Stress in fused glass can be caused by fusing glasses of different coefficients of expansion or by not allowing your piece to cool slowly enough, especially through it's strain point. Most glass used for fusing needs to be annealed at a temperature somewhere between 900 and 1000 degrees Farenheit. There's no magical process happening during this period, what you're really doing is giving the glass time so that it all cools at the same rate. Glass is a pretty good insulator, meaning the glass in the center will stay hot longer than the glass on the surface.
If the surface cools first and contracts over the still hot glass in the center, you will then have stress when that glass in the center cools, pulling on the glass at the surface. If it all cools at the same rate, it all contracts at the same time, so there's no stress to pull it apart.
The easiest and cheapest way to check for stress in your piece if to get 2 sheets of polarizing film and a light box. Your local glass shop should be able to get some film for you for between $5 and $10. Polarizing film is a filter that blocks light in one axis.
Here's what you do:
1. Place 1 sheet of the polarizing film on the lightbox (or you can use a sunny window)
2. Put your glass on top of the film.
3. Put the 2nd sheet of film on top of your glass.
4. Slowly rotate the top piece of film, you will see it getting darker or lighter as you turn it.
5. If you have stress in your glass, it should show up as little white clouds or streaks.
6. If you know the COEs are the same, you can fix it by heating it up to 1000 degrees and annealing
properly. If the COE's are different, it's game over for this piece.
Here's what stress looks like through the film:
This is Wissmach English Muffle fused to Spectrum FS100 clear glass. The COE's do not match.
With this technique you can experiment with different glasses, maybe find some of that older Spectrum glass works just fine for your fusing needs.
:)
Don
