Firing Tempered Glass
Tempered glass is glass that has been heated
and then cooled rapidly under controlled conditions so that it is
stronger than ordinary glass. If it breaks, tempered glass
will break into small pieces with rounded edges, rather than the
sharp edges that occur when regular glass breaks. This is why
tempered glass is usually thought of as being safer than glass that
has not been tempered.
Once tempered, glass cannot be cut without
shattering. It's also difficult to sandblast without causing the
glass to shatter. Because of these limitations to using
tempered glass, and also because excess tempered glass is sometimes
available very inexpensively, it is not uncommon to want to
de-temper glass so that it can be using for fusing, slumping, or
other kiln-forming activities.
De-tempering is a simple process. Just
fire the glass in a kiln to above the annealing range of the glass
-- above 1100F/600C will be sufficient for almost all commonly used
glasses. Then anneal using a basic annealing schedule and the
glass will no longer be tempered.
If desired, it's possible for the initial
firing to be to a temperature well above the annealing range, so a
piece of tempered glass can be fired to slumping or fusing
temperatures, then annealed and cooled using a standard schedule.
Just remember that when the glass emerges from the kiln it will be
ordinary un-tempered glass.
here for information on the two ways to temper glass.
Copyright 2005 Brad Walker.
All rights reserved.