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Compatibility Testing

This is part two of a multi-part tip on compatibility testing.  Click here to go to part one.

To conduct the test, you'll need to fuse one or more small squares of the glass to be tested onto a base glass.  In most cases the base glass selected is a clear glass of known origin (such as the clear from Bullseye's or Spectrum's glass lines). 

Begin by cutting a strip of the base glass around 1 1/2" (38mm) 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 in the kiln, then anneal and cool.

Once the test strip has been fused, sandwich it between two pieces of polarized film and hold it over your 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, then the glass is compatible with the base glass.   Greater amounts of incompatibility will be as shown by more and more light showing through the polarized film; this means that the glass is probably not compatible and is likely to crack with use.  See the photos below for an example.

Compatibility test prior to firing. The clear base glass is Bullseye tested compatible; the other glasses are (left to right) Spectrum System 96, clear float glass, Bullseye, and an unidentified art glass. 

Compatibility test after firing. Note that all four glasses appear compatible and have fused into the base layer.  Note also that while this size strip can work, you'll get more consistent results from using a slightly larger base strip.

The same glass viewed through polarized film. Note the halo around the Spectrum glass at far left and even brighter halo around the orange art glass at far right. The clear float glass has a much smaller halo and may be fusible with tested compatible Bullseye, while the Bullseye glass in third position has no halo and (as would be expected) is definitely compatible with the tested compatible Bullseye in the base layer. 

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Copyright 2007 Brad Walker.  All rights reserved.

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Copyright 2005-2006 by M. Bradley Walker.  All rights reserved.

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