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Mixing Green Beans with Brown

Part Five of a Five Part Tutorial on Fusing and Compatibility


Click here to go to the first part of this article.

To test for compatibility, you must fully fuse small pieces of the glass you'd like to test.  The simplest, but not necessarily to best, way to do this is to place one scrap of a glass you want to use on top of a slightly larger scrap of another glass, and then fully fuse the two pieces in your kiln. 

This will work, but a better way is to use a base glass of a known COE, and test your scraps to see if they work with the known glass.  To do this, cut a strip of the base glass 1 1/2" in width and several inches long. For each other glass you wish to test, cut a small square (about 1/2" by 1/2"). Place the test squares one-inch apart on the base glass. Fuse flat to around 1500 degrees Fahrenheit, then anneal and cool.

To conduct the test, you simply sandwich the already-fused piece of glass between the two pieces of polarized film and hold it over a light source (such as a light table or a flashlight). Rotate the lenses so that they are at right angles to each other and as little light shines through as possible. Now check the edges where the different glasses come together for a white glow. The brightness of this glow will determine the severity of the stress and the degree of incompatibility.

If there's no glow, or only a very slight halo, then the glass can be assumed to be compatible. Greater amounts of incompatibility (as shown by more and more light coming through the polarized film) means that the piece may be useable, but that it's more likely to crack with use.  Remember:  where there is light, there is stress.  And stress is what causes the cracks.

There are a couple of limitations to compatibility testing.  The most significant is that you can't use this method to test with two opaque pieces of glass, you have to have some transparent so that the light can get through.   That's why testing with a base clear glass is highly recommended.

A second limitation is that it's unrealistic to expect to test every single scrap of glass you have sitting around.  Instead, you have to test the larger pieces, or the colors you most want to use in a fused project.  Save the other scraps for non-fused projects.

And be careful when you mix green beans with brown.  You don't always get what you want.


Copyright 2005 Brad Walker.  All rights reserved.

This article was originally written in 1999 and was one of a series that became the basis for the Warm Glass website.  It has also been published in Common Ground: Glass, the newsletter of the International Guild of Glass Artists

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