There seems to be a popular notion
that newly kiln washed moulds must be cured before use. I'm not sure where the information comes from,
and no reasoning is given. It is
suggested that that quickly heating newly kiln washed moulds to 550°F (290°C) is
important.
If you want to make sure the mould is
dry, this may not be the best way to do it.
All ceramics have a quartz inversion at around 225°C. This a very rapid increase in volume of 2.5%
that often leads to cracks and breaks in ceramics when the rate of advance is
quick. The mould will react better and last
longer if the rate of advance is slow until that inversion temperature is
passed.
This is a reason to advance the
temperature slowly when slumping or draping with a ceramic mould. Another reason to heat slowly is to avoid
steam formation within the ceramic body.
If the steam is created over a short time, the force can be great enough
to break the ceramic. To ensure the
water evaporates, a soak at 95°C for a significant amount of time is a better,
safer option.
But in addition to all these
precautions, it simply is not necessary to cure kiln wash on slumping and
draping moulds made of ceramics. The
glass does not begin to move until after 540°C (about 1000°F). Therefore, the
kiln wash will be dry long before the glass gets near slumping temperatures.
Any vapor caused by evaporating water will escape through the vent holes in the
mould or under the glass at the rim, as it will not form a seal until higher temperatures.
newly kiln washed mould beside others already fired |
If you want to be sure your kiln wash
is dry before you put the mould in the kiln, you can leave it in a warm
ventilated space, or even on top of your kiln while it is being fired. Using either drying method will dry the kiln wash sufficiently before the glass is placed on the slumping mould.
Kiln drying ceramic slumping and
draping moulds is not necessary. It only adds another, unnecessary, step in
kiln forming. There are exceptionally good reasons to avoid rapid firing of damp moulds. The exceptions can occur
with texture moulds and those intended for casting that do not have vents.
Amen! :)
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