Photograph stability refers the ability of a photographic print or photographic film to remain visibly unchanged over periods of time. Different photographic processes yield varying degrees of stability.
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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.
Chromogenic dye color processes include Type "R" and process RA-4 (also known as "type C prints"), process C-41 color negatives. and process E-6 color reversal (Ektachrome) film. Chromogenic processes yield organic dyes that are less stable than silver, and can also leave unreacted dye couplers behind during developing. Both factors may lead to color changes over time. The three dyes (cyan, magenta, and yellow) which make up the print may fade at different rates, causing a color shift in the print. Modern chromogenic papers such as Kodak Endura have achieved excellent stability, however, and are rated for 100 years in home display.
Dye destruction prints are the most archival color prints, at least among the wet chemical processes, and arguably among all processes. The most well-known kind of dye destruction print is the Cibachrome, now known as Ilfochrome.
Some ink jet prints are now considered to have excellent stability, while others are not. Ink jet prints using dye-pigment mixtures are now common in photography, and often claim stability on par with chromogenic prints. However, these claims are based on accelerated aging studies rather than historical experience, because the technology is still relatively young.