My grandmother moved into a retirement apartment and we're in the process of selling her estate. I'm in the process of digitizing the slides my grandfather took of their travels (7,000-8,000 of them!) Every once in a while I'll have a batch of deep red tinted slides. What caused this? My initial thought is that it was my grandfather buying out-dated film. He notoriously pinched pennies with film, something that came back to bite him on several occasions. Furthermore, what are the mechanics for the red tint?