For the glass practitioner, the easiest way to think of
viscosity is to think of it as a measurement of how easy it is for a liquid to be poured
or stirred. The "thicker" a liquid, the more viscous it is.
Molasses, for instance, is more viscous than vegetable oil, which in
turn is more viscous than water.
With glass, as with most materials, the higher the temperature
the less viscous it becomes. (Said another way,
viscosity decreases as the temperature rises.) This simply
means that glass is harder to stir or pour at lower temperatures and
easier to stir as the temperature increases.
Because the viscosity of glass varies significantly depending on
temperature, viscosity plays a major role in
a number of kiln working processes. It's a significant
reason why slumping takes place at a lower temperature than fusing
(fusing requires more movement at a molecular level than slumping).
It's also responsible for the ability of air bubbles to rise through
glass when it's heated above fusing temperatures.
In addition, and perhaps most significant of all, it's important
to recognize that different glasses have different viscosities.
This is true even for two different glasses that are at the same
temperature in the kiln. In fact, viscosity is, like
coefficient of expansion, a critical indicator of whether or not two
different glasses can be compatible. Sometimes two glasses can
have the same COE but be incompatible due to different viscosities.
Coming soon -- more on the relationship between
viscosity and coefficient of expansion.
Copyright 2005 Brad Walker.
All rights reserved.
Portions of this discussion
adapted from Graham Stone's Firing Schedules for Glass.