It is often claimed that inadequate annealing of the fused blank can cause
breakage during a slump firing.
If annealing is the cause, it is likely to break on the rise in
temperature. Once the piece has reached
the annealing temperature, any breaks will be due to thermal shock on the way
down. An annealing break usually has a
hook at both ends of the break, although this is more difficult to determine in
a shaped piece.
Thermal shock tends to be along straight(ish) lines, often
between thick and thin, or strongly contracting colours. It tends to happen on the cool down.
Breaks on the rise or fall in temperature are difficult
to distinguish on slumps. The
temperature is low enough that there is little to distinguish the sharpness of
the edges. The real method of
determining, is to try to fit the pieces together. If they fit exactly, the break was during the
cooling. If they have even a little
variation in fit, the break occurred on the rise in temperature.
If the annealing of the slump is marginally inadequate,
it may break hours, days, weeks after cool.
The less stress the longer it will survive. This will not be the result of any inadequate
annealing of the fused blank. Only the last annealing is relevant to the
soundness of the piece.
How can you
ensure the annealing on a slumped piece is adequate?
You need to check the fused blank for stress before
slumping to ensure it has no or very little stress. The anneal for unstressed items needs to be at
least equivalent to, or longer, as for the fused blank.
Fire more slowly than usual for blanks with moderate
stress and anneal slumped piece more slowly than you did for the blank. This will help ensure the formed piece is
more adequately annealed than the mildly stressed blank.
Pieces with significant stress need to be returned to the
kiln to be annealed. Fire them
significantly more slowly than you normally would for a piece that thick. This may be one half or less the speed used
on the un-fused pieces.
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