Why do Ground Edges Appear on the Surface of the Glass?

"Why do the ground edges of my glass appear on top of the glass?"

The scum from grinding edges, which promotes devitrification, often appears in in fusing. But why does it appear on the surface? 

There is a lot of movement of glass edges during a firing. On the way up in temperature, the glass is relatively stiff and expands with a vertical edge during most of this phase.  At the top temperature the surface is expanding and pushes the edge flat to the separator.  Then, as the temperature falls the cooling glass surface contracts, pulling the edge to the surface together with anything it has collected from the separator.  Sometimes, cleaning the ground surface is not enough to prevent the rough surface from picking up some of the separator, and this is what is seen in the final product.

If you are going to grind to fit the glass pieces, you need to finish with a fine grinder bit. These are usually around 220 grit which might be fine enough if cleaned well, but run some tests to be sure. The safer grit to prevent the scum is 400, but the ground glass surface still needs to be scrubbed clean.

There are good reasons to avoid grinding, or when not avoidable, to fill gaps with powder or frit of the same colour.