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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?

  • I also want to add that I have a vintage PYREX dutch oven from France that also has a bluish tint and supposedly all European PYREX is Borosilicate, this is very confusing : ( – Neil Aug 14 '16 at 21:53
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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/

  • I've cracked borosilicate when making Jello as a kid -- made the mistake of using all hot water, then put it (9x13) in the fridge. Made a real mess. I'm not voting this up, as I broke the 'new' pyrex pan that I bought a few years back (not realizing it wasn't the same mix) ... just from pre-heating it w/ some oil in the oven to make a yorkshire pudding ... it shattered all over the place. I wouldn't recommend anything that risks doing that. – Joe Aug 14 '16 at 23:54
  • Maybe your container was too thick... even with chemistry glassware (where brosilicate is the norm), there is a reason that containers expected to be heated and cooled are made as thin as possible. – rackandboneman Aug 15 '16 at 9:50
  • It looks like borosilicate's refractive index falls in the range of reasonable values for normal glass, so that might not work too well. Though a saturated solution of sugar at room temp (a syrup that's has time to precipitate) should match borosilicate quite well. – Chris H Sep 14 '16 at 14:28
  • Sorry, @ChrisH, but that's simply wrong. The only other "glass" that falls near borosilicate is fused silica. Crown (aka soda-lime) is above 1.5, while 7740 pyrex is 1.474. physics.info/refraction (and many others...) – Ecnerwal Sep 15 '16 at 15:19
  • OK, you've found cooking pyrex itself. Optical borosilicate is higher (e.g. N-BK7 at 1.51). – Chris H Sep 15 '16 at 17:59
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I think the big question is what is considered vintage? I recently purchased several "vintage" Pyrex measuring cups that have no metric markings. I was thinking they must be fairly old. Marked on the cups, however, they say microwave safe. Microwaves were commonplace in the 1970s. Not so vintage in my book.

  • I think for this question vintage is in the difference of pyrex vs. PYREX and soda lime glass vs. borosilicate. – Neil Sep 14 '16 at 20:00
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As a chemist I don't think that the difference between the two glass compositions would make any difference to you as a cook.

The question is my mind is "vintage." It used to be that the handle on a measuring cup was connected at both ends. How the liquid heats in a microwave causes stresses on the glass. If you put a measuring cup where both sides of the handle are connected in a microwave then it is much more likely to break since a microwave doesn't heat the liquid evenly.

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    There have been a lot of consumer complaints about the new Pyrex formulation, with unexpected shattering. Consumer Reports did a big study on it a few years back, interviewing a couple hundred people who had incidents and ultimately recommending a government investigation after their own experience showed major differences in shattering frequency for "old" Pyrex vs. new in cooking situations. Most of these incidents are likely due to people not following the directions, but the Consumer Product Safety Commission has logged at least several hundred where there was apparently no misuse. – Athanasius Sep 15 '16 at 3:28
  • @Athanasius - Thanks for the additional information. "Pyrex" of course never meant that the glass container was indestructible. Truthfully it doesn't surprise me that a lot of measuring cups get broken. With our 24/7 world wide news coverage I'd wonder if reporting broken measuring cups somehow caught on as something to report. – MaxW Sep 15 '16 at 3:46
  • To be fair, I think most of the issues occur with glass baking dishes rather than measuring cups, but there are times when people also want to put a hot liquid into a measuring cup or something too. I also think the problem is created because of previous expectations -- consumers did "bad" stuff with the old borosilicate products and got away with it, but when they try it with the new stuff, it shatters a lot more frequently. – Athanasius Sep 15 '16 at 4:20

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