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You should never fuse on the kiln floor – among other things it makes heating the glass evenly very difficult.
Slumping and tack fusing is also inadvisable since tack fusing occurs higher than slumping and (probably the bigger issue) the two pieces of glass are likely to bend at different rates so when you do get to tack fuse temperature there’s a good chance you’ll trap a lot of air in the gap between the layers.
If you can get a 1/2″ thick piece of fiber board (Duraboard) then you can cut a bigger “shelf” and lay it on the existing ceramic shelf. Just make sure to use fiber paper between the glass and fiber board.
Helios Kiln Glass Studio