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I am using a 4x6 piece of fibreboard 1/2 thick. It has a3/4 inch edge around it. So basically a rectangle with the center cut out. I slumped my glass but the bottom is curved causing the dish to rock. I am just placing the board on my kiln shelf. Is the rocking due to the fact my board is not high enough?? Thanks

Your rounded bottom could be caused a couple ways.

One is that the slump isn't allowed to go long enough to get the glass all the way down. (most likely)

Second: the fiberboard may have some "give" to it allowing the glass to push into it.

You should, instead, use a fiber rectangle with the center cut out of it. That way the glass could slump down to the kiln shelf and flatten. As If it were a drop ring.

A flat lap or belt sander will correct the curved bottom.

Jim "The Hobbyist"

Is the blank you used 2 layers of glass or just one? Sometimes a single layer will not slump completely.