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Testing glass for compatibility

Fusing Compatibility of Glass

When glass changes temperature, it expands and contracts. The rate at which glass changes size is called the coefficient of thermal expansion. If you fuse two glass pieces together and one changes size faster or slower than the other, the fused piece may crack—even several months after fusing.

When different glasses have a close enough coefficient of expansion to fuse successfully, they’re called fusing compatible. Buy glass labeled fusing compatible. Or fuse glass that has been cut from the same sheet, which guarantees compatibility.

Fusing Compatibility Test

1. To test glass for compatibility, fuse small ½” square sample pieces of different glasses onto a larger base piece of clear transparent. It should extend beyond the small sample pieces by half an inch on each side. One of the sample pieces should be cut from the base piece.

2. Heat the glass to a temperature that completely rounds the edges of the small sample pieces.

3. After the glass cools, place a polarizing filter under the glass and another filter over the glass. Look at the glass with light shining through it (hold it over a lamp). Turn one of the filters until the filters are at their darkest.

Results of the Test

If you see a halo around the edges of the small glass samples, this usually means the glass is not compatible. If you see no halo, the glass is fusing compatible.

Why did we include a sample square cut from the base transparent glass? It tests for annealing. A halo around that piece means the glass was not annealed properly. Perform the test again, this time cooling more slowly through the annealing range.

Arnold Howard Paragon Industries, Inc.

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