If you fire polish right,
If you fire polish right, the edges will not round.
The grey scum of death that you refer to is probably due to not grinding the edge to a fine enough grit or not cleaning it well enough. Note: one way to prevent that is to use a devit agent on the edges. But always put the piece in water immediately after grinding and clean well.
Web Site http://www.kaiserglass.com
Glass Classes: http://www.Kaiserglass.com/classes.html