Just a quick post about the devitrification I have been experiencing on my fused glass work
I have been having quite a bit of trouble with devitrification on a number of items, not just cabochons. My first thought was to blame the firing schedule, then specific colours of glass.
In an effort to work this out I I talked to a much more experienced artist and found out something that was a little disappointing.

When we started our studio we purchased a large lot of Bullseye 90 fusible glass from a local glass business. As it was at a great price we were very happy with the purchase and probably didn’t do a great deal of research into its suitability for our needs. Took it all on trust, so to speak.
Well, I have now found out, and it has been verified by a couple of other artists, that Bullseye glass is prone to devitrification. I have been advised that it should always be capped, sprayed with anti-devitrification solution and crash cooled. Wow, does that explain a few things.
We are still very happy with the purchase of glass, just a little disappointed in myself that I didn’t do my research, and in the person selling it to us for not informing us of the issue. We did explain what we were using it for and also our lack of knowledge, so I thought he would have done the right thing. He did say he was focusing on COE 96 glass and now I understand why.
So a word of warning if you are starting out. Do your research, especially before committing to large purchases of materials that will be with you for a long time.
I was glad to see your followup blog entry explaining that you found one way to prevent devit on fired glass. http://rocketroseart.com/2013/05/04/the-importance-of-clean-glass/
The learning continues, though slowly sometimes. I still find devit an issue, regardless of how carefully I clean etc. The opals are the main issue but I hope to master it eventually.