These procedures are based on the observation that glasses compatible with the base glass are compatible with each other. This means that you can test opaque colours’ compatibilities with each other by testing each of them on clear strips.
These tests must be combined with an annealing test. This conists of putting two pieces from the same sheet of glass together - so you know they are compatible - and firing them along with your compatibility test.
Viewing the results of your annealing through the polarised filters shows whether there is stress left in your annealing. If there is, the compatibility tests are inconlusive as there is no difference in appearance of stress whether from incompatibility or from inadequate annealing. Once you have the annealing right, you can then interpret the compatibility tests done at the same time.
Cut a strip of base glass ca 25mm wide and as long as convenient for you or your kiln.
Cut clear glass squares of 25mm to separate the colours.
Cut 25mm squares of the colours to be tested
Start with a clear square at one end of the clear strip and alternate colours and clear along the strip finishing with a clear square.
Add a stack of two layers of clear to the kiln before firing. This is to test for adequate annealing. If the annealing is inadequate, then the whole test is invalid.
Test the result with polarising filters. Start with the clear annealing test square. If no stress is apparent, go to the test strip. But if stress is apparent in the annealing test, look to your annealing schedule as something needs to change. Usually the requirement is a combination of a longer soak at the annealing temperature and a slower annealing cool.
To test for compatibility, look carefully for little bits of light in the clear glass surrounding the colour. These are indications of stress – the more light or the bigger the halo, the greater the stress. Really extreme stress appears to be similar to a rainbow, although without the full spectrum.
You can use this test to determine if you annealing is satisfactory for larger pieces. In this case you should use at least 100mm squares. Stack them to the height of your planned project and dam them with fibre board or other refractory materials to prevent spread. Fire to full fuse and anneal. When cool check for stresses.
The tile method looks at compressive factors too.
Cut a 100mm clear tile
Cut two strips of glass 25mm wide and 100mm long for each test
Cut two rectangles of 25 by 50mm of the same glass for the two remaining sides
Cut a square of 50mm for the centre. The glass in the middle is normally the test glass. To be very certain of what has happened you can do the reverse lay up at the same time. You put coloured glass around the outside, but in this case the inside needs to be clear or transparent. At least one element needs to be transparent enough to view the stress patterns, if any. So you could have a clear middle and black exterior, and vice versa.
This test is a more time consuming process and you may wish to use it only for larger projects.
Also look at the use of polarising filters