Ever wonder about the process that causes newspapers and old paperback books to turn yellow and brittle? Basically, a reaction called acid hydrolysis is responsible for the so-called “small fires” consuming your paper materials over time.
More detail from the Library of Congress:
“Fibers made of cellulose chains degrade when exposed to an acidic environment in the presence of moisture. In this acid hydrolysis reaction, cellulose chains are repeatedly split into smaller fragments so long as the source of acid remains in paper. This acid hydrolysis reaction produces more acid in the process, and the degradation accelerates in a downward spiral.
The longer the cellulose chains that comprise paper, the stronger and more supple the paper. This type of paper is also more able to withstand degradation by acids and other abuse without showing visible signs of wear and tear. Conversely, the shortest fibers are the most vulnerable.
Early papers were made from cotton and linen rags. Most early papers, especially those made up to the middle of the 19th century, are still strong and durable, especially if they were stored properly under conditions that were not overly warm or humid. Cotton papers owe their longevity mainly to the length of the fibers used in their manufacture. Even when the length of these fibers is reduced on aging, it is still likely to be longer than that of fibers in relatively young, modern papers.
The shortest fibers are found in newsprint papers made from groundwood pulps; this pulp is made by the mechanical grinding of wood that is then made into paper without first purifying it chemically. Papers made by this process are substantially weaker than those made of chemically purified wood pulp, which is used to make the fine printing and writing papers that we often see in books.
Most modern book papers have a relatively short life span, which can be further reduced by improper storage environments. The exception to this general trend is alkaline paper — that is, paper that contains an alkaline reserve. This alkaline reserve, most frequently chalk, neutralizes acids and also makes the paper look whiter. Like cotton papers, alkaline papers can last indefinitely. Acids formed within the papers or those absorbed from the environment are neutralized before they have a chance to degrade the cellulose chains. Such papers often bear a permanence mark (an infinity symbol within a circle).
The primary source of acid in modern paper is the alum-rosin sizing agent introduced in the manufacturing process. Size is added so that writing and printing inks do not feather. In the presence of moisture, the alum in the sizing agent generates sulfuric acid. Acids are also formed in paper by absorption of pollutants — mainly sulfur and nitrogen oxides. Newsprint paper is particularly vulnerable to pollutants, which it absorbs vigorously from the environment as evidenced by the brown and embrittled edges of bound newsprint volumes and dime novels.
A new discovery made in the research laboratories of the Library of Congress shows that, as it ages, cellulose itself generates several acids, such as formic, acetic, lactic, and oxalic acids. Measurable quantities of these acids were observed to form within weeks of the manufacture of paper while stored under ambient conditions. This research also shows that these acids continue to accumulate within paper as they attach themselves to paper through strong intermolecular bonds. This explains why acid-free (pH neutral) papers also become increasingly acidic as they age. Acids are formed even in alkaline paper, although in this case they are probably neutralized by the alkaline reserve before they can do any damage to the cellulose molecule.
In addition to acid hydrolysis, papers are also vulnerable to photolytic degradation (damage by light), although newsprint papers are much more subject to this form of degradation than most other papers used to print books.”
The photo you see in this post is one of many photos which have been stored improperly for about 40 years and have been severely degraded by this process of acid hydrolysis. Preservation Week (April 22-28) is approaching so keep an eye on this space for helpful tips on how to save your stuff from the ravages of time.