Keeps breaking!

[Sarah999]

Hi, I am trying to make a slumped bowl.
I have a stainless steel mixing bowl from wilko's and I am trying to slum some glass over it. I fused lots of bits of glass together - all the same coe, to make a large circle, I would say about 10 inches across. and about 8mm thick.
I fused them all first, then places this on the underside of the bowl to slump. At about 350 degrees it broke in two. So I fused it back together, and put it back in the kiln to slump again, this time it brok when it got to about 120 (deg. f). So I have put it in to fuse back together again.

Is there anyway I will get this to slump? I tried to slump it on the pre programmed slow setting in my fire box 14. Do I need to programme in a very very slow programme? Or is it just likely to never work? I dont understand why it keeps breaking, any clues?
xxx

[llewennog]

maybe some slight coe issues, or heatspots in the stainless bowl? is it kilnwashed? What make of glass is it?
what sort of kiln is it?

[Zeldazog]

The problem with a "hump" is that if the glass contracts in cooling before the stainless steel does,  the the glass is smaller than the mould at one point...

I have used stainless steel bowls for moulds, but I am sure I slumped INTO it, rather than over the top of it.  This way, even if its the other way round (and I can't remember) and the mould shrinks first, its more likely to push the bowl up a little bit.  Over a hump, you're asking the glass to stretch over the now larger mould...

I can't remember which way round it is (Recommendations for best moulds for slumping in to, and over but I am sure there is something about stainless steel moulds on Brad Walkers Warm Tips pages, which materials for moulds, etc, heating and cooling rates but I am at Uni at the moment, so I don't have the link here, sorry

This is now going to be in something like third, forth or even fifth firing?? You're going to start struggling with this, I am sure Dennis Brady said something about re-fring glass more than that many times creates instability.... so DO slow down the annealing section even more, make your anneal soak longer.

[Margram]

http://www.warmglass.com/phpBB/viewtopic.php?f=2&t=35109&p=304720&hilit=slumping#p304720 This might help...? Similar problem to yours

[Sarah999]

Thanks, that helps! I am going to do a mega mega mega slow one, and see if that works, if not, I think I will just have to scrap it!
Does it matter how slow you doi it or can you do it as slow as you want?
xxx

[noora]

You can go slow up to a certain temp, then you have to go faster for a while as the glass reaches the devitrification temperature. If you go too slow around the devitrification temperature the glass can start to devitrify and go misty. If you're using bullseye glass, you can check documentation on their website where you can find out the devitrification range for the glass.

[Sarah999]

Me being an idiot didnt read this again before I just decided to try it, so I think I might have issues with the devitrifying thing, But I was planning to sandblast at least the bottom anyway, so hopefully this will be ok?!
xxx