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A polariscope is just two pieces of something fairly clear (so that light passes through easily) and polarized.

Polarized means that light can only pass through the polarized lens or material in one direction. If the light is coming in horizontally you can rotate the lens and it will gradually get darker as it blocks more and more of the horizontal light.

Sunglasses may be the best way to describe what happens with polarized lenses.

Light usually comes at us from all directions but...if light reflects off a horizontal surface like a road or the water, the light comes at us in a mostly horizontal direction.

A polarized lens blocks the light coming from all but one direction. In sunglasses, the lenses are put into the frames in such a way that they would block the horizontal light. The lenses will still allow light coming at them from a vertical direction to be allowed through the lens.

The reason the polariscope works is that stress in the glass will bend the light coming through the glass, changing its direction and allowing the light to "get through" the second lens, even when the second lens has been rotated around until it's dark from blocking the light. The stress shows up as lighted spots, or rainbows if it's a lot of stress.

To check glass for stress, you'll need two polarized lenses and a light to shine through them.

The most common things that are see-through, polarized, and inexpensive...or that you already may have lying around, are...

The way you use the two polarized lenses is to have light passing through one polarized lens, then through the glass you're testing and then through what we'll call the top lens.

You rotate the top lens until it's blocking most of the light and then watch for funny patterns of light in the glass. They may be rainbows or they may be black and white lines or patterns. One of the most common patterns is an "iron cross". You'll see this in the pics below.

If it's something like tempered you'll see a LOT of patterns, usually rainbow, through the lens. The rainbows mean LOTS of stress.

This is the best way to confirm if do you have tempered glass. Glass is tempered by creating lots of compressive stress on the outer surface. This is done by spraying the glass with compressed air when it's hot. If you want more tech detail on how that works and why have a look at the glass compatibility page under the annealing info about what happens when a piece of glass is cooled quickly on the outside. The same problem that would usually cause a fail in a glass piece can be used to strengthen the glass if it's done just right.

Using a cell phone with a polarized lens as a polariscope, you would put the glass on top of the cellphone with the screen lit up on the phone. That's your light, your bottom polarizing lens, and your glass to be tested. Rotate whatever you're using as the second or top lens until it's dark and you see stress...or not.

I found a video on you tube showing the cell phone polariscope in action testing for tempered glass:

When you do this test you'll almost always see some stress. The trick is knowing how much stress is acceptable for what you'll be doing with the glass.

Pictures of the Homemade Polariscope in Action

Some beads and the strip of glass used on the compatibility testing page. The strip hasn't been annealed where it was heated and cooled for the test so we'll see some stress. The top lens is not over these yet. Both lenses are the screw on polarizing filters for the front of the lenses on SLR (35mm)caneras. The whole setup is sitting on a light box like the ones dentists use to look at x rays.

Starting to rotate the top lens so it's blocking the light. I put one bead on there that has no stress for comparison.

beads and strip on bottom lens

More rotation. More light being blocked. Stress is really starting to show as light is being bent in a direction where it will pass through the top lens.

beads and strip on bottom lens

More rotation on the top lens. Look at the strip where it was heated up on the end.

beads and strip on bottom lens

Rotated to the darkest point. Stress is really showing. beads and strip on bottom lens