How to check for tempered glass

[scherzo]

I just tried this..

I had a pair (actually many pairs) of 3D glasses from movies that my kids go to. I know that they are polarized because that is how the 3D in theatre works.. anyhow..

It only seemed to work if I looked through the glasses backwards.

I had a pane of glass from an old french door and it clearly states TEMPERED. I also used a piece of scrap glass from my sump building attempt..

The NON tempered glass did turn completely black.. uniformly.
With the tempered glass it was hard to tell but you could clearly see that it didn't turn black and it was really evident along the edges where it looked like some kind of "stress" on the glass. Like a computer simulation calculating pressure or stress on an object.

Pretty cool! I think that this works best in the dark and would be very difficult to try on a tank full of water...