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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.

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