I bought 2 vintage PYREX measuring cups with green text from a thrift store, One large 1 liter and a smaller half liter measuring cup.
I read online that if it says PYREX in all caps then it's made from Borosilicate glass and that if it's lowercase pyrex then it's made from tempered soda lime glass.
What's unusual is that the measuring cups have different color tints to them. the larger one has a more bluish tint to the glass and the smaller one is clearer and more green.
On Wikipedia it's saying that the more bluish one is made of soda lime, but how can that be since it was made before 1998 and says PYREX?
https://en.wikipedia.org/wiki/Pyrex#/media/File:Pyrex_and_PYREX.jpg
Does anyone know what's going on or how I can tell which glass is used in these measuring cups?
Best Answer
The destructive test for borosilicate or not is as easy as fill with boiling water, wait a while for it to heat up, dump the boiling water, fill with ice. If it breaks, it's probably not borosilicate.
On the other hand, any tint is a probable indicator that it's NOT borosilicate, which has a very flat transmission characteristic in the visible range.
If you really want to get into the nitty gritty you can mix up some refractive index matching fluid, submerse the measuring cups, and see if they "disappear" - which seems to be a common means of testing for borosilicate, as it has a different index of refraction from quartz or soda-lime glass.
You can use a known borosilicate to help get the solution correct.
http://gr5.org/index_of_refraction/