Low Slumping Temperatures.

It is possible to have glass overhanging slumping moulds if you use low temperatures. The glass has the appearance of behaving differently at these low temperatures than at fusing temperatures.

Glass at low temperatures is affected largely by weight.  

At low temperatures it cannot quickly form exactly to the mould. It falls first in the middle.  Because the glass is not very plastic, the edges rise up from the mould at first, because the weight there is not great enough to allow the unsupported glass to bend.  The edges stay in line with the beginning of the bend in the middle. (apologies for the quick and dirty drawings)

If you have a mould with a rim, you will be able to observe this effect.  As the glass in the middle begins to slump, the glass at the edge rises from the rim.  This allows the central portion of the glass to settle and draw the excess glass into the mould.  

It only settles back to the rim with the heat work of the slump as the slumping soak continues.

It is useful to record the temperatures and times of these effects for different moulds.  It tells you when the slump begins, the intermediate stage(s) and completion of the slump.

Using the lowest practical slumping temperature gives the best results.

·         It allows glass with small overhangs of the mould to be successfully slumped. 
·         Low temperature reduces the mould marks on the back of the glass.
·         Fewer stretch marks are in evidence. 
·         Low slumping temperatures with long soaks reduce the uneven slump that is sometimes in evidence with deeper moulds.
·         Low temperatures allow different colours to heat more evenly.
·         Low temperatures reduce the thinning effect of a high temperature slump.