The thread test is a relatively easy way to check glass compatibility. Unlike the "stress-o-meter" (i.e. dual polarized lenses), the thread test can be used even when both glasses are opaque.
To conduct a thread test, use a torch to fuse together roughly equal amounts of each glass being tested. You only need a small sample of each and you want the glass fused together but not mixed.
Keeping the glass soft in the torch, use a tweezers to pull a single thread from where the two glasses meet so that the thread has a different glass on each side. Try to keep the thread as straight as possible when pulling.
If the glasses are compatible, both will contract at equal rates as the thread cools and the thread will remain straight. If the glasses are not compatible, one side of thread will have expanded more during the heating and will therefore need to contract more during the cooling. This will result in the thread curving towards the glass with the higher expansion rate.
It is worth noting that all compatibility tests have their relative strengths and weaknesses. While the thread pull test can help identify different expansion rates, these alone are not an absolute predictor of compatibility. It works best when comparing glasses within families -- for example when comparing two soda-lime glasses (most common glass) or when comparing two borosilicate glasses. It becomes less valid when comparing, for example, a soda lime glass and a lead crystal.
