The visual
evidence relating to this enquiry is a sharp-edged break through the middle of
the slumped piece. The two parts have slumped separately, and seem attached at
the rim, leaving the middle opened. A
moderate slumping temperature was used to fire the piece at the bottom of a stacked
kiln load.
This is used as an example of the kind of thinking required when investigating breaks in slumping.
The split occurred
before the slump was complete. We know this because the pieces no longer fit
exactly together. This means the crack opened
as the slump continued. There is other evidence.
The opening of the
crack cannot have happened at or after the annealing. It would have already
formed to the mould in a whole state. It would break completely across, because
it would be in a brittle state. And the
pieces would fit exactly together. But
they do not.
This piece was at
the bottom of a stack of shelves in a deep kiln. At the bottom, there is
no radiant heat, only side heat. This could be a major cause the kind of
break described.
It is possible that
the split was not across the whole piece. At the bottom of the kiln, the
glass is not receiving any radiant heat from the top. It is getting radiant heat only from the
sides of the kiln. That means the edges
were considerably hotter than the centre.
The edge may be in a plastic state while the centre is still in the brittle
state. The contrasts in expansions are
often great enough to break a piece.
From the evidence
we have, it can only be said the ramp rate was too fast for the conditions.
This little
exercise shows that a lot of information about layup, schedule, place in the
kiln, and any other relevant variation on the usual, must be detailed when
asking why something has not turned out as expected.
