The process of testing for tempered glass isn't hard to do. A car or truck windshield out in the sun and a set of polaroid sunglasses are all that you need.
Start by putting the car in a location where you can get a nice glancing reflection of the sun off of the windshield. This makes the sunlight very polarized so you have the lightsource of polarized light.
Next grab the sunglasses and verify that they do change the brightness of the reflection off of the windshield when the sunglasses are rotated about to do the polarization dimming trick. If so then you can then put the glass between and look at the amount of dimming that the glass does to the light. Try to get one area to dim fully and see if the rest also dims to match at the same angle. We're interested in the angle at which the max. dimming occurs, not that actual amount of dimming as different parts of the glass will be seeing different things behind the glass.
You will probably see polka dots or lines in the glass as you go through the dimming and those features will have a different angle at which they go to max. dimmness.
If you don't see any difrerence then the glass is well annealed and you can use it for making mirrors with no troubles.

• Testing for tempered glass - jkoptic - November 16, 2003 - 03:38 UTC