Drying Kiln Washed Moulds

A question about kiln wash. Do you have to let each coat dry while applying before applying the next coat?

 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 cristobalite inversion at around 225°C/437°F.  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.  But also note there is a quartz inversion at around 570°C/1060°F that is significant.

 

 This is another reason to advance the temperature slowly when slumping or draping with a ceramic mould.  A further 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/203°F 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/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.

 The other part of the question was about drying the kiln wash between applying coats. It is not necessary to dry between coats of kiln wash.  In fact, a better result is obtained by applying all the coats at one time. It is not like painting wood. The result of applying all coats is a smoother surface.  There is no dragging of the dry powder along with the wet kiln wash as it is being applied over the existing coats.

 Kiln drying ceramic slumping and draping moulds is not necessary. It only adds another, unnecessary step in kilnforming preparations.  There are exceptionally good reasons to avoid rapid firing of damp moulds. 

 Some extra care could be taken with texture moulds and those intended for casting.