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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.

Click here for information on the two ways to temper glass.

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Copyright 2005 Brad Walker.  All rights reserved.

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Copyright 2005-2006 by M. Bradley Walker.  All rights reserved.

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