Why Paper Yellows As It Ages - Scientific American

April 17, 2012

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Why Paper Yellows as It Ages

As paper ages, oxidation changes molecules in cellulose so that they absorb some wavelengths of light and make the paper look yellow. Sophie Bushwick reports

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As it ages, white paper turns a distinctive yellow. But why? To find out, scientists artificially aged modern paper to reveal the changes on the molecular level. The research is in the journal Physical Review Letters. [A. Mosca Conte et al., "Role of Cellulose Oxidation in the Yellowing of Ancient Paper"]

For 48 days, three unbleached paper samples aged rapidly in reactors that simulated different environmental conditions. The researchers then compared the artificially aged samples to the real deal: three pieces of paper crafted in 15th-century Europe. This technique allowed them to gauge the types and amounts of changes going on.

About 90 percent of the weight of old paper is cellulose, the sturdy material that makes up plant cell walls. But over time cellulose fibers oxidize. The process modifies parts of various molecules and turns them into what are called chromophores, which absorb light.

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White paper is white because it reflects all colors of light. Aging paper filled with chromophores reflects wavelengths that make it look yellow. The nondestructive technique used in this study could inform research to preserve and even whiten ancient texts and art. And give paper conservation a brighter outlook.

—Sophie Bushwick

[The above text is a transcript of this podcast.]

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