Temperature and relative humidity are the most important rate determining factors in the life expectancy of motion picture film.
By knowing the rate at which a typical cellulose acetate film decomposes at a known temperature and relative humidity(RH), it is possible to calculate similar cellulose acetate film decomposition at differing temperatures and relative humidities.
This is possible, however, only if the conditions remain static. In reality, there are slight fluctuations in conditions in the vaults and major changes if a film is removed from storage for any significant length of time. These fluctuations are cumulative and can significantly reduce a film’s life expectancy.
Significantly, it only requires one factor to change, either temperature or RH, to have an effect on the rate of reaction for the film. Keeping track of these changes in conditions is achievable using automated dataloggers. However, interpreting the extent that these changes will have on a film is far more onerous.
The Image Permanence Institute, Rochester Institute of Technology, has developed a series of tools — the Preservation Index (PI) and Time Weighted Preservation Index (TWPI) — that can be used to analyse the storage conditions to predict the effectiveness of storage environments.
Underlying the Preservation Index approach is the concept that temperature and relative humidity act in concert to speed up or slow down the deterioration in all organic materials (e.g.). PI values represent the approximate length of time that any organic material would last in any constant combination of temperature and humidity. Last is used in the sense of time before any deterioration becomes noticeable. PI values are time predictions based on experimental data obtained under accelerated aging conditions. PI gives a quantitative evaluation of how the sets of conditions effect the rate of reaction of decomposition. An abbreviated PI Table for new film is given in Table 7.1.
Time in years
High risk of mould growth
Time Weighted Preservation Index
Time Weighted Preservation Index gives a numerical measure in years of the cumulative average taking into account the variance in temperature and relative humidity. To obtain an accurate picture long term readings of storage conditions that allow for seasonal variations or other cycles to be included need to be gathered. TWPI reflects the fact that deterioration proceeds faster under some conditions than others. This prevents simply averaging the PI values to obtain an answer.
To calculate correctly a TWPI for changing conditions a greater weighting needs to be given to warmer and damper periods than the cooler and drier cycles. The time spent under bad conditions shortens the life of collection items much more than time spent under good conditions may extend their life. Doug Nishimura at the Image Permanence Institute has developed an equation that can be used to calculate the TWPI for a collection.
n = total number of time intervals
TWPIn-1 = TWPI after time interval n-1
PIn = PI measured at time interval n