Deterioration of paper occurs by a variety of mechanisms and paths, one or more of which may predominate under specific circumstances. In a truly comprehensive approach, we must consider not only chemical deterioration but also biologically-induced degradation as well as physically-induced loss of strength resulting from handling and use at various frequencies and “intensity.” The isoperm method is largely restricted to strength loss associated with the chemical reactions of cellulose hydrolysis and oxidation; conventional wisdom ascribed 90% or more of deterioration of paper to these two mechanisms.

Basically, the isoperm method arises from one idea: the rate of deterioration of hygroscopic materials such as paper is influenced by the temperature and percent relative humidity of its surrounding environment. The strength loss of paper resulting from the most common and important modes of chemically-induced degradation increases with increased temperature and moisture content. Conversely, lowering either or both temperature and moisture content of the paper reduces its rate of chemical deterioration and so increases the paper permanence. The isoperm method combines and quantifies the preservation effects of the two environmental factors, temperature and percent relative humidity, and presents the results in a readily comprehensible and usable graphical form.

What is at first sometimes confusing but is essential to an understanding of isoperms is that *relative* rather than *absolute* rates of deterioration (and paper permanence) are employed. That is, if r1 and r2 are the (absolute) rates of deterioration of a specific paper under two sets of temperature and relative humidity conditions we shall not deal with r1 and r2 individually but only with their ratio r2/r1 which measures the relative change in the deterioration rate resulting from the change in environmental conditions. To illustrate, suppose a certain decrease in temperature and/or relative humidity results in the initial deterioration rate, r1, dropping to a new lower rate, r2, such that r2/r1=0.5. This ratio carries the implication that all papers subjected to this change in environmental conditions will have their rates of deterioration cut in half. The rate reduction would be the same twofold value irrespective if a paper was short or long-lived. A paper which, for example, reached a given state of embrittlement in 45 years under the initial set of conditions would, because its rate of deterioration was halved, attain the same state of brittleness in 90 years under the new conditions. Similarly a paper with a 200 year life expectancy would see its permanence extended to 400 years.

It is the deterioration rate ratio that the preservation manager can control through changes in the temperature and percent relative humidity of the collection storage areas; it is not possible to change non-environmental factors such as fiber type, fiber length, degree of heating of the pulp, basis weight, thickness, and so forth, all of which influence the absolute rate of deterioration of a given paper.

The time a paper takes to reach a given level of residual strength is, of course, inversely related to the rate of deterioration. This life expectancy, which we shall refer to as permanence, rather than the deterioration rate, is the more apt and useful term to describe the effect of environment. The ratio of the two permanence values, i.e., the relative permanence, is mathematically the inverse of the deterioration rate ratio:

**Erratum:***The following formula corrects an error in the printed edition.*

p2=1=r1 __ _____ __ P1 r2/r1 r2 (1)

Here, as in the earlier illustrations, a precise definition of permanence has not been given; it is only essential to understand that it denotes the time required to attain some specific state of deterioration or residual strength. In other contexts it has been found useful to define permanence as the time required for a paper to drop to a strength of 1 MIT double fold at a 0.5 kg (kilogram) load, but other measures may be used.