The usual approach to evaluating the preservation quality of storage areas is to set specific targets for climate control, monitor the actual climate conditions, and act to correct any divergence from the targets. However, interpretation of climate monitoring data can be difficult, and correcting divergence from target values can be challenging.
Preservation professionals are increasingly arguing for a different approach. Rather than considering the climate in terms of a right or wrong set point, collections managers should determine what the climate is in each storage area (via monitoring), evaluate the effect of those conditions on the collections, and make changes or redistribute collections accordingly.
Evaluating the specific effect on collections of a particular storage area can be very difficult, however. For example, how do you determine the overall effect on collections of a storage area that cycles between 70°F and 80°F every day? How do you decide whether a storage area that is consistently 75°F and 40% RH is better or worse than one that is consistently 65°F and 50% RH?
Research conducted over the last few years by the Image Permanence Institute (IPI) has focused on answering these types of questions, and on evaluating the risks or benefits to collections of individual sets of environmental conditions.
To evaluate the risks associated with specific environmental conditions, IPI has developed two tools: the Preservation Index (PI) and the Time-Weighted Preservation Index (TWPI). The PI provides a comparative measurement of how long vulnerable organic materials (e.g., those with short life spans, such as magnetic tape or newsprint) might survive under various environmental conditions. The baseline for this measurement is 68°F and 45% RH, which results in a PI of 50 years.
As the temperature and humidity are raised and lowered, the PI changes (for example, lowering the temperature to 60°F and the humidity to 35% results in a PI of 110, while raising the temperature to 75°F and the humidity to 65% results in a PI of 18). It is important to remember, however, that this is a comparative measurement, not a literal measurement of years. The numbers are meant to illustrate how different storage environments can slow down or speed up deterioration.
In real storage areas, however, temperature and relative humidity do not remain the same from day to day, or even throughout a single day. Therefore IPI developed the Time-Weighted Preservation Index (TWPI), which takes into account changing conditions over time, as well as the cumulative effect of deterioration, for a particular storage area. The resulting measurement is called the TWPI.
See IPI's Step-By-Step Workbook: Achieving a Preservation Environment for Collections (PDF) for a more detailed explanation of PI and TWPI.
IPI has also developed a specialized datalogger (the Preservation Environment Monitor) and associated software (Climate Notebook) specifically for cultural institutions. These tools record data and calculate PI and TWPI, providing an additional level of data analysis.
The TWPI is not just an average of the PI values in a storage area, but also takes into account that time spent under poor climate conditions has more of an effect on the lifespan of collections than time spent under good conditions.