4 BASIC UNDERSTANDING OF PATTERNS OF DETERIORATION

The effort to establish general principles regarding the various ways in which the properties of materials can change over time has great significance to the practicing conservator. First of all, if one wishes to estimate the possible condition of an artifact some time in the future, it is important to understand whether the changes that have already taken place are likely to continue at a steady pace, apparently slow down, or seriously speed up in time (the latter is often labeled autocatalytic or autoaccelerating behavior). The degradation of cellulose triacetate movie film appears to be of the latter character (Allen et al. 1987); so does the deterioration of cellulose nitrate plastics (Shashoua et al. 1992; Derrick et al. 1993). Within a day of opening a vitrine in which cellulose nitrate materials have been kept in reasonably satisfactory condition for a number of years, serious fracturing can occur; the rapid rate of disintegration then becomes a matter requiring immediate attention (Derrick et al. 1993). The autocatalytic stage can be said to have begun with the opening of the vitrine. Research must attempt to answer many questions: Why do the physical changes occur? Can the behavior be successfully retarded? Can it be circumvented? Is it too late to carry out satisfactory treatment once a particular stage of deterioration has been reached?

The practical importance to the conservator in realizing that a particular deterioration process tends to speed up in time is to emphasize noticing the process and initiating treatment at the earliest possible stage, catching potential trouble at an early point in time. Moreover, when devising accelerated-aging tests to evaluate the potential stability of different types of materials, one must appreciate that it is fundamentally a difficult task to evaluate the relative merit of those that age in the manner of figure 2b versus 2c or 2d.