I regard this as a prime example of customer betrayal. The company set up the brand to promote a specific product(borosilicate glass cookware) After many years of this specific promotion, to the point the pyrex was interchangeable with borosilicate glass in customers minds. That brand stopped being made of borosilicate glass resulting in a product that looked exactly the same but with distinctly different mechanical properties.
Final thoughts: I don't think PYREX vs pyrex painted on is enough of a differentiator and my understanding(as the parent post pointed out) both types of glass are used with the lowercase trademark. I think glass cookware should have a standardized indicator stamped into the glass itself as it is very tricky to tell otherwise.
There is the mineral oil IOR test, but... the IOR ranges of the two glass formula actually overlap so it is very tricky to tell for sure. There is the heat shock test, but... that will destroy the item if it is tempered glass. I suspect you could use a polarized light test to identify if it is tempered glass. but none if the threads I have read on the subject have mentioned it probably because it requires specialized equipment.
I've seen the new pyrex in stores and it's readily apparent just how green it is. My old pyrex is clear. But apparently that has nothing to do with the type glass used, both kinds can be green.
Final thoughts: I don't think PYREX vs pyrex painted on is enough of a differentiator and my understanding(as the parent post pointed out) both types of glass are used with the lowercase trademark. I think glass cookware should have a standardized indicator stamped into the glass itself as it is very tricky to tell otherwise.
There is the mineral oil IOR test, but... the IOR ranges of the two glass formula actually overlap so it is very tricky to tell for sure. There is the heat shock test, but... that will destroy the item if it is tempered glass. I suspect you could use a polarized light test to identify if it is tempered glass. but none if the threads I have read on the subject have mentioned it probably because it requires specialized equipment.