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|
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|
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.
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.
Bigourdan, JL and Reilly, J Environment and Enclosures in Film Preservation