[jzelinskie]

I watched a video recently that dove deep into this as well[1]. It turns out there's not an easy way to figure out if it's borosilicate other than if it has "made in France" on it or if you know it was purchased in Europe. AFAICT, you can't really buy borosilicate Pyrex in the US.

The video does also show off a cool "mineral oil test" to tell the difference, but probably is only effective if you had something to compare it against.

My takeaway though was that I need to thrift some Corningware, though!

1: https://youtube.com/watch?v=2DKasz4xFC0

[foofoo12]

Mineral oil test bookmark: https://youtu.be/2DKasz4xFC0?feature=shared&t=873

It's a non destructive test. Quote from the video (with funny youtube transcription spelling errors):

"Without getting too technical, the gist is if you put the mineral oil in a vessel made of boroilic and then dip another glass made of boroilicate into it, that glass will seem to disappear while others will not. So I filled a vintage what I think is made of borosilicate Pyrex vessel with mineral oil. Then it dipped in a vintage what I think is made of boro silicut loaf pan and it seemed to disappear right before my eyes. Eureka I thought the experiment works well until I dipped a new Pyrex piece lowercase that I know is not made of borocyic and it disappeared too. Once again, another spokesman at the Cording Museum of Glass that I reached out to said that even the mineral test isn't a sure thing. According to Brady Spalling, he says in order for glass to quote unquote disappear in oil, the glass being submerged must have a similar refractory index, which allows light to pass through both without significantly bending. Mineral oil and borosyic do have similar refractory indexes. So what you've heard is correct. This method is often used to quickly ascertain whether a glass object is borosyicate. However, variability of glass recipes makes it difficult to rely solely upon this method. In short, it may work and it may not."

[s0rce]

I'm surprised that there is enough difference between borosilicate glass and soda lime glass refractive index. They are also both slightly mis-matched to mineral oil but I guess good enough for a visual check. Appears its not consistently reliable.

Also, its refractive index not refractory.

[JKCalhoun]

In the video he claims it does not work in his case — but I saw a very clear (ha ha) difference between the two. You could easily see the edges of the non-boro glassware.

[fmajid]

Soda-lime glass has a greenish hue (look at it sideways). Borosilicate is blueish.

[KingMob]

If you'd read the article, they literally address this. They said the colors are due to unrelated impurities, and are not a foolproof guide.

[s0rce]

This depends on the source of the raw materials, the green is typically from iron and you can have higher purity soda lime glass that is much more clear. I didn't realize there was a visible blue tint in consumer borosilicate glass, I wonder what causes this.

[zahlman]

I've recently been getting this exact video suggested to me frequently in the sidebar, and I have no idea why.