Why do old photographs turn yellow?

Traditional photographic prints contain metallic silver that forms the image. Over decades, sulphur compounds in the air, leftover fixer chemicals and acids in the paper slowly convert some of the silver into yellow or brown silver sulphide. Albumen prints, popular in the 1800s, are especially prone because egg-white albumin yellows with age. Heat, humidity and light accelerate the process. Colour photos from the mid-20th century can also shift toward yellow or magenta as the organic dyes in their layers fade at different rates. Proper archival storage in cool, dry, acid-free conditions slows degradation.