Over time many images will gradually fade away.
The rate at which fading progresses depends on the quality of the initial processing, the substrate on which the emulsion layer is deposited, and general storage conditions.
This picture was made around 1930, and was fading out of existence. Since it was stored in a frame under glass it had virtually no physical damage. Recovery was straightforward with no complications.
These originals had lost much of their fine detail to fade. High end scanners can discern minute differences in gray levels that can be then spread out to reveal remaining detail. The pictures had a few artifacts that were revealed after the fade recovery. The sepia tone was added after the recovery process was completed to provide a more 'old timey' look.
