Slumping Glass 6mm and 10mm

by David
(Melbourne Australia)

I have been producing small pieces of slumped float glass in under 2 hours. They have survived successfully but I can see light brown colors around some of the textured areas. Could this be stressed? How do I eliminate this problem?
P.S: this is a very cool site!!!!

Milly's reply: Hi David, thanks for your enthusiasm about my site, you can come back!
As for your glass - well, I'm amazed. Amazed that you're managing to produce slumped glass in that short time scale, and amazed that it's not broken even before you open your glass kiln.
Stressed it will be. Have a look at my Kiln Firing Schedule on this page to see why I'm amazed. Fired glass - especially glass that has been taken to high temperatures and been asked to 'do' something (slump or fuse, for example) - needs to go through what's called an annealing cycle.
'Annealing' means holding and cooling down the glass at an appropriately slow rate to allow it to safely make the transition back from a viscous liquid to a solid state. All its molecules are in disarray and need time to realign! There's no rushing it. You can't over-anneal something, so it's always better to be conservative.
You don't say how big your pieces are, but even bead makers anneal their work, so it's definitely something you need to research. At 6mm and 10mm it's reasonably thick, too.
Float glass isn't all the same, sadly. Find out from the manufacturers of the glass you're using what their annealing temperature is, and ask them for a couple of generic kiln firing schedules to get you started. Then start experimenting - your kiln will have hot and cold areas, too, so it's all a bit complex. Bet you wished you'd never asked now?! I hope this points you in the right direction.

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