There are thousands of different recipes for white or "milk" glass (at least!). Even a small change in the recipe can alter the behavior of the glass.
Most are soda lime glass, including Bullseye's fusing glass. However, glass for historical neon signs, for example, was often lead glass, which has a very different behavior and melting point.
Do you know where the owner of the local glass shop ordered her glasses? Can her supplier provide more information about its melting point?
In addition, some glasses change after being heated, so three firings would accentuate that.
The key is testing your glass with small quantities before you melt your finished piece.
I'm sending a couple scans of pages from books that deal with opacifiers and fusing problems.
1) Bray, Charles. Ceramic and Glass: a Basic Technology. Sheffield: Society of Glass Technology, 2000. (attached)
2) Walker, Brad. Contemporary Fused Glass. Clemmons, NC" Four Corners Intern'l, 2010.
3) Fenton, Dan. "Fusing Problem Solving" 1993.
They are too large to attach to this message, so I will send them to you via WeTransfer.
For more information about dealing with devitrification and other potential problems, I attached our bibliography.
If you wish to obtain copies of library items, please contact your local library. The Rakow Research Library is a member of the Online Computer Library Center (OCLC). Your library can contact us through OCLC or they may send an Interlibrary Loan form to us by e-mail. We send copies of articles and will also loan certain books if a second copy is available in the library collection. Books are loaned for a four week period. For further information, please see our web page: https://www.cmog.org/research/library/about/interlibrary-loan.
If you would like additional information, please let me know.
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