Thursday, 15 December 2011

Uneven Slumps

Uneven slumps are an occasional feature of kiln forming. There are a number of possibilities that can cause this.
A mould that is not level will lead to an asymmetrical slump. You need to make sure on each firing that the mould is level, as well as your kiln and the shelf or supports. A three-way level can check this quickly.
Glass that is placed unevenly on the mould can cause uneven slumps. The glass should be placed on the mould so that it is equidistant from the edges of the mould all the way around. The glass needs to be level too. Again the three way level can help.

A slight bevel on the bottom edge can help avoid any catching of the glass as it moves within the mould.
Baffling or shielding parts of the glass can keep the heat off areas that tend to heat up faster than the rest and so begin moving before other parts of the blank.

Deep moulds most often present difficulties with uneven slumping. The best approach here is to use multiple, progressively deep slumps.

It also is possible to reach in with a wet stick and move the glass back to an even slump during the firing. Please observe all the safety requirements.

Most importantly, slow temperature rises solve most of the problems of uneven slumps. Fast temperature rises allow the component parts of the piece to absorb heat differently for different colours and glass of different viscosities. Slow rises in temperature help avoid any unevenness in the way the kiln heats the piece. Slow rises give more control and achieve the result at a lower temperature.

Note that a number of the solutions require observation while firing. The best results come from observing what is happening in the kiln. It allows you to make corrections either while firing, or – more often – in the next firing of a similar project.