In
general, the blank should be no larger than the thickness of the
glass over the mould. So a 6mm blank would have no more than 6mm
overhang.
In
the case of steep sided moulds, the glass should be entirely within
the mould to avoid any hangup on the edge, leading to uneven slumps
and needling on the edges.
But,
if you need the glass to be the size of the mould, you can make a
collar to go around the mould, which will support the glass while it
begins to slump into the mould.
Make
a donut shape that will fit around the mould (whether round, oval or
rectangular) and extend beyond. Support the collar on kiln furniture
to be as high or slightly higher than the top of the rim of the
mould. This makes a kind of drop out ring, allowing the glass to
drop into the mould.
Donut ring suitable for placing around a circular mould |
This arrangement is suitable for placing around a mould of the same diameter as the interior of the ring |
Make
sure that the collar is well covered with kiln wash to ensure the
glass can move along the fibre board. This includes both the surface
and vertical edges of the collar.
As
the glass softens and begins to fall into the mould, the glass at the
edge does not have the weight to bend down and so raises off the
collar and begins to slip into the mould.
And
finally, you need to ensure that the mould is not so steep as to trap
the glass inside. This is more of a concern on steel with its
greater expansion and contraction than ceramic.
A steel mould likely to trap the glass inside with its vertical sides |
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