Silver Halide
Black-and-white negatives and prints made by the silver halide process are stable so long as the photographic substrate is stable. Some papers may yellow with age, or the gelatin matrix may yellow and crack with age. If not developed properly, small amounts of silver halide remaining in the gelatin will darken when exposed to light. In some prints, the black silver oxide is reduced to metallic silver with time, and the image takes on a metallic sheen as the dark areas reflect light instead of absorbing it. Silver can also react with sulfur in the air and form silver sulfide. A correctly processed and stored silver print or negative probably has the greatest stability of any photographic medium, as attested by the wealth of historical black-and-white photography.
Read more about this topic: Photograph Stability
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—Henry David Thoreau (1817–1862)