Bob has been kind enough to give me a large dish mould in a shape and size that I haven't used before (it measures 30cm X 20cm) so I have tested the mould using float glass. There's no point in designing a piece using expensive glass if I make a mistake with the first fusing.
First I drew a template of the dish and cut two pieces of float glass, grinding them to shape. Next I flat used the two pieces up to 842 degrees c (1547.60 degrees f) and then put it back in the kiln to fire polish the edges.
Finally I slumped the glass piece in the mould so that the glass would slump down into the shape of the dish. I also tested a new soap dish mould in the same firing.