Origins Of Paper Discoloration

Cellulose oxidation takes the blame for the unsightly yellowing of ancient paper, according to a study published in Physical Review Letters that identifies some of the key functional groups responsible for the discoloration (DOI: 10.1103/PhysRevLett.108.158301). By weight, ancient paper typically consists of more than 90% cellulose, which is made up of crystalline bundles of polysaccharide chains, each of which is built from hundreds to thousands of glucose molecules. To identify the chromophoric groups linked to paper discoloration, physicist Adriano M. Conte of the University of Rome, Tor Vergata, and colleagues compared samples of 15th-century French and Italian paper with modern reference samples that had been aged at elevated temperature and various levels of humidity. The group measured ultraviolet-visible reflectance spectra and analyzed the results through quantum mechanical calculations. They found that most of the yellowing was due to glucose oxidation that formed carbonyl units adjacent to aldehyde groups, ketone units, and conjugated diketone groups. The analysis method could serve as a nondestructive diagnostic tool to validate chemical methods to restore ancient art and paper, the group suggests.