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The article reports on a study which reveals why old paper turns yellow. It says that "aldehydic" chromophores, or those with a carbon atom double-bonded to an oxygen atom and single-bonded to a hydrogen atom, cause paper yellowing. This was discovered by researchers who examined the wavelengths of light emitted by both old and new paper and compared them with signature wave lengths of different chromophores. It adds that this finding could help restorers to reverse yellowing of old paper.