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Devitrification, a whitish scum that sometimes appears on the top surface of glass that has been fired in the kiln, occurs when glass remains at too high a temperature for too long.  In most cases, devitrification is considered a nuisance, and glass artists will go to great length to prevent its formation.

To understand why devitrification forms, we must first understand the nature of glass at room temperature.  Although a sheet of glass appears quite stable and unchanging, it's actually delicately balanced between two states of being.  On the one hand, there is an equilibrium in the glass between its various constituents (sand, soda ash, limestone, etc.).  On the other hand, a tension exists as these individual components (especially the sand) have a natural tendency to return to their original states.

When glass is heated, this equilibrium is interrupted.  The particles of the glass are heated past their solid state and become increasingly liquid.  So long as the glass is allowed to return to its solid form fairly quickly, the molecules are able to return to the normal configuration and the delicate balance is restored.

However, if glass remains at too high a temperature for too long, then the normal process of establishing equilibrium is interrupted and the molecules in the glass are prevented from regaining their delicate balance.  Instead, the high temperature causes some of the elements in the glass to burn off.  The glass crystallizes along the top surface, forming a crystal (called devitrite).  A mild case of devitrification  results in a dull whitish deposit on the glass, while more severe cases can cause the top surface to break down and even deteriorate completely. 

Click here to go to the next part of this discussion, which deals other factors which contribute to devitrification, as well as tips for prevention and treatment.


Copyright 2005 Brad Walker.  All rights reserved.

Portions of this discussion adapted and expanded from Keith Cummings's excellent book, Techniques of Kiln-formed Glass.

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