Polarized Filters (or a Polariscope) can be used to detect internal stress in glass. The plastic sheets have grooves that filter the light in one direction. To use them, place one sheet on a light source, add your glass, and place the second sheet on top, with the grooves at a right angle to the bottom sheet.
Internal stress can be caused by a few things, annealing issues, incompatibility, and tempering. Here are some examples:
Here are two mixed bottle glass fused samples, the top sample shows not incompatbility issues, the bottom sample shows flares of light around the edges of the fused pieces, indicating the bottles are not compatible with each other.
Polarized film can also be used to test for unmarked tempered glass. Here’s a piece of plain float glass:
and a piece of tempered glass (and a nice reflection of my light fixture):
There’s a nice aurora effect across the glass through the filter, and the edges glow nicely. But, my most favorite and, I think, coolest use for polarized film is to identify tempered glass vintage plates, here’s a not tempered plate:
Although the sloped parts of the pattern pick up light, there isn’t an overall aurora effect. Now, check this out:
How cool is that? The pattern shows up slightly, and the stress patterns are super obvious.
This one is very similar to the one I have. I bought a 12″ x 17″ sheet and cut it in half.