Slumping sizes

A question about slumping into a bowl mould.

·        Will the end result always be that the glass drops down into the mould and so the bowl size is smaller than the mould?

·        Does the firing schedule affect the outcome?

You need to think about what is happening to the glass in the slumping process to understand what the outcome of a slump will be.

During slumping, the glass is being bent rather than stretched (very much) into the mould.  In other, higher temperature processes the glass is being both stretched and thinned.

The reason for using low temperatures is to reduce the thinning effect while obtaining the shape of the mould.  If you use higher slumping temperatures, the glass will slide down the mould more than at low temperatures, and will begin to thicken, or create an uprising, near the lower portion of the mould.

Take the simple case of a ball mould. One which has no flat bottom but a simple curve from edge to edge.

If the glass was 300mm diameter when flat, it will still be (approximately) 300mm from edge to edge on the bottom.    If you put a flexible tape measure around the bottom from one side to the other, it will be very near the 300mm diameter around the curve.

This means the new diameter of the rim will be smaller than the flat diameter.

Steep moulds have less change in diameter, but greater change in the height of the piece in relation to the mould.  Again, the glass measured from side to side on the bottom will be about the same as the flat piece.  To achieve this, it will slide down into the mould more than a shallow one.

More detailed information is available in the e-book: Low Temperature Kilnforming.