Compatibility - WarmGlass.com

Compatibility

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Linda Blackburn
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Location: Georgia

Compatibility

Postby Linda Blackburn » Thu Jun 28, 2012 11:55 pm

I've read the info on BS about compatibility, and I guess I grasp that on an intellectual level, but how do I apply that to pot melts? I just had one break for no other apparent reason than incompatibility. Can someone please explain this to me?
Linda Blackburn

"Art flourishes where there is a sense of adventure." - Alfred North Whitehead

Bert Weiss
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Re: Compatibility

Postby Bert Weiss » Fri Jun 29, 2012 1:18 am

As glass gets hot, the chemicals in it go through various reactions that result in changes in the compatibility aspects of the glass. For instance, glass made in a particular furnace in the morning could be incompatible with glass made from that same furnace in the afternoon, even though the materials put in to the furnace remain the same. The longer cooking factor changes the glass. Some colors are more vulnerable to these changes, like yellow, orange, and red.

If you read the blurbs on the Bullseye website, they state that their compatible glasses can be heated 3 times to 1500ºF. Since potmelts are hotter than 1500, all bets are off. Yes they often remain stable, but they sometimes do not. If you read reports here, there are a few glasses that tend to present problems when heated above 1500. This is one of those situations where you might get away with going outside the safe boundaries, but if you encounter problems, it is not all that difficult to assign blame.
Bert

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Warren Weiss
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Re: Compatibility

Postby Warren Weiss » Fri Jun 29, 2012 9:16 am

Linda,
Besides what Bert mentioned, inadaquate annealing is just as likely to cause the crack. Lots in the archive about annealing.
Warren
Last edited by Warren Weiss on Fri Jun 29, 2012 11:06 am, edited 1 time in total.

Mary Lou
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Re: Compatibility

Postby Mary Lou » Fri Jun 29, 2012 10:54 am

Linda, what was your schedule?

Linda Blackburn
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Location: Georgia

Re: Compatibility

Postby Linda Blackburn » Fri Jun 29, 2012 2:11 pm

This was the third firing for this piece. 1st firing was the pot melt, which came out 1/4" thick as I had planned but didn't quite spread to fill the form. The 2nd firing I added frit around the edges to fill in the corners, still maintaining 1/4" thickness. The third firing was when the piece broke. I cold worked the edges and was firing to fire polish. Here are my schedules:

1st firing - Pot Melt:
250 > 1000 > 15
250 > 1100 > 15
450 > 1700 > 60
9999 > 1520 > 15
9999 > 900 > 180
100 > 800 > 60
100 > 650 > 15

2nd firing Full Fuse for added frit:
100 > 1100 > 0
50 > 1250 > 45
250 > 1480 > 20
9999 > 900 > 180
100 > 800 > 60
100 > 700 > 15

3rd firing - same as above. I did not open the kiln until room temp. The break occurred during cool down. The piece is 8.5" square and made of a variety of BE greens, white, and clear. The break is a straight diagonal line that crosses through the various colors. Thanks for your suggestions. I am really trying to educate myself on why these things happen for no apparent *to me* reason.
Linda Blackburn

"Art flourishes where there is a sense of adventure." - Alfred North Whitehead

Mary Lou
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Re: Compatibility

Postby Mary Lou » Fri Jun 29, 2012 3:46 pm

I think you allowed enough time for annealing, actually 3 hours seems more than enough. In my kiln I would have held at 1050/1100 for 30/45 min, skipped the bubble squeeze, (that was done during the fusing process), gone a little faster to process temp to avoid devit, not gone as high as 1480 for FP or hold as long, annealed for 2 hours then not held at 800 and gone 180 to 700 no hold. That's only my experience using my kiln.
So if it cracked on the way down, then perhaps you have to look at the previous firings.
Your schedule for the pot melt seems unusual, here are a couple of references I've used with success.

http://www.clearwaterglass.com/Tutorial ... ePour.html
http://www.fusedglass.org/downloads/sch ... hedule.pdf

I'm sure more experienced opinions would be more helpful.

Linda Blackburn
Posts: 14
Joined: Thu Jan 24, 2008 11:48 pm
Location: Georgia

Re: Compatibility

Postby Linda Blackburn » Fri Jun 29, 2012 4:08 pm

Thanks, Mary. I'll look at those schedules. The one I'm using came from Laurie Spray, Bonnie Doon Glass. I've used it a lot and have never had trouble with it. I think it something with that particular combination of glasses.
Linda Blackburn

"Art flourishes where there is a sense of adventure." - Alfred North Whitehead

Morganica
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Re: Compatibility

Postby Morganica » Fri Jun 29, 2012 5:10 pm

From your description, it may not be a compatibility issue. Compatibility fractures more often outline the incompatible glass pretty precisely, then streak off to the nearest edge. If the break is across multiple colors and never actually outlines a full color, it could be compatibility...but it could also be something else.

Possibly the glass got slightly hung up on a worn-out kilnwashed spot when it was expanding or contracting, or that the glass wasn't quite annealed enough in the second or third firing. (Annealing cracks--there's a tutorial on this site that explains this pretty well--tend to look like long, lazy S's that snake across a piece)

On compatibility: The thing that makes glass interesting to artists--its ability to slowly deform with heat (i.e., expand/contract/flow/slump), instead of suddenly shifting from solid to liquid to solid like most solids--is also responsible for compatibility issues.

Compatible glasses (mostly) deform at the same rate* so that they all move together in the same piece. When one glass deforms faster or slower than another, it will tend to pull away from the others. If if does it a little bit, you get some stress in the piece. If it does it a lot, the incompatible glass will try to break out of the piece entirely, and you get compatibility cracks.

Compatibility changes because glassmakers add different chemicals to change color, opacity, softening points and other things. These chemicals can be heat-sensitive, so that the more heat they receive, the more likely they are to physically change (for example, if you overheat red transparent glass it will often go opaque. When they change, they often change the way the glass will deform...which is why it becomes incompatible with the glasses it used to work with. What Bert was referring to, the 3 firings at 1500F stuff, is basically that Bullseye tests its glass to make sure that those changes won't happen UP TO that point. After that, you're on your own.

To fix the incompatible glass you must completely remelt it, which means you get it up to 2300F or thereabouts. Our kilns rarely go past 1700F, so that's not really practical. And in any case, if you reheated your potmelt to that level you'd wind up mixing glasses and coming up with something entirely different anyway. Even if it worked, some of the changed chemicals don't change back, so you wouldn't get the original glass back. That's why--when you have incompatibility cracks--the best thing to do is move on.

*or at least so that the movement of one compliments the other
Cynthia Morgan
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Lynne Chappell
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Re: Compatibility

Postby Lynne Chappell » Thu Jul 05, 2012 12:29 am

My compatibility failures have usually been on a third firing, so I would certainly be suspicious. Can you not do a test with polarized lenses on the suspicious party?


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