I wonder if they are glass negatives.
The pictures taken by Ernest Shackleton's photographer, Frank Hurley, rival in clarity any taken today. They are magical.
http://www.shackleton-endurance.com/joomla/
I have studied the adventure of the Endurance for years, but just found this well made page:
http://www.kodak.com/US/en/corp/features/endurance/