Quote Originally Posted by panastasia View Post
.............The "silvering" is the result of compression on the photo surface caused by other photos or things stacked on top of the print. This is one hypothesis I've been told. I'm sorry I can't validate this with any reference, though.
It's a chemical process. This means that placing the photo in an area with restricted air flow and against other materials that contain certain active chemicals can lead to silvering. So having a print in a pile of ordinary paper or cardboard can lead to the problem, just as trapping a print in an unsuitable frame can.