Why Paper Turns Yellow With Age

An old newspaper left in the sun for even a single day can turn noticeably yellow, while a book printed on higher-quality paper can stay white for decades. The difference comes down to a substance called lignin, a natural compound found in wood that gives trees their structural rigidity. Most paper is made from wood pulp, and cheaper paper, like newsprint, is produced by grinding up whole wood fibers without removing most of the lignin they contain, since that extra processing step is expensive. Higher-quality paper, by contrast, goes through a chemical process that removes most of the lignin before the paper is ever made. Lignin itself is not yellow to begin with, but it's highly reactive to both oxygen and light. Over time, and especially when exposed to sunlight, the lignin molecules break down and rearrange into new chemical compounds called chromophores, which absorb light differently than the original lignin did and reflect back a yellowish color to the eye. This reaction happens continuously in paper that still contains lignin, but sunlight and heat both speed it up considerably, which is why a newspaper left in direct light yellows far faster than one stored in a dark drawer. Paper with the lignin removed during processing simply doesn't have the raw material needed for this reaction, which is why it resists yellowing for much longer.