How can I determine whether my borosilicate glass is fake or not?
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.
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.