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Devitrification
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.
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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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