Wednesday, 23 March 2016

Kiln Forming Myths 20

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”.