Why are old photographs often yellow or sepia?

Many older photographs were made with chemical processes that are less stable than modern digital files. Over time, exposure to light, humidity, oxygen, and pollutants can alter dyes, silver compounds, or paper fibers. That degradation may cause fading, yellowing, or a brownish sepia tone. Some photos were intentionally printed with sepia toning because it improved archival stability compared with plain black-and-white prints. Proper storage in cool, dark, dry conditions can slow the aging process significantly.