Glass Overhanging Moulds

Glass Overhanging Moulds

Glass that overhangs moulds by too great an amount is likely to break upon cooling.  Even during slumping,with its lower than fusing temperature, glass expands.  Unless the overhanging glass is drawn into the mould while slumping, there will be some draped over the edge of the mould. When cooling, it begins to contract. 

This post shows the way in which glass behaves during the slump.  This may help with determining how much of an overlap is allowable.  If the mould has a broad, nearly horizontal rim, the glass will not rise and slip down into the mould enough to avoid  an overhang during the firing.  However, a mould with a small rim can accommodate an overhang.  Circumstances vary, but generally a 6mm overhang on a narrow rimmed mould will be safe.  For broad rimmed moulds, no overhang is safe.

The risk of breakage is not so great on steel, where the metal is contracting more than the glass.  However, all of us normally use ceramic moulds which expand and contract less than the glass.  This means the glass will trap the ceramic until the stress is relieved by breaking a part of the overhanging glass.

There are methods to support the overhanging glass during the slumping, as described in this blog post, which will eliminate the risk of the glass trapping the mould.