PE, Yes the edges are roughed with a stone or file. I've actually seen guys just rough the edges with another piece of glass. For ambrotypes this is sufficient but if you are doing a glass negative and redeveloping the negative to build density then the pyro in the developing solution will cause the collodion to shrink and at times lift it from the glass. By subbing I mean that a thin coat of albumen is put down on about 1/8" around the surface edge of the plate. There's nothing worse than getting a nice plate and then watch the collodion slide off during development or intensification. I rough the edges and sub the edges with albumen and have never had one lift. I take one egg white and dip a Q-tip in distilled water and then in the egg white and sub the edges, let it dry for a minute and the plate is ready to pour. That's not much effort to insure the collodion don't lift.