This is part two of a
multi-part tip on compatibility testing. Click here to go to
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
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.
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
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
Copyright 2007 Brad Walker.
All rights reserved.