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Mixing Green Beans with Brown
Part Three of a Multi-Part Tutorial on Fusing and
Compatibility
Click here to go to the
first part of this article.
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.
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Copyright 2005 Brad Walker.
All rights reserved.
This article was originally
written in 1999 and was one of a series that became the basis for the
Warm Glass website. It
has also been published in Common Ground: Glass, the
newsletter of the International Guild
of Glass Artists.
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