Paper deterioration is still a problem, but thanks to years of scientific research by the library community and beyond, it is no longer a mystery.

Why is 500-year old paper often in better condition than paper from 50 years ago? In other words, what makes some papers deteriorate rapidly and other papers deteriorate slowly?

The rate and severity of deterioration result from internal and external factors: most importantly, the composition of the paper and the conditions under which the paper is stored.

Paper is made of cellulose -- a repeating chain of glucose molecules -- derived from plant cell walls. One measure of paper quality is how long the cellulose chains, and subsequently the paper fibers, are: long-fibered paper is stronger and more flexible and durable than short-fibered paper.

In the presence of moisture, acids from the environment (e.g., air pollution, poor-quality enclosures), or from within the paper (e.g., from the raw materials, manufacturing process, deterioration products), repeatedly cut the glucose chains into shorter lengths. This acid hydrolysis reaction produces more acids, feeding further, continued degradation.

Before the mid-19th century, western paper was made from cotton and linen clothing rags and by a process that largely preserved the long fibers of the raw material. While fibers may shorten with age, rag papers tend to remain strong and durable, especially if they have been stored properly in conditions not overly warm or humid.

Starting in the mid-19th century, wood replaced rags as the raw material for paper manufacture. Wood is processed into paper by mechanical or chemical pulping, which produces paper with shorter (compared with rag paper) fibers.

Mechanical pulping produces paper with the shortest fiber length and does not remove lignin from the wood, which promotes acid hydrolysis. Newspapers are printed on mechnically pulped paper. Chemical pulping removes lignin and does not cut up the cellulose chains as thoroughly as mechanical pulping, yielding a comparatively stronger paper, but which is still not as durable as rag paper.

Wood pulp paper from before the 1980s also tends to be acidic from alum-rosin sizing (added to the paper to reduce absorbency and minimize bleeding of inks), which, in the presence of moisture, generates sulfuric acid.

Acids also form in paper by the absorption of pollutants -- mainly sulfur and nitrogen oxides. Book leaves that are more brown and brittle along the edges than in the center clearly illustrate this absorption of pollutants from the air.

Research by the Library of Congress has demonstrated that cellulose itself generates acids as it ages, including formic, acetic, lactic, and oxalic acids. Measurable quantities of these acids were observed to form under ambient conditions within weeks of the paper's manufacture. Moreover, paper does not readily release these acids due to strong intermolecular bonding. This explains why pH neutral papers become increasingly acidic as they age.

Acids form in alkaline paper as well, but can be neutralized by the alkaline reserve.

Besides acid hydrolysis, paper is susceptible to photolytic (damage by light) and oxidative degradation.

Photodegradation appears to progress more severely and rapidly in poorer quality papers.

The role of oxidative degradation appears limited compared with acid hydrolysis, except in the presence of nitrogen oxide pollutants.

Generally speaking, good quality paper stored in good conditions (cooler temperatures; 30-40% relative humidity) are able to last a long time -- even hundreds of years.