Thursday, 25 October 2012

Rounded edges on slumped pieces


Often people want to know how to get rounded edges during slumping of a single layer piece, especially when trying out techniques with their spare art glass.


Achieving a rounded edge on a slumped piece is a combination of temperature, thickness, larger top layer and cold working.

Rounding of the edges of a piece of glass occurs at tack fusing temperatures, which are beyond the slumping temperature. It is possible to take the glass to a tack fuse within the mould as long as you are prepared for some consequences.
  • More mould marks are evident on the bottom of the vessel.
  • Mould life is reduced.  You get a lot more mould marks on the bottom of the glass because the bottom of the glass is softer than in a standard slump. These marks will be directly related to the surface texture of your mould.
  • You need to re-coat the mould before the next slumping to avoid the kiln wash sticking to the glass.  Ceramic based moulds last a long time if fired below 680C. But numerous firings at tack fusing and higher temperatures increase the possibilities of glass sticking to the mould and occasionally, thermal shock. If you insist on tack fusing in your mould, you need to renew the separator each time, as the kiln wash breaks down at tack fusing temperatures leading to it sticking to the bottom of the next piece you fire in that mould.
  • You may get an uprising at the bottom, as the glass slowly sinks down the mould and pushes the glass up at the bottom in any mould other than a simple, shallow shape.  

These things indicate that it is best to tack fuse first and then slump at the lower temperature.

Of course the best result can come from using 6 mm of glass, with the top layer 6 mm larger than the bottom layer. This allows the upper layer to sink over the outer edge of the lower one, giving a rounded edge with no sign of any differences between the two layers.

Cold working solution 
If however, you want to work with a single layer, you need to realise that the edge will be the same when it comes out as when it went in. So you need to cold work the edge before slumping. You do not need sophisticated machinery to do this. A few diamond grit hand pads will do the job. Start with one at about 100 grit to shape the edge. Make sure you keep the pad and working surface damp. If you begin to get a white paste appearing, you need more water.

After shaping the edge satisfactorily, take a pad of about half the grit size (twice the number) and begin the smoothing of the scratches created by the shaping. When finished with one grit move on to the next. You can use a paint marker to help tell when one grit is finished. [Paint and cold working 20/12/12].

This process will give a sheen that will change to shiny during the slumping.