Its really odd how some people get rings, and others donīt. Ive been printing with my negative between glass for several years now. When I make enlargments, I place the negative between two sheets of glass as my negative carrier, and when I am contact printing, I use various types of cheep picture frame glass in my contact frames. Iīve NEVER had rings appear!
Now that I mention this however....they will probably start showing up the next time I print! :-P