What causes paper to yellow as it ages?

A great deal of our cultural history has been preserved on paper. However, this heritage faces inevitable damage due to the passing of time. As the centuries pass, paper must be kept under ideal conditions in terms of humidity and sunlight to prevent its yellowing and cracking. Dr. Adriano Mosca Conte of the University of Rome Tor Vergata and collaborators began a search to identify what molecular structures arise in paper that contribute to its yellowing. They write about their results in Physical Review Letters for April 9, 2012. With the knowledge gained in their study, the process used to preserve ancient manuscripts gets a boost.

The oldest surviving examples of paper originate in China in the 2nd century B.C. The treatment of plant material to create paper is believed to have originated in that region. From there, it spread through the Middle East and eventually found its way to Europe by the 13th century. The cheap mass production of paper during the 19th century substantially increased literacy rates in regions participating in the Industrial Revolution and, it could be argued, form the basis of our educated society.

Paper in good condition is primarily composed of cellulose, whose molecular structure consists of a long chain of carbon, hydrogen, and oxygen. These fibers are typically around a micrometer (0.0001 centimeters) long and wrap around each other to create paper. Cellulose forms the structure of cell walls in plants making it a perfect ingredient for canvas material.

However, the structure of cellulose breaks down over time by interacting with oxygen in the atmosphere. Oxidation, the loss of electrons through interaction with an oxidizing agent – oxygen in this case – is a common form of material corruption.

Fire and rust are other examples of oxidizing reactions, and oxidation of cellulose is not as well understood as these more common examples. In particular, it is not well understood what the exact products of this reaction are, i.e. what paper turns into when it degrades in this fashion. Cellulose breaks down, via oxidation, to molecular structures generally known as chromophores. Chromophore, however, is just a generic term referring to the portion of a molecule which can emit or absorb visible light; that’s why paper turns yellow when it ages. The exact chemical structure was not known until Conte’s work.

Conte and crew studied the light absorption properties of healthy cellulose versus that in degraded paper in order to ascertain what chemical structures are present. The two states of paper show markedly different light absorption bands, pointing to the different molecular structures present in the different paper states. By matching the observed absorption bands with calculated models, they were able to identify which hydrocarbon chains are responsible for damaging paper.

The products of the oxidation reaction are simply rearrangements of the hydrogen, oxygen, and carbon atoms to form different chemical bonds. By sampling manuscripts from 15th century France and Italy, Conte and his team found that cellulose from this era mostly broke down to Carbon-Hydrogen-Oxygen chains belonging to the aldehydic group. See picture. With this knowledge, it is possible to devise chemical treatments to preserve paper by preventing these degradation channels. This experiment also provided a non-destructive method of ascertaining the chemical composition of the paper samples.

Bottom line: Dr. Adriano Mosca Conte of the University of Rome Tor Vergata and collaborators conducted a study whose goal was to identify the molecular structures that cause yellowing in aging paper. Writing in Physical Review Letters for April 9, 2012, they describe sampling manuscripts from 15th century France and Italy and their subsequent discovery that cellulose from this era mostly broke down to Carbon-Hydrogen-Oxygen chains belonging to the aldehydic group. Their hope is that, once the correct molecular structures are identified, researchers will also find appropriate chemical treatments that can be applied to aging paper to prevent its further change of state.