As a consumer, it is important to note that `pyrex` and `PYREX` are not the same thing [0]:
"Corning used borosilicate to produce all Pyrex products. However, the company that purchased the cookware products switched to soda-lime glass, adopting the name pyrex (spelled with all lowercase letters).
Corning continued to make its lab tools with borosilicate, dubbing these products to be PYREX (spelled with all uppercase letters)."
All of the glass examples in TFA are borosilicate all-caps PYREX, while most of what you can buy today in the store is lowercase pyrex (Europe is an exception where the all-caps variety can be found).
> All of the glass examples in TFA are borosilicate all-caps PYREX, while most of what you can buy today in the store is lowercase pyrex (Europe is an exception where the all-caps variety can be found).
Using all-lower or all-upper case is not a good indication of the type of glass used.
A recent video (Sep 2025) from the I Want to Cook channel, "PYREX vs pyrex -- What's The Difference & Why It Matters", went into the history of this:
Specifically, he found the following at the Corning Museum of Glass site:
> The short answer is that the change from Pyrex trademark upper to lower case signified a re-branding of the trademark Pyrex® in the late 1970s but is not a conclusive way to determine, historically, what type of glass formulation the product is made from.
So if someone goes to thrift stores looking for borosilicate via the 'old way' of spelling the name, there is no guarantee it will be borosilicate.
See 16m11 of the video for advice if you want borosilicate glass: in Europe, it is all borosilicate; in US, import it yourself, look for "Made in France", or use another manufacturer (e.g., Oxo names the glass they use).
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.
> Europe is an exception where the all-caps variety can be found
'can be found' is too weak, it just is all-caps & borosilicate. (Perhaps imported lower-case stuff 'can be found', but it's not the norm at all, you won't see it in shops, and - I just checked - I haven't been suckered with it on Amazon either.)
"Corning used borosilicate to produce all Pyrex products. However, the company that purchased the cookware products switched to soda-lime glass, adopting the name pyrex (spelled with all lowercase letters).
Corning continued to make its lab tools with borosilicate, dubbing these products to be PYREX (spelled with all uppercase letters)."
All of the glass examples in TFA are borosilicate all-caps PYREX, while most of what you can buy today in the store is lowercase pyrex (Europe is an exception where the all-caps variety can be found).
0: https://www.corning.com/worldwide/en/products/life-sciences/...