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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 check whether I had worked the process out correctly. Here is the cross-section, enlarged and with some drawn arrows I will explain later. The glass is a layer of clear Spectrum 100SF at the bottom, above a layer of white opal 200SF, and pattern pieces of cobalt blue 130.8SF, 130.8SF, Uroboros black iridized and 132SF in that order.
Here is the analysis.
   With just two layers (clear and white) the glass is at the thickness it really wants to be - about 6mm. Gravity and surface tension are in balance. Any thicker and fluid glass will tend to flow outward (gravity stronger than surface tension); any thinner and it will tend to gather together like raindrops or mercury (the surface tension is stronger than the spreading influence of gravity). So the pattern pieces disturb this balance.
   As the temperature is raised, the glass becomes more and more fluid. At or just below 700°C two things begin to happen: the bottom edge of the pattern piece begins to stick loosely to the base glass, and the surface tension begins to slightly round off the top corners 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 by tension on the surface. However the glass that was exposed stays exposed; it just moves slowly. Notice that this means that the top side and the edges 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 edges. As it does so, it pushes the now fluid base glass out sideways (have another look at the picture and the arrows showing the directions of movement). The bottom of the piece stretches and the top compresses.
   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 under them and oozed up between them. This sample was held at 770°C for 10 minutes to retain some surface texture, and the process was therefore frozen part-complete. Large pattern pieces will not fuse as flat since the glass in the base has a lot further to flow.
Wherever the process is stopped, the underside of the pattern piece is the only side that fuses (and should be cleaned for that reason). The topside and the edges form the top of the result, and are therefore at risk of devitrification and should be cleaned and treated for that. This can be clearly seen when fusing small black iridized glass pieces, since the top surface will end up with the iridescent layer slightly compressed, and a thin black border around the edges from the uniridized cut sides.
Since glass can't disappear, the edge of a small fused pattern piece may be slightly outside the original edge of the cut piece, perhaps by 1mm. This is because it has ended up thinner than before even if only at the edges of large pieces. There is nothing you can do about it except use a light fusing schedule. (When fusing on to single 3mm sheets of glass, surface tension plays a dominant role and the pieces shrink. I will have to explain this separately.)
Sharp points are rounded off for two reasons: firstly the surface tension pulls the point back before the underside fuses to the base, and secondly the flow during fusing will be as fast at right angles to the strip as out to the point. There is not a lot you can do about this either, except use a light fusing schedule so the second mechanism is inhibited.

© Copyright  2001  AHJ Sale
Page last modified on 2001 October 24