Conserving historic photographs is a very complicated business because of the variety of methods used in early photography, and the science itself is a fairly recent development. This fascinating article traces the history of photo conservation and the complications involved. Research into photo conservation was spurred in the 1990s by the explosion in value of historic and artistic photos and the discovery of some frauds.
What astounded me was that there have been over 150 different photographic processes developed in the 187 years since Joseph Niépce first took a photograph of the view from his window. The article lists some of these including processes i've heard of like daguerreotypes and also others i hadn't like ambrotypes and kallitypes!
Flickr Commons, British Officer from Sir William Dixson's collection of ambrotype portraits, ca. 1857-1858, possibly by Thomas Glaister (State Library of New South Wales)