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I've been making small bowls (5 1/2" diameter circles in a Bullseye ball mold) with
good success EXCEPT that every time I try to use a screen melt circle as a starting point
they crack early in the slumping process. Even starting my kiln at the lowest setting
this still happens. Using the same mold, the same kiln, I have good success. So, there
must be something I don't understand. Any ideas?

Thanks,
Emily

More information is necessary.

Are the melt disks the same thickness as the other ones that don't break? If they are thicker they will have to be ramped up much slower especially in the tight confines of that little kiln. That would explain why the break appears to be from thermal shock.

The break also has the appearance of being caused by stress from poor annealing or incompatible glass. More information about how your melts are made will shed some light on those possibilities

Jim "The Hobbyist"

What is your firing schedule? Which coe glass are you using?

Jim and Joe -

I'm using Bullseye only. I think Joe's point about annealing could be the answer. I measured the
pieces and although I gave the glass plenty of time and space to spread, the thickness is between
7 and 8 mm instead of the usual 6 mm. In terms of annealing and firing schedule, since it was a screen
melt I went fairly quickly to the process temp, but on the way down I did my usual which is to just let
the kiln cool at its own rate since it is a small (8x8 Evenheat Studio Pro) kiln. That works fine for small
pendants, etc, but I an see now that for a screen melt that I plan to slump, I should probably turn the
kiln back on when it gets down to about 900-1000 and hold it there for a while.

Does that make sense?

Thank for the advice.

Emily

You're on the right track but "and hold it there for a while" isn't going to give you repeatable results. BE has an annealing chart. Use it.
Also realize that all the various melts- pot, screen etc. have been fired higher than BE's test firings and that some colors may shift in COE and viscosity, making them potentially incompatible.

What you are going to try with future annealing is the way to go. Spend a bit more time to save time. Only other suggestion would be to add some baffling between the edge of work and the side elements. Depending on what you have on hand is where you may need to get creative with the baffling problem, could be just a ring of ceramic fibre supported by posts. peace, haydo

I agree with Haydo about the baffling. I have to baffle in my smaller kiln, and now I use baffles in every single firing in that kiln. There is a lot of heat directed to the edges and the center is still waiting for the heat. Enormous difference in heat between the edges and center, no matter how slow you go.

Beautiful melt by the way!