re-fusing a thermals shocked piece

[Nicole Hanna]

Okay, I've spent hours searching, can't seem to find the answer I need, so I gotta ask. I stupidly took this piece out of the kiln too soon...I figured since it's about 98 degrees in my studio these days, it wouldn't hurt since the kiln temp said it was down to 102. Obviously I was wrong. I know how to avoid thermal shock, I need to know how to save poor Bessy here! It fits back together perfectly, so I think I can re-fuse it, and I know I need to go slow, but should I dam this to ensure it doesn't spread?
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[Nicole Hanna]

Auugh, the image didn't get in...try this again!!

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This is what she's suppose to look like:

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[Brad Walker]

You absolutely have to dam it when refiring. Otherwise the two halves will separate.

You will prob find that even with damming the line between the two halves will be visible. But with luck you may be able to make a repair and re-fire again if needed.

Go very slow on the refires and make sure to anneal conservatively.

[Kevin Midgley]

Unless there is a specific requirement for a specific size that does not allow for spreading, I would place another piece of clear glass the same size as is the broken piece or perhaps 1/8" larger and overhanging all sides. The flow in theory will be in the top piece of new glass rather than the pattern area. You may find the outer edges will go to clear glass as it flows.
Otherwise, what is to stop the panel flowing outwards and distorting at fusing temperatures other than dams with all their after firing clean up issues?
Without the top piece of glass there is zero chance of the line being invisible imho.

[Nicole Hanna]

Thank you so much for the replies, guys! I was pretty sure I needed to dam it, but Kevin your idea of putting a piece of clear over it never occurred to me. I think I will do some experimenting with something from my scrap heap and see which works better. I spent a lot of time on this crazy cow, I really would like to save her if I can! I will post results...
Thanks,
Nicki

[Marty]

Your thermal shock didn't happen at 102F (unless you threw cold water on it). What's your schedule?

[Kevin Midgley]

My guess is that there is/was a kiln post in the center of the shelf below the center of the glass.
It goes to show you that there was a huge amount of thermal mass in that area that was not the same as the air temperature of the kiln.
Thus the hooks in the split at the center where the center was not fully annealed.
The glass may have been cool enough to even handle when the center portion you would not dared to have touched.
You were right to ask that question Marty, I'd thought about it and forgotten to post about it.
The annealing schedule must take into account the thickness of the kiln shelf when calculating an annealing schedule and in this case time would have had to have been added for the thickness of the non cooling kiln post below it.
It is all in Stone's book of firing schedules.

In some ways breaking the cow was a good thing to do as it makes us all more aware of the intricacies of firing glass.

[Nicole Hanna]

Oh my gosh, I didn't think of that. The things ya forget to think about! I used a different shelf that is much thicker than any of my others. I have no idea what kind it even is, it was in some stuff I bought from someone else, but it's at least twice the thickness of the other shelves I typically use. I've used it before with no problem, but then, I usually let the stuff sit in the kiln for quite a few hours at room temp before I attempt to take it out---because I know that things need to be evenly and completely cooled! This is the first time I've rushed it...I needed the kiln for another piece! Ugh!
I did not touch the middle or much of it at all...I took it out by the edges, and promptly set it on a towel on my workbench and went on to do other things. Several hours later I went to pick it up to inspect it and it fell apart in my hands.

Well, I have two tiles in the kiln now...one is dammed and the other has a piece of clear over it, on a very slow schedule, so we'll see what happens with the repair experiment! Thanks again so much for your valuable knowledge! You guys are always so good about replying, I really appreciate it!
Nicki

[Valerie Adams]

The other thing to consider is that you've got white glass sandwiched on either side with black glass; those have very different viscosities.

[Nicole Hanna]

Thanks Valerie. I did think about that before I even started on it and considered not doing a black and white, but I just didn't want a brown cow!

[Nicole Hanna]

Well, the kiln is done on my test tiles. I used tiles that were previous goof ups in my reject bin. I cut them in half, butted them back together, then I dammed one with fiber paper and kiln posts, the other I just put a layer of clear Tekta over the top of it, about 1/8th inch larger than the tile, but didn't dam it. Both of them turned out pretty great...there is very faint line on the back where you can tell it had been two pieces, but nothing is showing on the front, it looks perfect. The one with the dam will need cold working the edges though, where as the one with the clear, as Kevin predicted, just has about 1/8th inch of a clear border around it. Which could also be cold worked off, but it's nice and smooth so it wouldn't have to be.
I tried to take pictures of them, but the faint line doesn't show up on the photo's so there's really nothing to see!
Now to try this on Miss Cow!

Nicki

[Nicole Hanna]

Bessie is back together! I couldn't decide which option to take, so I did both...put a layer of clear over it, and dammed it. I'll have to cold work the edges now, but there is no trace showing that it had ever been broken. I used a very slow schedule of:
150-900-15
300-1100-0
100-1250-30
AFAP-1420-5
AFAP- 960-1hr 45m
50-800-0
100-700-0
200-100-0

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[Marty]

great save!

(I've got some minor issues with your schedule but we'll save that for another time...)

[Nicole Hanna]

Thanks Marty!
But, please do elaborate on your issues with the schedule when you get a chance!

[Buttercup]

Cool! I've been wondering about Bessie. Great result! Jen