Accelerated Aging of Paper: Can it Really Foretell the Permanence of Paper
Introduction
After more than a century of scientific investigation into the permanence, or lack thereof, of paper (Kantrowitz, 1940; Fellers, 1989), this field of inquiry is more fertile than ever. We, and our predecessors before us, have done our utmost, and we still wonder about exactly how paper ages. This is a tribute to the complexity of paper and the everchanging, constantly advancing technology of its manufacture.
Many issues remain to be resolved, but few merit as much concern as the development of a consensus around a sound and practical accelerated aging methodology. Such a development would in itself serve to resolve many other long-standing issues.
The present NISO, as well as ISO standards for permanent paper are heavily weighted towards specifying the composition of paper, rather than its performance (NISO, 1992; ISO, 1994). While the recent revision of ISO 9706 improved upon the NISO permanent paper standard by incorporating an accelerated aging test, it did not do so to replace any of the composition related requirements.
Composition-based standards leave much to be desired. From a fundamental perspective, the consumer, whether a librarian, an archivist or a conservator, is ill-equipped to tell the paper maker how to make paper. However, the consumer does know the end qualities and performance he/she would like to see in the product. The real problem in the case of permanent paper standards is that the consumer does not have a credible set of tests for quality control on which he/she can depend. That is why the consumer is forced to depend on the composition of the product, for which presumably adequate test controls are available. However, a composition based set of requirements is always less efficient and more vulnerable than a performance-based standard.
Consider, for example, the NISO standard, which mainly requires:
- a minimum alkaline reserve of 2% calcium carbonate equivalents,
- a maximum limit of 1% on lignin content and
- minimal tear resistance, which is adequate enough to allow the printing of paper.
A paper product can meet all of these requirements, and still not be permanent if it were contaminated with significant concentrations of copper, iron or other oxidative catalysts, free radical chain initiators or oxidants. Any of these contaminants would have the potential to accelerate the degradation of paper in spite of the presence of the specified alkaline reserve. Even if other testing requirements were to be introduced to cover this gaping hole in the standard, the consumer would still be none the wiser regarding the effects of any number of paper chemicals and additives that could have been introduced in its manufacture, or in the processing of the recycled content.
Not only does this composition-based standard allow the possibility of an unstable paper being passed off as permanent, but more importantly, it excludes truly permanent papers, such as a good rag paper, which may last indefinitely even if it lacked an adequate alkaline reserve. Let me hasten to add that the NISO Committee that revised the standard recently, was well aware of these problems, but had little choice. Therefore, the only real solution that would be in the best interests of the consumer and the manufacturer alike, is to develop an accelerated aging test, or a set of such tests, that will distinguish between permanent and impermanent paper in a reliable, reproducible and efficient manner. But that seems like a tall order today.
Protestations against accelerated aging are not hard to come by in the literature on paper permanence and in personal communication among scientists and conservators. A few of the more recent, as well as eloquent works are cited here.
Bansa and Hofer subjected a naturally aged paper to further aging at temperatures ranging from 50 to 95°C (Bansa, 1989). Their work convinced them that there was no correlation between natural and accelerated aging. They suggest that if at all we must depend on artificial aging, it must be at 80°C and 65% RH.
In a more recent work, Bansa expresses further frustration with accelerated aging techniques (Bansa, 1992). With the aid of extensive accelerated aging data, he has attempted vainly to find reproducible degradation patterns with different papers. He is convinced that accelerated aging experiments can lead to conclusions that may not only be doubtful, but can even be "deceitful". He points to several findings from his data that perplex him: These include the lack of any predictability in the observed decay patterns (some are curved upwards, others downwards, while some remain flat), the unexplained stability of a paper sample with a pH of about 5, and also the stability exhibited by alkaline groundwood papers.
Stroefer-Hua (1990) has presented a thought-provoking examination of basic assumptions on which accelerated aging experiments are generally based, and questions their validity. He believes that Arrhenius plots for the aging of paper are not straight lines, and therefore the mechanism by which paper degrades at higher temperatures is probably quite different from that which prevails under ambient conditions. For these reasons, accelerated aging of paper seems nothing more than a futile exercise to him, and he finds that even relative inferences drawn from such tests may not have any merit. He presents a thesis that each paper has a unique history that determines how that paper will age in the future, and accelerated aging experiments cannot predict that pattern.
Regardless of rampant skepticism in accelerated aging techniques, there is no substitute for the process in the research laboratory. Scientists engaged in research on the permanence of paper have no choice but to employ one or more of the artificial aging techniques presently available, and to temper their observations from such tests with caution. It behooves us therefore, to look a little deeper into one of our best tools and hone it as best as we can. Any efforts in this direction are bound to be richly rewarding.
Table of Contents - Introduction - Status of Accelerated Aging of Paper - Research in Accelerated Aging of Paper - Comparison of Accelerated Aging of Paper in Stacks and Sheets - Aging of Paper Sealed within Polyester Film - Inadequacy of Single Sheet Accelerated Aging Methods - Accelerated Aging within Sealed Enclosures - Comparison of Accelerated Aging Methods - Accelerated Aging under Light -Measurement of Rates of Degradation - Conclusion - References - Supporting Documents