- Film Preservation Handbook Contents
- Film Construction
- Base Polymers & Decomposition
- Gelatin
- Image Forming Materials
- Damage to Films
- Cold Storage of Film
- Preparation for Long Term Storage
- Occupational Health & Safety
- Condition Reporting
- Film Identification & Handling
- Film Repair
- Conservation Treatments
- Photographic Duplication
- Disaster Planning
Cold Storage of Film
Cold Storage
It is known that 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, it is possible to calculate how long a similar cellulose acetate film will take to reach a similar level of decomposition at differing temperatures and relative humidities. This is possible 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 access purposes for any significant length of time. These fluctuations are cumulative and can significantly alter the life expectancy downwards.
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.
Preservation Index
The Image Permanence Institute, Rochester Institute of Technology, has developed a series of tools, the Preservation Index and Time Weighted Preservation Index, that can be used to analyse the storage conditions to predict the effectiveness of storage environments.
Underlying the Preservation Index (PI) 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. cellulose acetate film). 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.
| %RH | Temperature C o | ||||||
|---|---|---|---|---|---|---|---|
| 2 | 7 | 13 | 18 | 24 | 29 | 35 | |
| 20 | 1 250 | 600 | 250 | 125 | 60 | 30 | 16 |
| 30 | 900 | 400 | 200 | 90 | 45 | 25 | 12 |
| 40 | 700 | 300 | 150 150 | 70 | 35 | 18 | 10 |
| 50 | 500 | 250 | 100 | 50 | 25 | 14 | 7 |
| 60 | 350 | 175 | 80 | 40 | 20 | 11 | 6 |
| 70 | 250 | 125 | 60 | 30 | 16 | 9 | 5 |
| 80 | 200 | 100 | 50 | 25 | 13 | 7 | 4 |
Time Weighted Preservation Index
Time Weighted Preservation Index (TWPI) 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 used to calculate the TWPI for a collection.
 |
where: | |
| n | total number of time intervals | |
| TWPIn-1 | TWPI after time interval n-1 | |
| PIn | PI measured at time interval n |
