The Deterioration and Preservation of Paper: Some Essential Facts
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. The preservation strategy for paper materials at the Library of Congress continues to evolve as our scientific understanding of deterioration mechanisms has progressed.
Factors that Promote Paper Deterioration
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.
Accelerated or Artificial Aging Tests
One can readily observe that papers from hundreds of years ago are often still in good condition and that papers from not even 30 years ago are often brittle or perhaps even unusable. So as not to rely on anecdotal observation, scientists have developed experimental methods known as accelerated or artificial aging to collect data over weeks or months on how materials may age over years or decades.
- Artificial or accelerated aging tests for paper is continually evolving and is viewed as more or less meaningful, depending on the test parameters and what information is sought from the data.
- The U.S. National Bureau of Standards, now the National Institute of Science and Technology, carried out early accelerated aging tests on paper in the 1920s-1930s, which involved inducing aging with elevated temperatures. In the 1940s, William Barrow used elevated temperature aging methods to project lifetimes of paper samples. Barrow's predictions have since been proven to be erroneous and illustrate the importance of the test parameters and of framing questions appropriate to the test design.
- Today, artificial aging tests combine elevated temperatures and elevated humidity, recognizing the essential role that moisture plays in the acid hydrolysis of paper.
- A five-year research effort completed in 2000 at the Library of Congress showed similar chemical products formed during natural and accelerated aging of paper. Moreover, when naturally-aged papers were subsequently subjected to accelerated aging in the laboratory, the degradation products from natural aging simply increased in concentration by predictable proportions; no new degradation products were identified. These results suggest that well designed artificial aging tests can provide a good approximation of natural aging and is a valid and reliable test for predicting paper longevity.
- Artificial aging tests show that the rate at which paper degrades increases with time as acidic degradation products accumulate in the paper.
Other Experiments and Findings
- Experiments with cotton rag papers (see Henk Porck, "Rate of Paper Degradation") have found that the center of the leaf in a book tends to be weaker than the edges and that paper bound in books ages faster than loose single sheets of paper. (Not to be confused with the observation of brittle edges that result when the paper absorbs, starting at the edges, industrial pollutants from the air, which catalyze acid hydrolysis.)
- In other words, the moisture contained within the paper contributes to acid hydrolysis and paper deterioration. In an closed airtight environment, the paper retains the acidic degradation products, which accelerate aging.
Applying the Scientific Findings to Library Practice
- In the 1980s, paper manufacturers began adding alkaline buffers to wood pulp papers intended for lasting use and today this is common practice. Alkaline buffers retard or prevent acid hydrolysis by neutralizing acids that attack the cellulose chains. Alkaline wood pulp papers stored under good conditions are long lasting. Since the 1990s, books published in the U.S. that conform to ANSI/NISO paper permanence standards (e.g., ANSI/NISO Z39.48 - 1992) are likely to be printed on chemically purified wood pulp alkaline paper.
- Alkaline papers made of chemically purified wood pulp (high alpha cellulose) mixed with cotton are also available and are recommended for permanent documents.
- The useful life of retrospective collections on acidic paper, but which are still serviceable (i.e., not yet brittle), can be extended many times by neutralizing or removing the acids in the paper.
- Neutralizing the acids with an alkalizing agent is an approach that can be scaled to address millions of items (mass deacidification) and is an option for loose sheets as well as for bound items.
- If mass deacidification treatment is carried out while the paper still has significant measurable strength, and the treated items are then stored under proper conditions, these once-acidic items are projected to remain in usable condition for several centuries, rather than becoming brittle and unusable in only fifty to a hundred years.
- Improving environmental storage conditions, regardless of whether the item can be deacidified, will also significantly slow the rate of degradation and extend the useful life of paper items.
References
Shahani, C. J., S. B. Lee, F. H. Hengemihle, G. Harrison, P. Song, M. L. Sierra, C. C. Ryan, and N. Weberg. "Accelerated Aging of Paper." In ASTM Paper Aging Research Program, ASTM Research Report (RR:D06-1004), available as Adjunct to D6789 Standard Test Method for Accelerated Light Aging of Printing and Writing Paper by Xenon-Arc Exposure Apparatus. West Conshohocken, PA: ASTM International, 2002.
Shahani, C.J., and G. Harrison. "Spontaneous Formation of Acids in the Natural Aging of Paper." In Works of Art on Paper: Books, Documents and Photographs. Edited by V. Daniels, A. Donnithorne, and P. Smith, 189-192. London: International Institute for Conservation of Historic and Artistic Works, 2002.
