Isoperms–Introduction

The challenges faced by the preservation administrator of a research library or archive in choosing among preservation alternatives are daunting. Typically faced with a varied collection and very limited budget, an administrator must make decisions and allocate resources among alternatives ranging from preservation microfilming to engineering a preservation environment of controlled temperature and relative humidity. Adding to the complexity and uncertainty of making these preservation decisions are nascent or near-future processes such as deacidification and paper strengthening as well as rapidly evolving technologies such as optical digital storage and format transfer that promise enhanced access. Any means of quantifying the preservation outcome of employing even one of these approaches would provide a powerful tool for the development of an overall preservation strategy. This paper describes one such quantitative tool–the isoperm method–that quantifies the effect of the environmental factors of temperature (T) and percent relative humidity (%RH) upon the anticipated useful life expectancy of paper-based collections.

This isoperm method provides ready answers to a large variety of questions related to environmental conditions such as: How much longer can I expect my collection to be preserved if I follow the recommendations of my conservation staff to change from the present storage conditions of 73 degrees and 50% RH to 60 degrees F and 30% RH? What are the preservation consequences of allowing wider swings in temperature and percent relative humidity about the established set point conditions, since my HVAC (heating, ventilating, and air conditioning) consultant says tight limits are expensive in both capital and operating costs? Should I cycle between summer and winter storage conditions; if so, what risks and advantages are entailed? How can I convincingly show to curators, directors and funding agencies the preservation gains expected to result from modification and improvement of existing or construction of new storage facilities?

The basic theory underlying the isoperm method has been described in detail elsewhere (see Sebera in “Readings and References”). This publication is intended to serve as an aid in understanding the concepts so they may be used with greater confidence. My experience in presenting the isoperm concept has been that at first reading the science and mathematics appear complex and often intimidating. I urge continuing on to the applications where all the difficulties will disappear!

Finally, it should be noted that with appropriate modifications the method can be applied to other hygroscopic materials such as textiles, parchment, and so forth. In fact, it already has been extended to the preservation of film negatives. (IPI Storage Guide for Acetate Film, Image Permanence Institute, Rochester Institute of Technology, 70 Lomb Memorial Drive, Rochester, NY 14623-5604)