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|