If the marks are on the back (not the emulsion side) you may be able to clean them with simple glass cleaner. Just be certain not to get any on the emulsion side while doing this (and try not to get finger prints on the emulsion side either). Apply the glass cleaner to a lint-free cloth first, and sparingly, and then carefully wipe the spots rather than spraying the cleaner on the glass.
I've got a number of glass plate negatives in my collection and most all of them have some small amount of frilling of the emulsion around the edges, which leads me to believe that the adhesion of the emulsion could be fairly easily damaged if not handled with care. If in doubt, just leave it be.
- Randy
