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Is there actually a way to determine whether the sheet of glass that one has received from a Chinese supplier on eBay is actually a sheet of Borosilicate glass, and not just a piece of normal glass intended for glazing purposes?

Are there any laminations, or markings/features, visible under a certain light or if the sheet is viewed at a particular angle?

I ask because most of the 200 x 300 mm sheets on eBay are priced at around £16 - £26, but there are a few priced at just £10, which still claim to be toughened glass, although they do not mention the word Borosilicate, such as this one, 300x200x3mm 3D Printer Heated Bed Toughened Glass Clear Build Plate UK STOCK.

To paraphrase, "Usually if things seem to good to be true, then they are probably fake." However, I wondered whether it was worth a punt as it is only £10, and then if I would be able to verify its veracity once it was in my hands.

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    $\begingroup$ Good question, I worked a while in labs. I think borosilicate glass was not looking greenish from the side (but I am not 100% sure). Other than that, the density is lower than of regular glass. $\endgroup$ – dgrat Oct 16 '18 at 13:31
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Google turned up this thread: http://arstechnica.com/civis/viewtopic.php?t=96214

There don't seem to be any easy, definitive tests. You have two main options, both of which seem to require a fairly high level of laboratory skill (but nothing fancy in terms of equipment).

The refractive index of Borosilicate glass is very similar to vegetable oil (implying no reflection from a boundary if you have a pool of oil on the glass).

The density of the glass can be measured too, using a water bath. This should also be able to give a fairly accurate answer.

Stress testing seems the most reliable way to determine if you have a 'good' sample, but may be more expensive.

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All you have to do is submerge your sample of glass into mineral oil. If it seems to disappear, it's borosilicate.

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    $\begingroup$ Hi and welcome to SE.3DP! Do you have a link, image or video to demonstrate this visual effect? In addition, please keep comments friendly... if you disagree with someone else's answer there is no need to denigrate it. You may also wish to edit your answer, and fix the typos. Thanks :-) $\endgroup$ – Greenonline Oct 10 '18 at 10:40
  • $\begingroup$ exploratorium.edu/snacks/disappearing-glass-rods $\endgroup$ – user77232 Nov 23 at 14:05
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"toughened" glass,

Lead, cadmium and barium would have that effect. It goes without saying that you wouldn't want that kind of glass anywhere near your neighborhood.

Even those items that mention borosilicate glass in the product description may have never seen any boron.

Some studies have found that more than 90% of items sourced from China are fraudulently labeled. There is no enforcement.

While stats say nothing about any particular sample, I'd certainly keep them in mind.

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    $\begingroup$ It might be worth sharing some of said studies - also while it says what else the glass could be, it dosen't really help OP with working out if its borosillicate glass. $\endgroup$ – Journeyman Geek Oct 16 '18 at 7:44
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    $\begingroup$ Hi and welcome to SE.3DP. Can you provide links, sources to back up your claims? It goes without saying that you wouldn't want that kind of glass anywhere near your neighborhood. Pyrex will be found in your kitchen probably. Some studies... what studies? Do you have a link? Your answer would have more worth with sources. Thanks. Also, you aren't really answering my question, but rather questioning the morals behind Borosilicate. I just want to know how to check that it isn't just plain old vanilla glass. $\endgroup$ – Greenonline Oct 16 '18 at 7:47

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