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Last week I was slumping a sheet of glass that I worked on years ago. On the way up it cracked. The ramp rate was 300° an hr. It must not have been annealed correctly.

Today I put 2 sheets of glass in the kiln both 14 X 14. Clear over Black rainbow irid. Again I got thermal shock on the way up. This time it was the top layer, the clear, that cracked. The bottom layer, the black irid did not shock. Both pieces of glass were thin and I had the old bullseye thin fire on the shelf. The clear healed when it got hot, that is how I know if happened on the way up, also by the fact that the black was not cracked.
Here are pics of the piece out of the kiln. http://f2.pg.photos.yahoo.com/ph/the1gl ... 5b&.src=ph

I have done this firing many times before. Both with the same types of glass and at the same rate. This time it was 9999 to 1450°.

I have not fired the kiln in between these 2 firing... So, is it coincidence or trouble?

Hi Stuart!

To help eliminate the controller as a potential suspect, you may want to run your firing program again and calculate the time it SHOULD take to ramp to temp and compare that to the time it takes to actually get there.

Of course, you don't need glass in the kiln to do this.

-stro

That's a strange thermal shock crack. For it to curve around like that it must have traveled through the glass quite slowly. I don't know what that might mean though. Maybe there was a small check in the clear that you missed and it ran through during heat up.

AFAP to 1450 is risky for glass this big if you ask me. How close to the elements were the corners of the glass?

ch

The kiln is a GL22. The shelf is near the bottom. So, I would think about 8 inches.

Stuart

Wow, you can actually go that fast? How long does it take to get to 1450 in your kiln? I've gone 500/hour with only occasional cracking but anything over that is a problem. The glass doesn't seem to have separated very far, so its not like any thermal shock that I have experienced. Usually the glass moves apart at least an inch, sometimes a lot more than that, sort of like an explosion. And it's usually the bottom piece that cracks, but that may be because it's usually the larger piece.

The shape of the crack looks like the glass was pretty poorly annealed. I think that the thin glass is not really annealed in the factory.

So I guess you're not going to know until the next firing if this is just the new order of things with your kiln. You may have to slow down like the rest of us.

Lynn,
By going 9999 on the Bartlett and Orton Controllers is basically the same as AFAP on others. What it does is fires the relays at 100% instead of coming on and off to keep the temp going at the rate that you want. It also turns off the part of the controller that allows for checking for too much or too little.
I think on reason the piece did not crack wider was because of the irid under the piece.
I think that I have other pieces of clear from the same bullseye run so I will try the same glass again. If people don't know how to read the Bullseye codes you can look at the small price tag looking label and see what day the sheet of glass was made. And then the white or black ink, tells you want number sheet you were in that run.
I am going to contact Bullseye with that info and see what they say about that crack.