You were right! Yippee, the cracks filled in. I measured pre firing and it was indeed 8mm. so all is good.
Before slumping, can you recommend the best cold working technique? The wet belt sander or the lap grinder I've been reading seem like the most logical next purchase. In the meantime with my current piece, is it best to even out the edges with the tile saw and then slump? The edges were a bit rough.
I'm so happy how this is coming out. I can't wait to try another "design".