From the Article:
Old manuscripts can decompose due to acidity, moisture, insects and mould. Early films on cellulose stock deteriorate before finally turning into a rust-like powder. According to the U.S. National Film preservation Board, part of the Library of Congress, fewer than 20% of American silent films still survive, and half of the American films produced before 1950 no longer exist.
In the U.K., the British Film Institute is calling for donations to raise the £2 million necessary to restore and preserve nine early silent films by “Psycho” director Alfred Hitchcock, while the recent rediscovery in Argentina of a more-complete version of Fritz Lang’s 1927 masterpiece “Metropolis” is minus a couple of scenes due to damage to the reels.
Source: Wall Street Journal