I had an overhang [on a texture mould] and I heard a pop
and opened kiln and saw it cracked off the mold. … [The piece] is 2 layers
Bullseye irid placed face down and Tekta [on top]; the mold was sprayed 3-4
times with zyp and Thinfire; and I put mold on kiln posts. [I] fired to
1440[F].
Diagnosis
The overhang of the glass caused the break. As the glass heats it expands. The ceramic does not expand as much as the glass. This means even more glass will hang over the edge than at the start. As the glass reaches slumping temperatures, it begins to drape over the edge. At the same time the glass on the interior is beginning to slump into the textures. When the glass has fully taken up the texture, the overhanging glass will be touching the outer sides of the mould. This means at the end of the heating and soaking part of the firing, you have the ceramic mould partially and tightly encased in glass. The glass has formed around the ceramic.
Credit: theavenuestainedglass.com
The physics of the two materials are that glass expands more than ceramic. On cooling, the glass grips the sides of the ceramic mould tightly. This is because it shrinks more than the ceramic. In this case, the ceramic was stronger than the glass and the strain caused the glass to break. Upon occasion the opposite can happen.
Two other notes.
The temperature of 781°C is higher than needed. You will need to do a bit of experimentation
to find the right combination of temperature and time for each mould. You could consider 630°C as an
initial temperature with a 90-minute soak.
Bob Leatherbarrow (p.161) describes a method of scheduling a sequence of
slightly higher temperatures with soaks.
If the texture is not forming (as determined by observations), you can advance to the
next segment with a higher temperature and see how that goes. When the appropriate amount of texture has
been achieved, advance to the cooling to anneal segment.
Iridised surfaces provide a very good separator. With the iridisation down against the mould,
it may be unnecessary to use Thinfire, especially when you already have used boron
nitride as the separator.
Further information is available in the ebook: Low Temperature Kiln Forming.
