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I had posted the item on a potentially failing thermocouple on my Evenheat GT-18. I have replaced it and I *think* things are ok now. I need to do some more testing to be sure, but I wanted to ask about some oddities I saw with my test piece.

This was 2 layers of System 96 clear with lots of shards on top. I kept firing this piece as I tried to debug my problems, so it got taken up to (I think) about 1400-1450 four times. It also failed to anneal properly in each of these firings (and had serious cracks in it).

In the firing with the new thermocouple, it appears that I got a full fuse, but I noticed that previous cracks are still visible inside the glass, and some of the surface, where the shards did not cover, devitrified (unusual for S96). With this method I usually find the shards flow over the full surface of the piece.

So, my question is - does the glass remember its past mis-treatment as far as recovering from not being annealed? I note that the piece still exhibits one crack on the surface after its full fuse - all the other cracks seem to be trapped in the body.

Also, I know one only has a few high temperature firings before devit sets in (so there is some memory) - does devit inhibit the flow of glass, so when I finally did get a full fuse, the shards would not flow over the devitrified surfaces?

I'm trying a slump with this piece now through a Ceramguard drop ring to practice cutting off rims from bowls.

In lampworking, at least, you want to take the glass up to a fully molten state to get rid of a crack... I suspect that it's the same thing, that those cracks will always exist unless you melt it down completely...

froggee501 wrote:In lampworking, at least, you want to take the glass up to a fully molten state to get rid of a crack... I suspect that it's the same thing, that those cracks will always exist unless you melt it down completely...

Glass does not have memory of past annealing. What can happen, is it can go through changes due to overfiring which can shift COE because oxides have burned out and changed the chemical composition. If you just heat above 1000 nothing is supposed to happen or be remembered.

Could you be seeing just the seam rather than an internal crack? Often the "scar" remains even when there is no longer a crack.

I took some pictures before trying the slump I mentioned. I peeked during the flash vent and noticed the glass "tore" on slumping. Must be one really ticked-off piece of glass. I'll post the story and pictures after the cooling finishes.

I'll fire another piece tonight - I guess I still have no complete evidence that the new thermocouple has fixed the original problem.

Richard

For closure. The piece I slumped committed suicide - the glass "tore" while slumping and the piece further cracked.

For details, see

http://www.stanford.edu/~dubois/stained ... ermocouple

I have another (fresh) piece cooking now to see if this episode was restricted to this poor mistreated piece of glass or if the kiln is still sick.

Richard

The cracks you see may have only partially healed unless you took the piece above the normal fuse temperature. What looks like a cracks may have been long thin trapped air bubbles. The devitrified surface of the glass will have a higher surface tension than the surrounding glass. Higher surface tension will inhibit the movement of the glass.
I looked at you images and it looks like you should have taken the piece higher. The slumping problem is most likely due to inconsistent thickness throughout the glass. When you have this much variation in the glass thickness heat up very slowly.

Tim

The note says slumped at 1375 at 750 per hour. Really? That's too fast. And if you were going that fast below 1000 then it's really too fast. I'm not a believer in "tearing". I'm pretty sure that's a crack that opened up.

I think after all those firings, that you had better just start again to test your new thermocouple. I've had glass shift COE after multiple firings and then it's just plain hopeless.

Hi Lynne,

Well, I started off at 500 dph in the ramp. For about a year, in fact. Then got a bit impatient and tried 600. Then 800, then 1000. Might as well see for myself, I said! At 1000 a piece broke on the way up, so I backed off to 600. For these tests, I bumped it back up to 750 because I didn't want to wait so long for these tests on dud glass (and it seemed ok before)...

I'll back off to 600. And a fresh test piece with the new thermocouple (and 750 dph ramp) turned out just fine. Back on the air again!

Thanks,

Richard