Warm Glass
COMPATIBILITY TESTING
Gross glass incompatibility will show up as small cracks or fracture lines where the two types of glass come together. These cracks will likely increase with time and may eventually result in a complete separation. The kind of incompatibility that shows up as small cracks is seldom a problem because it’s obvious that the glass can’t be used. More insidious incompatibility problems aren’t obvious to the naked eye and need to be discovered through compatibility testing.
There are four main ways to test for compatibility:
Click here to see a photo illustration of a compatibility test.
To perform this test, often used by lampworkers, small pieces of each of two glasses to be tested is heated in a flame and then stretched to form a thin rod. Once the rod cools, it is examined to determine if it is straight, indicating compatibility between the two glasses used.
While this test may be accurate for lampworking, it is not recommended for fusing. It requires a level of expertise to accurately pull the threads. More importantly, it doesn’t replicate the fusing process and may give false results. Use one of the other recommended tests instead.
This is more of a general test for stress than a true compatibility test, but it has the virtue of being very easy to perform and not requiring any special equipment.
After you’ve fused the different glasses together, and the piece has cooled to room temperature, you simply put it in the freezer for at least a day. Then you remove it and let it return to room temperature. If it hasn’t cracked, it’s probably stress-free enough to use.
One variant on this test is to remove it from the freezer and place it in the dishwasher and let it run through the cleaning cycle. Any piece of glass that can survive the stress of freezing followed by the stress of a hot dishwasher is almost certainly stable enough to take on the real world.
This test requires that you obtain two polarized lenses. You can buy a device made for this purpose or you can easily make your own. Just get two pieces of polarized film, each about 6" square. Polarized film is available from some glass suppliers (C & R Loo, for example) and from companies like American Science and Scientific.
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 slight halo, then the glass can be assumed to be compatible. Greater amounts of incompatibility (as shown by more and more light) mean that the piece may be useable, but that it’s more likely to crack with use.
This test builds on the polarized lens test already described and is generally considered the most accurate of all the tests. It requires that a clear glass (called a "base" glass) be used as a control glass against which to test all other glass pieces.
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. Anneal and cool.
Place the fused test strip between the polarized lenses (or film) as previously described. The degree to which a halo or bright spots appear is directly related to the degree of incompatibility. No halo means no stress.
If you save strips of your base glass, you can do subsequent tests with different test glass squares and know that all of the compatible glasses are compatible with each other.
PHOTO ILLUSTRATION OF GLASS COMPATIBILITY TEST
Compatibility test prior to firing. The clear base glass is Bullseye; the other glasses are (left to right) Spectrum, float glass, Bullseye, unidentified art glass.
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Compatibility test after firing. Note that all four glasses appear compatible and have fused into the base layer. |
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The same glass viewed through polarized film. Note halo around Spectrum glass at far left and even brighter halo around orange art glass at far right. The window glass exhibits minimal incompatibility and may be fusable with tested compatible Bullseye, while the Bullseye glass has no halo and (as would be expected) is compatible with the tested compatible Bullseye in the base layer. |
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