Wow - thanks everyone for your helpful suggestions! I apologize for the belated response; had family troubles shortly after my initial posting so I did not have time to check the board.
Just to provide a context to the proposed installation - the wall of "fading" photographs will be mirrored on the opposite wall with "archival" photographs. So it starts off looking like two identical walls, but will change as the viewer revisits the installation later on.
I think the trick is to ensure that the photos on the "fading" wall deterioriate at different rates. It sounds like I will have lots of experimenting to do in the darkroom!
Thanks again for your helpful suggestions!