There are many different production methods and recipes for making glass. As a result, there are almost as many different types of glass as there are glass artists who want to use them. Types of glass range from basic window glass (called "float glass") to brightly colored stained glass (also called "art glass"), and many of the types of glass come in numerous sub-types and categories.
In addition, there are many types of glass coatings, such as iridescent and dichroic, which have unique properties when applied to glass. New types of glass and glass coatings are constantly being developed.
All of these different glass types are candidates for fusing, slumping, and other kiln-forming processes. Some can be used off the shelf, but others require testing to make sure they will work in the kiln. That’s because it’s likely that you will want to combine more than one different sheet of glass in your projects. If so, then you’ll need to make sure the glass you select is "compatible." Using incompatible glass may cause cracking or even shattering of the piece when it cools.
To better understand compatibility, let’s consider what happens when glass gets heated in a kiln. Like many other substances, glass expands when it gets hot and contracts when it cools. This change in density, which occurs at the molecular level, 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. 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 a 32 COE. Standard window glass, referred to as "float" glass by the glassmaking community, has a COE that is usually around 84-87, while Effetre (Moretti) glass, commonly used for lampworking, has a 104 COE.
These differences in expansion and contraction may not sound like much, but they are very significant on the molecular level. A 10 inch length of Bullseye glass, for example, will shrink about 0.046 inches (about 1 mm) in cooling from around 950 degrees Fahrenheit to room temperature. By contrast, a 10-inch piece of Spectrum glass will shrink about 0.049 inches over the same temperature range. That difference - .003, or three thousandths of an inch - sounds trivial, but it’s enough to ensure that you can’t fuse Bullseye and Spectrum together.
Two glasses with considerably 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. In the example below, Bullseye (90COE) and Spectrum (96 COE) glass has been fused together. All looks fine to the naked eye, but viewing the glass with a polarized film shows the underlying stress.
You can sometimes get away with using two different glasses where the COE is only one or two apart (say, a 90 with a 91), but not always. Sometimes even two glasses with the same Coefficient of Expansion can not be fused together. That’s because the laboratory test that determines COE takes place at a different temperature than the one the warm glass artist often uses.
There are really only two ways to know if your glass is compatible:
• Use glass that has already been "Tested Compatible" by the manufacturer.
• Conduct compatibility testing on your own. This is a matter of fusing small squares of the glass to be tested to a base glass of known COE, then examining the fused strip by sandwiching it between two strips of polarized film.
The advantage of testing for compatibility is that a broader range of colors and textures are available. Also, most manufacturers charge a premium for "Tested Compatible" glass, so it’s generally less expensive to use glass that has not been tested.
PHOTO ILLUSTRATION OF GLASS COMPATIBILITY TEST
Top left: Base clear glass with three fused squares: the dark blue are 96COE Spectrum, while the middle square and the base glass are both 90COE Bullseye. To the naked eye, the glass appears to have successfully fused.
Bottom left: The same glass sandwiched between two sheets of polarized film and viewed on a light table. Note the bright halos around the Spectrum squares, indicating stress due to lack of compatibility. The Bullseye square in the middle has no halo, demonstrating compatibility with the base glass, which is also Bullseye.