Re: cracks after firing

[shegan]

Also posted in the kiln casting section, but didn't want to "hide"
Hi there all:
I'm working on staircase treads in a pie shape, 7" on the small end, 22 on the outside, 35" on the edges. 7/8 inch thick. Pretty pig piece, hope you can imagine it. Seattle fused batch annealing temp 910. Annealling schedule after firing 4 hour soak at 1000, 13 hours to 800, 6 hours to 600, 4 hours to room (takes longer of course). I have not tested the kiln to see exactlyhow the manufacturers annealing temp aligns with the glass itself. Stainless steel molds lined with paper, set on floor of kiln. So far so good.
After I remove the treads from the oven, after a day or two, the surface that was in contact with the paper (away from the heating coils) starts to crack, not all the way through the piece, and not many, but they're there. I was going to lower the soak to about 930, then go 13 hours to 800, or something like that. I figure that it has to be an annealing problem since there are no inclusions, and all the glass comes from the same batch at the factory.
Any suggestions?
Scott

[robertb]

if your annealing temp is 910 why the 4 hour saok at 1000? Am I missing something? Would not your 4 hour soak be at the annealing temp?
robert

[shegan]

I thought that the annealing temp was higher - around 965, and just found out today the correct figure. The reason for the high soak is advice from glassmakers that soaking 25-35 degrees higher than the annealing temp for four hours is helpful. No you are not missing anything, but I was.
Scott