Self-portrait of Leonardi da Vinci, Red chalk/ Wikipedia

ScienceShot: Why Old Paper Turns Yellow

Possibly the only formal self-portrait of Leonardo da Vinci resides in the Royal Library of Turin, Italy. Some of its details are obscured, however, thanks to 500 years of paper yellowing. Scientists have long known that such yellowing can stem from cellulose—the main component of old, handmade paper—which oxidizes over time to develop colored molecular structures known as chromophores. Until now, however, researchers haven't identified which chromophores are responsible. In a paper due to be published this month in Physical Review Letters, researchers examined the wavelengths of light emitted by both old and new paper, and compared them with signature wavelengths of different chromophores. The results reveal that "aldehydic" chromophores—those with a carbon atom double-bonded to an oxygen atom and single-bonded to a hydrogen atom—are the most prominent paper yellowers. The researchers believe the findings could help restorers to reverse yellowing, perhaps with the use of "reducing" chemicals that make the chromophores revert to their nonoxidized state. But the team warns that such chemicals also risk damaging valuable artifacts. So it might be best to leave Leonardo's self-portrait alone.

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