Friday, 20 April 2012

Single Layer Slumping

Contrary to expectations, single layers are more difficult to slump than multiple layers, as you do not have so much mass for the gravity to act upon. Especially if the pieces are small, the slump will be slower than for large pieces.
It is possible to take the glass up in temperature very quickly without breaking it, but that will not give you much control over the slump. You may not get the curve you want, and you may over fire it so that it distorts.
If you feel the need to go quickly, slow down from 630C to a rate of advance of 40 or 50C/hr. For the first firings and new moulds, observe how the glass settles into the mould. This will give you the top temperature.  In future you can back off 10C or more and soak for about 30 minutes to get less marked pieces.
You can drape or slump a single layer over a mold or drape two overlaid layers on or over a mold – assuming the layers are 2 mm to 3 mm thick.  Draping double layers together is a popular way to make two colour double layered handkerchief vases with art glass. But when you fire one piece of glass on top of another, the air between the two pieces acts as an insulator, slowing the heat transfer between the two pieces of glass. So you should advance at about half the rate you use for a single layer. Anneal soak and cool for 6 mm glass.
Note that there are different behaviours between single and double layered pieces.
Draping and slumping processes work by taking advantage of the action of gravity on the just pliable glass. This means that thicker, heavier glass slumps and drapes more quickly than thin light glass does. You will need longer soak times for thin glass than for thick glass. You can go higher, but the effects become less controllable.
The size of the opening of the mould has a strong effect when slumping single 3 mm layer glass. You need much more time – often as much as three times as long as for a 6 mm piece over a small diameter.
The effect of the shape of the mould has little effect on simple shapes. But when angular or complex curves are part of the mould the single layer will have much more difficulty in conforming to the mould than the thicker pieces.
Annealing the 3mm piece can be a bit quicker than for 6 mm, but a reduction of more than a third in the soak time and more than doubling the cooling rate may cause problems.