Piece fusing

Analysis

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.

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.
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.

Consequences

The bottom of the pattern piece (and only the bottom) will fuse to the base. The top and the sides will be exposed and subject to devitrification, and will need cleaning and possibly treatment to reduce this if it is a problem.

That nice iridized glass (on its top face) is going to end up with a narrow black (un-iridized) border all around it. Similarly for clear iridized glass and dichroic glass.

Since glass can't disappear, the edge of a fused pattern piece on a 6mm or thicker base is likely to be slightly outside the original edge of the cut piece, perhaps by 1mm. This is because it has ended up thinner than before. There is nothing you can do about this except use a light fusing schedule. (If fusing pattern pieces on to 3mm glass, surface tension plays a dominant role and the pieces may shrink. This deserves a whole extra page to explain.)

Sharp points are rounded off for the two reasons: firstly surface tension pulls the tip back before the underside has a chance to fuse to the base, and secondly the flow during fusing will be as fast at right angles to the strip as out to the tip. There is not a lot you can do about this either, except use a light fusing schedule to inhibit the second mechanism.

Two pattern pieces placed with adjacent cut edges are unlikely to fuse together except at some accidental places. The first reaction of the fusing process is for the sides to pull away from each other, aided by base glass trying to come up in between.