The
recommendation is that you put the shelf on supports to keep it above
the base of the kiln and allow air to circulate around both the top
and bottom of the shelf.
The
question remains, why do the shelves break. There are at least two
reasons: physical impact and thermal shock. It is possible to knock
the shelf while moving it around the studio. This impact does not
always cause a break, but sometimes creates a stress point that later
can develop into a crack and break. You can sometimes see the start
of the crack from the edge of the shelf. In this case, you can
either continue to use the shelf with support under the crack or
dispose of it immediately, because at some point during a firing it
will separate.
The
thermal shock that causes the break occurs because (usually) uneven
cooling. It seems the shelves are pretty resistant to rapid heating,
but less tolerant of rapid or uneven cooling. In general un-dammed
fusing and using moulds elevated a little from the shelve do not
create that uneven cooling.
However
placing a large refractory mould directly on the shelf can promote
cracking either immediately or on subsequent firings.
The
main culprit in any breakage seems to be large or heavy and damp
refractory moulds directly on the shelf. The mould is giving off
water vapour which cools the immediate area around the mould. So as
the temperature rises, the covered part of the shelf stays cool, in
addition to being shielded from the general heat of the kiln while
the uncovered parts of the shelf rise in heat. At some point the
temperature differences in the shelf are too great for its strength
to resist. The solution is to remove the shelf from the kiln and
place the mould, slightly raised, on the floor of the kiln. The
bricks, being softer, do not react in the same way as shelves to
uneven heating.
For
thick fusing with dams all around, it seems best to do this on a
shelf that almost fully covered with glass and dams. This promotes
more even heating and cooling of the shelf than having a small part
of the shelf covered. It does mean having different sized shelves,
but then you may already have some of them due to the breaking of
other shelves. Just cut the broken shelf to the size you want on a
tile cutter.
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