Films, especially silent films, made during the years before sound have almost been lost forever. Somewhere between eighty and ninety percent of these films have been destroyed. These movies themselves were filmed on highly flammable cellulous nitrate film stock. The stock requires careful storage to slow down the decomposition over film, but the films actually shot on this nitrate were not preserved. As a result, as the years went by, the prints turned into dust. Many were recycled because of the silver, thrown away for more storage space or destroyed in studio fires. Its because of all of these circumstances listed about that film preservation is something that needs to be taken seriously.
Film stock is extremely fragile and the proper preservation of it involves storing the prints in climate-controlled places. Most films were not stored like this, which is why there is barely any left from the 1920s, whatsoever.
But, film decay is not limited to just movies made on cellulose nitrate. Many have found that films made on less expensive processes of color decay at a rapid pace, also. A number of highly-regarded only come to exist today as copies from original production or exhibition because the originals have decomposed beyond repair. Monochrome films, or those made on cellulose triacetate stock--they were the replacements for nitrate. These films suffer from Vinegar syndrome instead. When preserving color films of this nature, it usually involves a compromise of some sort. Low tempatures inhibit the color from fading, but increase the effects of Vinegar syndrome. But, if a room is normal tempature, the color will fade.
Listed now are the main problems with restoring film: scratches, dust, dirt, tears, color fade, color change, film grain noise (a copy of a film that has all of the film grain noise from the original as well as film grain from the copy itself), missing scenes, missing sounds, censored or edited films that were re-released and shrinkage.
To read more about film preservation or what some organizations are doing to keep these films around for decades to come, please visit either the National Film Preservation Foundation or the Moving Images Collection, which are both government-funded programs. If you want to see some preserved films, you can either buy a box set that has been released by the National Film Preservation Foundation or check out their website for video clips.