So what is this thing called incompatibility? It doesn't happen with beans. Why does it happen with glass?
To understand compatibility, you have to go back to high school physics. You may have been asleep during the discussion of thermal expansion (I'm sure I was), but perhaps somewhere in the fog of your past you recall that things expand when they get hot. And, if you really reach back, you probably remember that different things expand at different rates.
And glass is no exception. Like almost every other substance, glass expands when it gets hot and contracts when it cools. This change occurs at the molecular level -- unless you have a very strong microscope, you probably can't see it. But it can be measured in a laboratory. A typical one inch piece of Bullseye brand glass, for example, will expand 0.0000090 inches for each 1 degree Centigrade (about 1.8 degrees Fahrenheit) increase in temperature. That's nine-millionths of an inch!
This rate, which is commonly known as the Coefficient of Expansion (COE), is usually expressed as a whole number, rather than as a long decimal figure. (Even scientists are lazy.) Most Bullseye glass, for example, is said to have a coefficient of expansion of 90, and you will often hear glass artists refer to it as COE90 glass. Spectrum, another common glass, has a COE of around 96, while Corning's Pyrex glassware has around a 32 COE. As you can see, different kinds of glass have different coefficients of expansion.
These differences in expansion and contraction may not sound like much, but they are very significant on the molecular level. Two glasses with different COEs are said to be incompatible. They cannot be fused together and should be kept in separate areas of the glass studio to prevent their accidentally becoming intermingled. This is especially critical because you can't always tell incompatible glasses just by sight. And you usually can't detect incompatibility until the different glasses have been fused together.
Click here for the next section, which discusses how to test for compatibility.