Pyrex

[GLC]

While it might seem attractive to try to acquire some "old" Pyrex in hopes of getting borosilicate, there's no reason to expect that anyone under the age of about 45 here (barring real collectors) can be sure that they've ever seen a piece of borosilicate Pyrex cookware, even in momma's cupboard. Perhaps someone somewhere actually knows which particular pieces of which post-1940 years are borosilicate, but if they are out there, they're not writing about it. I have doubts that the makers could tell you, at least not without some deep digging in the archives.

If Corning began phasing in soda-lime Pyrex in the 1940's, you'd have to know how to date pieces to pre-1940 to be sure. The fact that is years old and hasn't failed doesn't mean it's borosilicate. One would be hard put to tell the difference without very sensitive instruments or destructive testing. The two glasses have refractive indices in the same range, and the difference in density is very subtle. Soda-lime glass is fractionally harder. Spectral transmissions are almost identical.

I suspect someone reported their Pyrex failing, and someone discovered that there had been a change from borosilicate to soda-lime, and they didn't realize the change was long ago. Borosilicate Pyrex may fail above 450-degrees, and I imagine there have been plenty of failures from abuse that were just put down to the user having messed up and mishandled it.

Who has a piece of Pyrex they absolutely know is borosilicate, and how do you know? (Documented date before 1940 is one way, of course.)