Crash cooling will harm your kiln or break your glass.
Crash or flash cooling was often a requirement in the
early days of fusing to avoid devitrification. The kilns used were ceramic ones
that did not lose heat very quickly. The
glass also was more subject to devitrification than the glass being made now.
Since those early days, kiln design has advanced so the kilns lose heat
more quickly, although still well insulated; and the glass is more resistant to
devitrification. Thus, crash cooling is
no longer advised.
If you have a brick lined kiln, crash cooling is hard on
the bricks. The cold air causes rapid
shrinking of the brick. The more rapidly
the brick heats and cools, the more fractures will develop in the brick. This effect will take place over many firings
before there is any noticeable damage to the structure of the brick. However, if you have brick tops or lids,
there is the increasing likely development of crumbs of brick falling onto your
work. Brick lids and tops should be
vacuumed frequently to remove the crumbs as they form.
Crash or flash cooling from top temperature toward
annealing temperature is unlikely to break any glass other than thick glass
pieces. However, when using glass
formulated for kiln forming, you do not need to crash cool. The crash cooling
may be more useful when using glass that is not formulated for kiln
forming. The purpose in this case would
be the same as that for the early fusing – avoiding devitrification by moving
as quickly as possible through the devitrification range.
Sometimes flash/crash cooling is required to fix a free
drop in place. If allowed to cool on its
own, the glass will continue to move for a while. If the extent of the drop is critical, crash
cooling is required. This should be to a
point below the slumping but above the annealing temperature. The flash cooling will cool the outer
portions of the glass, stopping any further movement. Meanwhile the inner
portions are still hot. This sets up
significant stresses. By stopping the
cooling just below the slumping temperature, you allow the internal and
external temperatures of the glass to approach one another before going into
the anneal soak where the temperature equalises throughout if the differentials
are not too great from the flash cooling.
All myths have an element of truth in them otherwise they would not
persist.
They also persist because people listen to the “rules” rather than
thinking about the principles and applying them. It is when you
understand the principles that you can successfully break the “rules”.
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