ASPECTS OF CHEMICAL RESEARCH IN CONSERVATION: THE DETERIORATION PROCESS
ROBERT L. FELLER
6 EARLY DETECTION OF IMPENDING DETERIORATION
It is possible that the subtle processes taking place during the “apparent induction period” can be monitored successfully. In the case of rubber cement, it was shown that an increase in weight and in hydroperoxide concentration could be detected before serious discoloration took place (Feller and Enke 1987). Moreover, a “sticky” stage was also observed before serious loss of solubility took place. These observations held out the possibility that one could find and measure properties that were changing during a period when few or no visually observable changes were occurring. More recently, Shashoua et al. (1992) report that they are able to follow the early loss of nitro (NO2) groups relative to carbonyl (CO) in the infrared spectra of nitrocellulose adhesives that had been applied to objects over a period of 30 years. The loss of the camphor plasticizer can be similarly monitored (Derrick et al. 1993). There are also early stages in which odors are detectable or exudations observed. Perhaps one day these emissions can be precisely measured to monitor condition.
Studies regarding the deterioration of cellulose triacetate motion picture film, representing more than 30 man-years of effort at the Centre for Archival Polymeric Materials, Manchester, England, have confirmed that the process is autocatalytic and that it tends to exhibit an induction period. The Centre has since demonstrated that early changes within the film can be monitored by analysis of the moisture content by means of a “novel technique of microwave spectroscopy,” a nondestructive method (Edge et al. 1992, 147). This example and that of the cellulose nitrate studies are cited to indicate that progress is indeed currently being made in our ability to monitor the subtle changes that can take place in organic materials before serious physical deterioration sets in.