Quartz is a crystalline form of silica
that's usually found in ceramic and clay items such as slumping
molds. It's a very complicated material that undergoes a
number of significant changes (called phases) when heated or cooled.
Most of these are of no importance to the warm glass artist, but
there is one phase (called quartz inversion) that sometimes impacts
our work in the kiln.
Quartz inversion occurs at approximately
1060F/570C. Basically, the silica molecules in the mold
material change form, causing rapid expansion or contraction.
The exact amount of the change in size depends on the type of clay,
but as a general rule most mold materials will expand or contract
approximately 1%. Because this change happens very quickly, it
can sometimes cause the mold to crack. This is much more
likely when the material is cooling than when it is expanding, but
breaks are possible when heating or cooling.
The effects of quartz inversion can be
avoided by heating or cooling more slowly through the quartz
inversion range. This is most important when using hand made
molds, which are more likely to have problems with cracking than
commercial molds. If your clay molds crack in the kiln, it's
often due to quartz inversion.
Copyright 2006 Brad Walker.
All rights reserved.