Glass
Compatibility
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.
UNDERSTANDING COMPATIBILITY
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.
Click
here to see a table that summarizes the Coefficient of Expansion for many
different glasses
(and
even for some common metals, such as copper).
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. |
Click here to go to
the next page of the tutorial, which discusses the many different types of glass.
