Tools For The Glass Artist

Main page

Our other sites

Contact us

Polarized COE Stress Tester

It is easy to check you fused glass jewelry and artwork for internal stress caused by incorrect COE (coefficient of expansion). Just place your glass piece between the two pieces of polarized film and hold up to the light.

You will be amazed at how easy it is to spot problems. You may also be amazed that the glass you thought was the right COE turns out to be something else.

What is COE and why does it matter?

Glass (and just about everything else, for that matter) expands when it is heated and shrinks when it is cooled. The rate of expansion is called the coefficient of expansion (COE). A 1 inch long piece of 90 COE glass expands 0.0000090" for each degree Centigrade  increase in temperature (about 1.8 degrees Fahrenheit). It also shrinks the same amount when cooled.

If glass with different COEs are mixed, the glass will shrink different amount as it cools resulting in internal stresses. Sometimes these stresses are enough that the piece will crack immediately. Sometimes it will wait weeks to crack. I made a fused glass paperweight out of odds and ends of glass. It set on my desk for over a month before it cracked. When I checked it with the polarized film it was obvious that I had mixed glass with different COEs.

How do you check for COE stresses?

Using 2 pieces of polarized film it is very easy to find mismatched COEs.

1. Fuse a test piece of the glass. Use a piece of clear glass that you know is the right COE. On top of this piece place small pieces of the unknown glass. Fuse in your oven just like you would do for a small piece of jewelry.

2. place the piece of glass between the 2 pieces of polarized film and hold up to the light.

3. Areas of stress in the glass will show light. Areas of no stress will be black. You may need to rotate the top piece of polarized film 90 degrees.

The above picture shows the test strip of glass with one piece of polarized film behind the glass. I used a fluorescent light to take these photos, but any lighting will work.

There are 5 samples of glass on top of a 90 COE clear glass base in this test strip. From left to right: a piece of the same 90 COE clear glass (barely visible), black glass from the same supplier, dark green glass sold at 90 COE, a piece of picture frame glass, a piece a wine bottle.

The above picture shows the test strip of glass between the two pieces of polarized film. The areas of no stress show up dark. A areas of stress in the glass show white.

The left 2 samples show no stress (no light showing through)

The sample 3 (the glass that was sold as 90 COE) shows a lot of stress. I am not able to tell from this test what the COE is of the unknown glass is, but I know now that it is not 90 COE.

Samples 4 and 5 (the picture frame glass, and the wine bottle) show stress, but this was expected.

This photo is just showing the top piece of polarized film slid half way off the test strip. This gives you an idea of how well the areas of stress show up.

These photos where not PhotoShopped to exaggerate the white stress areas. This polarized film will show the stress in the glass very clearly. You will not have to squint and guess what you are seeing. If the glass is the wrong COE, this polarized film will show it very clearly. Your satisfaction is guaranteed or your money back.


Polarized COE Stress Tester. $11.00

2 pieces of high quality polarized film 3 1/4" x 3 1/4" .03" thick. Includes free shipping in the US.




  graphics from templatemo.com