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Newbie question: all other things being equal (using Bullseye glass) is a thicker piece more likely or less likely to crack during a slump? I'm trying to think it through and can see an argument for either way.

it all depends on your schedule and set up. as a rule i think that the thicker the piece, the more conservative the heat up needs to be to achieve thermal evenivity. r.

"Cracking during a slump" ?? have not had that problem in the last 20yrs. Low and Slow and as Rosanna said adjust for thickness. I think many or even most of the newbie problems are the result of too fast and too hot! You didn't elaborate on you schedule or the thicknesses you are using so am unable to offer specific help.

The thicker the piece of glass the more difficult it is to heat evenly. You must not get the surface too much hotter than the core. So, slow down...

"All things being equal" is a pretty sweeping assumption, but I'd say the thicker it is, the more likely you are to have cracking. There's simply more chance for a big difference between temperatures in the core of the piece and the surfaces.

I don't know that I'd characterize it as a brittleness problem, though.

Yeah, I know I didn't give any real details. I had two pieces that were virtually identical except one was 6mm thick and the other was 7.5mm; fired identically the thicker one cracked apart during a drape, and judging from the positions of the pieces it broke during the ramp up to process temp.

(Once I realized it had broken I aborted the program and opened the kiln. When the pieces were cool enough to handle I put them on a table, where they promptly exploded. First time that's happened to me, it was a little exciting.)

Anyway, thank you for the input.

Somewhere I saw a value that the core and surface (or any two points in a piece) must be kept wthin 20C 36 F below the transition range. In annealing the delta is 5 C. There we are worried about permanent frozen in stress. In warming up we are worried about temporarily tolerable stress.

Above transition range the glass is supposed to 'give' and no stress may accumulate.

In draping -or pate de verre over acore it may tear apart, not break.

My 0.02€
-lauri

Morganica wrote:"All things being equal" is a pretty sweeping assumption, but I'd say the thicker it is, the more likely you are to have cracking. There's simply more chance for a big difference between temperatures in the core of the piece and the surfaces.

I don't know that I'd characterize it as a brittleness problem, though.

I agree with Cynthia that there is a semantic issue. Brittle is not the best word. Stresses are cumulative. So if a piece has existing stress from a poor anneal or compatibility issues, and you add thermal shock stress, it will break. A well annealed more compatible piece might be able to withstand a bit more shock before it destructs.