Re: cracks after firing

shegan · Post by **shegan** » Wed Apr 07, 2004 12:26 pm

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

jerry flanary · Post by **jerry flanary** » Thu Apr 08, 2004 7:51 am

Are the cracks in all five of the surfaces that were in contact w/ paper? I would lift the mold off the floor with bricks or something just to get more even heat all around. Look at it w/ a polariscope .

elizglass · Post by **elizglass** » Thu Apr 08, 2004 9:58 am

Id go much slower on cool down. can you do a Strain Point test on your glass to get the propper annealing temp? I've worked with annealing mottled glass that I was told had various annealing temps, so I held at different ranges- so, you could soak fro 4 hours at 1000, then go SLOW to 930 and soak 4 hours, and then slow to 800 and soak etc. When I'm doing work of the size you are describing, I like to take 26-28 hours to go from annealing to room temp. The idea of lifting your mold up is good- Sounds like a great project- good luck!
Liz

charlie holden · Post by **charlie holden** » Thu Apr 08, 2004 1:33 pm

What Jerry said. Get air under the glass.

How much insulation is in the floor of you kiln? To fire this thick and broad a piece directly on your floor you need either lots of insulation or lots of mass, so the floor cools more slowly than the interior of the kiln.

Think of it in terms of; heat is always moving out of the kiln through the floor. Once your kiln floor gets to annealing temp, (presumably shortly after your glass gets to annealing temp, maybe even before), it has to pull heat out of your glass to stay at annealing temp. The floor is always losing heat to the outside. If your glass needs to soak for a while longer you have a problem, since the floor is now cooling the glass down from below. The top suface of the glass is hotter than the bottom.

ch

shegan · Post by **shegan** » Thu Apr 08, 2004 3:40 pm

If I suspend the stainless forms on bricks, the forms will bend and warp. So I suppose you would use kiln shelfs to suspend them, but wouldn't that in turn rob some of the heat from the piece?
Scott

shegan · Post by **shegan** » Thu Apr 08, 2004 3:42 pm

Maybe rather than suspension on birck, I'll insulate it from the floor with an inch of frax.
Scott

charlie holden · Post by **charlie holden** » Thu Apr 08, 2004 10:32 pm

An extra inch of frax insulation might be enough. To be sure you could raise the frax off the floor and use it like a kiln shelf.

ch

jerry flanary · Post by **jerry flanary** » Fri Apr 09, 2004 9:06 am

You could use silicon carbide shelves; they are strong enough and their effect the heat transfer is negligable.