How does a pattern piece fuse into a sheet of glass? Find out.
It may help you understand what happens and therefore design better projects.
I became interested in understanding just how pattern pieces became fused in my tile technique. So when I had a failed tile, I sliced it with a diamond saw to see if I had worked out the process correctly. Here is the cross-section, enlarged and with some drawn arrows I will explained later. The base is a layer of clear Spectrum 100SF at the bottom, with a layer of white opal Spectrum 200SF above, and pattern pieces of cobalt blues 130.8SF, 132SF and Uroboros black iridized (all System 96 glasses).

  1. With just two layers (clear and white) the glass is at the thickness it wants to be, about 6mm. The pull of gravity trying to spread the fluid glass and the pull of surface tension trying to bead it up just balance. Thicker glass will tend to flow outwards, and thinner (such as single thickness of 3mm glass) tend to contract. However, the pattern pieces disturb this balance.
  2. As the temperature is raised, the glass becomes more fluid. At or just below 700°C two things begin to happen: the bottom face of the pattern piece begins to stick loosely to the base glass, and the surface tension begins to round off the top edges of the piece. Raise the temperature still further, and the underside becomes tightly fused to the base (certainly by 760°C), and the edges round off still further. However, the glass that was exposed stays exposed, and the glass that was in contact stays in contact. This means that the top face and the side faces (the 'edges' of the glass in colloquial terms) are going to form the top face of the fused piece. However, the glass can't just contract as it rounds off. The surplus glass must go somewhere, and where it goes is into the base. The piece begins to sink into the base like an iceberg, more in the middle and less on the sides. As it does so it pushes the now fluid base glass sideways (have another look at the picture, and the arrows which show the direction of movement). The bottom of the pattern pieces stretches and the top compresses.
  3. If we keep on heating the glass, say to 790°C and hold long enough, the surface will end up more or less flat with the pattern pieces recessed into the base glass which has thinned out underneath them and oozed up between them. This sample was held at 770°C for 10 minutes, and traps the process part-complete. Large pattern pieces will not fuse as flat as the glass under the middle has a lot further to flow.
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 Copyright 2003+ Arthur Sale
Last Modified: 27 December 2010