The firing isn't over until the kiln is at room temperature and the glass must not be removed from the kiln until it too is room temperature. Add the above to Brock's "leetle incompatibility" and you have a crack in some fine fused work. You have a bunch of time and materials invested in fused glass so peeking, although tempting just isn't worth it.
Hope it turns out. Kevin

Hey Kevin-

This peek was in the brick kiln and was only about a 3 inch lift of the lid. Matter of fact, it was so quick that the temp never changed on the controller.

Now on to the results.......it came out with NO CRACKS AT ALL!!!!! I changed my anneal to start at 990 instead of 960 to accomodate the predominance of transparents and it looks to have worked.....woohoo! I can proudly say that incompatibility was not the problem and my over worked brain can now take a rest.

Only minor prob is that I dammed it about an 1/8" away from the piece so I could get a bullnose edge instead of it hanging up on the fiber paper and everything shifted into a slight lean, so the pattern is not straight up and down anymore, rather leaned looking. If you look at it from the side you can see through it, but from straight on it is a bit distorted........but hey it worked and for me that is all that counts! its still pretty cool anyway. A big lesson learned.

Lisa

[Cynthia]

Fabulous! Glad that my hunch was wrong and that you salvaged the piece.

The learning curve sucks with new kilns. You'll figure this one out so you can fire stress free (pun intended) in your new baby.

[Lisa Allen]

Fabulous! Glad that my hunch was wrong and that you salvaged the piece.

The learning curve sucks with new kilns. You'll figure this one out so you can fire stress free (pun intended) in your new baby.

Thanks, Cynthia! I wouldn't say it is salvaged completely, because it shifted too much for it to be saleable to me, but the important part was figuring out what was going wrong and that I did........whew.

Now on the Denver, I will not only add a 990 seg when firing trans slabs, but also a seg from 700 to 400 to control the cooling better. Do you include a segment under 700 in your Denver?

Lisa

[Cynthia]

...Now on the Denver, I will not only add a 990 seg when firing trans slabs, but also a seg from 700 to 400 to control the cooling better. Do you include a segment under 700 in your Denver?

Lisa

I do when firing pieces that are thicker than the standard. Usually if I have a piece that is thicker than 6mm it's because I have tack fused elements. Then I add a ramp with a decrease of somewhere around 200 dph from 750 to 400. I don't honestly know though if my kiln is cooling more rapidly than that in this range (I know, I should be paying closer attention ) I seriously doubt it though since it holds onto the heat like nobodies business. I add the ramp for insurance.

Do what you know works. Sorry that I could only offer the "Hindsight is 20/20" kind of advice. I really did want to soften the learning curve for you...

The road to Hell is paved with good intentions...or something like that

[Lynne Chappell]

Re the annealing cycle, the Bullseye folks seem to think it is better to soak below the annealing point rather than above - I can't exactly remember the reason.

It makes sense to me for you to start your anneal at the highest annealing point of the glass you have used, especially since your piece is thick, but perhaps a very slow descent would be better than multiple soaks?

Probably the problem was the fast cooling of the fiber kiln. I think with a new kiln, it is beneficial to watch the entire cooling cycle to see how fast it cools naturally.

[Lisa Allen]

Re the annealing cycle, the Bullseye folks seem to think it is better to soak below the annealing point rather than above - I can't exactly remember the reason.

But 960 leaves you soaking below the transparent annealing point (990) and above the opaque annealing point (935), so you end up accomodating the opaques more than the transparents since the descent is typically controlled below 960. And I don't know about everyone else, but I have always raced 9999 down to 960, skipping right by 990.


It makes sense to me for you to start your anneal at the highest annealing point of the glass you have used, especially since your piece is thick, but perhaps a very slow descent would be better than multiple soaks?

I agree it needs some tweaking. i was just in overkill mode trying to salvage this piece and learn something hopefully

Probably the problem was the fast cooling of the fiber kiln. I think with a new kiln, it is beneficial to watch the entire cooling cycle to see how fast it cools naturally.

I agree that the fast cooling of the fiber kiln was a factor in the big crack, but this piece came out of the brick kiln in its first firing and had cracks, smaller, but there nonetheless. The salvage firing with the mega anneal segments healed everything and there were no more cracks, which leads me to believe that 990 needs to be considered in thicker, primarily transparent pieces. I sure don't want to go through this again!

[Tony Smith]

Lisa,

As a point of reference, it's not unusual for two thermocouples to deviate 20 degrees or so in the middle of their range. They are typically pretty linear, but when you cover more than 1400 degrees, it can happen.

Tony

[Amy on Salt Spring]

which leads me to believe that 990 needs to be considered in thicker, primarily transparent pieces. I sure don't want to go through this again![/color]

Lisa I was just planning to do a piece with very thick transparents so thanks for sharing this info.!
Amy