Glass compatibility testing should be carried out if you are in doubt regarding the correct Coe of your fusing glass.
Fusing incompatible pieces of glass can lead to breakages and cracks.
Glass from various makers can have differing Coefficients Of Expansion (COE) which means the glasses will expand and contract at different rates if fused together. This will produce internal stresses in the glass leading to cracks and breakages.
Glasses with widely different COE's are said to be incompatible and should not be fused together. It is generally not possible to tell the difference between glasses just bv sight.
Kiln fused glass can be tested by placing the fused glass between two pieces of polarized film, stresses in the glass will appear as a halo around the edge of the fused glass.
To test the glass you will require two pieces of polarized film and a light source. You will need to cut squares from the unknown glass and fuse them to a base glass of which you know the correct coe.
Fuse these pieces in your glass fusing kiln using your normal fusing schedule.Once the glass has been fired and cooled, place it between the film and hold it over the light source and check to see if a halo appears.
If a halo appears you will know that the glasses are not compatible and should not be fused together.
The following video contains a very good demonstration of compatibility testing from Kaiser-Lee who also have a polarized test card for sale.