This is a really old thread, but I know how to test glass, so I thought I would share the technique, it is easy... This test can be used to compare the COE between two samples of glass. It can't tell you the exact coe, only show if there is a difference.
So mixing different glass in a project successfully involves many different factors. COE is the big one, but it is not the only one. How solid your final object is plays a role, so tac fused shards can mix better than the same mix fused into a large solid shape.
COE and viscosity both contribute to compatibility, with COE being the most important.
There is a way to measure COE, but measuring compatibility between two samples of glass is actually easy. To do this you need to fuse a small sample of the darker of the two glasses onto a somewhat larger sample of the most transparent of the two samples. So I take some green glass about an inch and a half across, and a shard of brown bottle glass that is and inch by a half inch. I fire these to either a tac or a full fuse, either works fine. I use a microwave kiln for this because I can see the sample in 30 minutes. This is the only thing a microwave kiln is good for, but it is worth having just for doing these tests...
So once you have a fused sample, you need two polarizing filters (standard linear polarizer, not circular). I bought a pack of 10 sheets of polarizer film for $8 on amazon. Your LCD computer monitor has a polarizer built in already, so you can use that as the first polarizer and the light source.
You orient the polarizers so that they are black, and put your cooled test glass between them. If the glass is at a different COW, you will see a glowing halo on one or both sides of the junction between the two types of glass. Any halo means they are not compatible, but you CAN get away with taking them together as long as they can flex a bit.
The halo is the glass acting like a polarizer, caused by it being bent under pressure from the dissimilar COE. The bent glass will alter the light in a way that it comes through the second polarizer twisted off-axis from the first one.
You can also do this test with polarized sunglasses and a computer monitor with a white page. Turn the glasses till they are dark against the computer screen.
If you wear polarized glasses, and notice patterns in some cars rear windows, what you are seeing is the stress intentionally built into the tempered glass. Tempered glass in the side and rear windows is designed to break into tiny cubes, instead of large daggers, and you can see it with a polarized lens.
The halos and patterns are not visible if the glass is of the same COE. You can mix any glass, but that stress means the glass wants to break. So depending on how far apart the COE and viscosity, your fusings might be loaded springs waiting for an excuse to break apart. When they do, they are under spring pressure, and glass shards can be sent flying.
Test your glass for compatibility, it is easy enough to do. You can fuse any glass together, and have it not crack right away. But it can be a dangerous ticking time bomb just waiting to sent shards flying the moment something taps it wrong.
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