PI and TWPI

Preservation Index: The "Richter Scale" for Natural Aging

Preservation index (PI) is a concept introduced by the Image Permanence Institute in 1995 to express the “preservation quality” of a storage environment for organic materials (carbon-containing materials like plant and animal products, plastics, paper, dyes, etc.). Such materials tend to spontaneously decay, a phenomenon often described as natural aging. Natural aging is a chemical process. Heat energy causes chemical reactions to go faster; therefore, temperature is the most important environmental factor in natural aging. Because many chemical decay reactions involve water, relative humidity (RH) is also very important. Together, the temperature and RH of a storage area determine how rapidly or slowly organic objects will deteriorate. Pollutants, light, and mishandling can speed the decay of organic materials, but every organic object will tend to spontaneously decompose, even in the absence of these factors.

Temperature and RH data for a hypothetical storage area, gathered over a 16-month period.

PI values, expressed in units of years, show us the combined effect of temperature and RH on the decay rate of vulnerable organic materials in collections and give us a general idea of how long it would take for them to become noticeably deteriorated, assuming that the temperature and RH did not change from the time of measurement onward.

PI for the same storage area, for the same period.

PI helps us to quantify how good or bad the environmental conditions are at that moment. If conditions change over time, the life expectancy of an object also changes. (This is where TWPI comes in.) PI is not meant as a predictor of the useful life of any particular object. It is simply a convenient measure of the effect of current environmental conditions on the overall life expectancy of a collection. PI is for natural aging what the Richter scale is for earthquakes: a way to communicate the magnitude of effect in quantitative terms.

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In designing the PI concept, IPI chose to use as a benchmark the approximate lifetime at room temperature of a typical “preservation problem object.” Rag paper, with its inherent long life at room temperature, is not a problem for most preservation managers. Acidic, wood-pulp paper, color photos and movies, nitrate and acetate film, herbarium specimens, and magnetic tape are all examples of preservation problem objects. They all deteriorate significantly in about 50 years at room temperature and moderate RH. PI values in years were designed so that the PI of 68°F (20°C), 45% RH is 50 years, to reflect the behavior of preservation problem objects. The PI of a storage condition represents how long it would take the typical preservation problem object to deteriorate significantly in that condition. (Note: “significant” deterioration means noticeable discoloration or embrittlement or other changes that involve a serious loss of appearance or functionality. It does not mean “crumbling to dust.”) If a storage condition has a PI of 100 years, it means that a preservation problem object like acidic paper would require 100 years to discolor to the same extent that it would in 50 years at room temperature and moderate RH. PI values can still be used in a purely relative sense. If a storage condition has a PI of 200 years, then organic materials would last four times longer in that particular condition than they would at room temperature where the PI is only 50 years. This relative use of PI applies to all organic materials. PI values have meaning in a relative sense for all organic objects, and have meaning in a literal sense as life expectancy values for short-lived preservation problem objects.

PI can be helpful in planning new storage areas, but its main practical use is for calculating TWPI, which is a kind of “average PI” for situations where temperature and RH vary over time.

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Time-Weighted Preservation Index (TWPI): A Cumulative Measurement

Nearly every storage environment is dynamic, changing with the weather, with the seasons, or by conscious actions taken to save money or to be more comfortable. It is difficult enough to know the effect of any given static condition on the decay rate of a collection; the total effect of changing conditions over time was impossible to obtain at all, until the conception of the time-weighted preservation index (TWPI). The TWPI makes it possible to measure the effects not of just one set of conditions but of fluctuating conditions, over a period of time, expressed as a single value. TWPI is an average of changing PI values over time. If PI values are obtained at regular time intervals, a relatively simple recursive calculation (one that is repeated again and again with new data) can produce a single number that accurately expresses the average rate of deterioration for the time period. This number is the TWPI. It represents the approximate length of time, in years, that vulnerable organic materials would last if every time period in the future were just like the one during which the TWPI value was measured. TWPI values can represent the cumulative effect of a week’s, a month’s, or several years’ worth of temperature and RH conditions.

TWPI for the storage area, for the same period.

PI and TWPI are based on the science of chemical kinetics, which deals with the rates of chemical reactions. For some time, preservation scientists have used the laws of chemical kinetics in formulating environmental analysis tools. The isoperm concept, invented by Donald Sebera, formerly of the Library of Congress, used accelerated-aging data and kinetics principles to apply relative “permanence-factor” rankings to different combinations of temperature and humidity. TWPI does likewise, but it goes further by allowing for life expectancy values expressed in years instead of relative factors. The main advantage of TWPI is its ability to condense a whole period of changing temperature and RH conditions into one bottom-line value by properly averaging or “weighting” how much each interval of time contributes to the decay rate overall.

PI and TWPI are covered in greater depth in New Tools for Preservation: Assessing Long-Term Environmental Effects on Library and Archives Collections.

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