# Synopsis: Only on Paper

A study of ancient and modern papers reveals how cellulose reacts with light to discolor documents and art over time.

Though books and pamphlets written centuries ago survive, we know that paper degrades over time due to physical damage, exposure to humidity and sunlight, and biological agents, such as termites. Any advance that mitigates this damage is a boon to archivists and librarians. Writing in Physical Review Letters, Adriano Mosca Conte at the University of Rome “Tor Vergata” and colleagues have identified the molecular structures in paper they believe cause yellowing.

Most old paper, produced from cotton or linen, is over $90%$ cellulose in weight. When paper decomposes, it yellows because of an oxidizing process in which cellulose fibers develop light-absorbing molecules, called chromophores. This chemical change is complex, particularly since ultraviolet rays affect ancient paper differently than modern paper.

Conte et al. approached the problem by comparing oxidation in three types of paper produced in Europe in the 15th century to three modern unbleached samples from the Netherlands. The latter were artificially aged, with a 48-day stay in a reactor that substituted for the unknowable ravages of time and conditions. The controlled setup, aided by density-functional calculations, allowed the team to identify the chromophores that likely yellowed the ancient samples. Their diagnostic technique could lead to improvements in the conservation of fragile and ancient paper. – Sami Mitra

Corrections (10 April 2012): The original version incorrectly stated that cellulose fibers act as chromophores and that the team used a “a couple,” instead of three, modern paper samples.

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