For years we've looked for a good easy test to tell if glass is tempered or not. Here's a way to tell using a flat screen monitor and polarized sun glasses.
If you have a polarized light source, such as a flat screen monitor, and a polarizing filter, such as sun glasses, and if you hold the tempered glass in front of the monitor, then all kinds of strange patterns show up, apparently from the tempering furnance.
If the glass is not tempered, then no change in the light is seen. It's either black or clear. But with tempered glass the effect is plain as day.
The shadows in the glass are eerie to say the least. I am pretty sure they are ghosts of dead people. I found one that looked a lot like Nixon.

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