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As temperatures drop the movement of molecules slows down and lowers the rate at which chemical reactions can take place.
The life expectancy of a film can be lengthened by lowering the temperature under which the film is stored. In 1889, Savante Arrhenius showed that the rate constant of a reaction was dependant upon temperature.
As temperatures drop, the movement of molecules slows down and lowers the rate at which chemical reactions can take place, reducing the rate of the decomposition reaction. In practical terms the rate approximately halves for every 5°C drop in temperature and conversely doubles for a 5°C increase.
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. 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.
Time in years
High risk of mould growth
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
The ability of air to 'hold’ moisture changes according to temperature, with warmer air able to hold more water than cooler air, measured as % Relative Humidity (%RH). The temperature at which the air becomes saturated and no longer able to hold any more water is known as 'dew point’. The dew point temperature relates to the air temperature and the absolute moisture content. Air with a high temperature and high moisture content will have higher dew point temperatures than air at low temperature and low moisture content.
When moving a film from a low temperature controlled environment to a warmer uncontrolled environment there is a risk of condensation on the film’s surface. Condensation will form on any surface that is colder than the dew point. A few common temperature and relative humidity conditions and the approximate dew point for each is given in Table 7.2. For determing exact values the Dew Point Calculator can be used.
Equilibration will take place between the films’ internal moisture content with the water in the atmosphere. This will take quite a long time depending on the initial water content of the film, temperature and the % Relative Humidity (%RH). However thermal equilibration is quite rapid and may only take an hour or two to be substantially achieved.
|Temp °C||Relative Humidity||Dew Point °C|
Table 7.2: Approximate dew points at common temperature RH conditions
As can be seen the risk is greater at higher temperature and RH conditions. If the vault temperature is above the dew point then there is little, if any, need for any acclimatisation. However, if the vault is below the dew point then some period of time should be allowed for the film to thermally acclimatise before opening the can.
Non-vented film cans restrict the flow of air quite significantly but provide very little in the way of thermal insulation, irrespective of whether the can is made from plastic or metal. What this means is that as long as the film can is not opened the acclimatisation time necessary for a film coming out of a controlled environment vault needs not be too long, just sufficient to raise the temperature of the film above the dew point.
As has been stated film has a very low specific heat and will thermally equilibrate quite rapidly. Table 7.3 tabulates some warm up times for 35mm motion picture film, the temperature differential was -16°C to 21°C.
|Enclosure||Length||Warm-up times in minutes to achieve various % thermal equilibration|
|Vented plastic can||1000’||40||60||85||120||165|
Table 7.3: Thermal equilibration – Bigourdan, JL and Reilly, J Environment and Enclosures in Film Preservation
Moisture equilibration takes considerably longer and may take several weeks to achieve equilibration. This may create an interesting situation in materials being initially placed or returned to controlled storage. The water content of an object equilibrated at a set temperature and relative humidity if moved to a different temperature will behave independently of external relative humidity until moisture equilibration is reached.
Below freezing point the moisture content of emulsion gelatin does not freeze, even at temperatures as low as -60°C.
Fig 7.1: Temperature, %RH and Equilibrium Moisture Content
Bigourdan, JL and Reilly, J Environment and Enclosures in Film Preservation
In uncontrolled cold storage below 0°C, the %RH may be in the region of 60-70%. Assuming conditions of -4°C and 60%RH the emc is approximately 4% by weight. This is equivalent to the emc at 20°C 73%RH. Since photographic materials can absorb more moisture at colder temperatures the risk of water damage as the film is thawed increases. If the cold storage environment has a very high %RH then it is conceivable that the gelatin could swell sufficiently to ferrotype or adhere to adjacent layers. In extreme cases the gelatin could move beyond its glass transition point.
A desiccant is a drying substance or agent. Zeolites, also known as 'molecular sieves’, are passive sorbents that can absorb decomposition products from within the film can microenvironment. By absorbing the decomposition products from the air inside the can, diffusion of the decomposition products from within the film will increase. This has the beneficial effect of lowering the level of decomposition acids trapped in the film, which in turn has a slowing effect on the rate of the decomposition reaction.
Fig 7.2 Typical zeolite structure
Molecular sieves are slow to absorb the decomposition products. However, molecular sieves also provide a desiccant action that will quickly reduce the %RH inside the film can microenvironment, reducing the amount of water available for the decomposition reaction and biological factors. Other desiccants are also valuable in the preservation of film. The main problem is determining when the desiccant is saturated. Silica gel often has an indicator built in that changes colour from blue to pink when the gel has absorbed all the water it can. Some desiccants, like silica gel, can be regenerated by heating at around 100°C.
Packaging of the desiccant is important. The packaging must be robust enough to contain the desiccant and prevent dust from the desiccant damaging the film. The packing should not harm the film, which rules out most paper materials unless they do not contain lignin or other sources of acid. Terylene fabric, which is just polyester under a commercial name, is suitable, but sometimes the effort involved in saving a few dollars can actually cost more than an off-the-shelf solution!