My test tiles were perfect!
They've just come out of the kiln for a second time.
The fire time was a "full fuse", at a higher temperature, where the coloured pieces were fused together with a clear piece on top.
The second time was a "contour fuse", as I added some noodles and stringers (thin glass rods).
I wanted them to partially fuse into the bar, but still stand out on the surface.