The rate of decomposition of cellulose nitrate films roughly doubles with every increase of six degrees Centigrade, so the higher the storage temperature, the faster the film will deteriorate. Storage temperatures therefore need to be as cold as possible.
In addition, the [legacy-smartlink:relative humidity] must be also be low as practicable to slow down the deterioration of nitrate materials. At higher temperatures and humidities nitrogen dioxide gas is more quickly formed which reacts with water in the atmosphere and in the photographic emulsion to form nitric acid which will attack the film.
The FIAF Preservation Commission recommends:
|Storage temperature||4°C (39°F) +/- 1°C||l4°C (39°F) +/- 1°C|
|Relative humidity||50% +/- 2%||50% +/- 5 max.range 40 to 60%|
|Rate of fresh air in-take||The rate of fresh air in-take should allow the air to be gradually renewed over a period of five hours at 4°C. Note: The author (Henning Schou) has not been able to find any research material which confirms this (apparently) arbitrary figure. However, the following empirical guideline applies: if it is possible to smell the nitrate then air replenishment is insufficient.|
By reducing the storage temperature by 6°C, the production of nitrogen dioxide gas will be reduced by approximately 50 per cent, i.e., by lowering the temperature from 24°C to 4°C the amount of nitrogen dioxide is reduced to less than 1/10th of that at 24°C. The rate of fresh air in-take should be greatly increased if the temperature is allowed to rise.
Nitrate films must always be stored away from other materials because of the formation of harmful nitrogen dioxide during decomposition.
Safety motion-picture film on cellulose acetate base has been found chemically damaged on occasion by storage in the same can with unstable nitrate film. As a result, the effects on acetate film of the three nitrogen oxide gases produced by the decomposition of nitrate film have been investigated. Nitrous oxide and nitric oxide were found to be harmless, but nitrogen dioxide, even at low concentrations, is seriously damaging to safety film. This is to be expected, since nitrogen dioxide reacts with moisture to form nitrous and nitric acids.
Nitrogen dioxide is known to react with finely divided silver as follows:
2 NO2 + Ag -> NO + AgNO3
Nitrogen dioxide also reacts with water:
2 NO2 + H2O -> HNO3 + HNO2
Then nitric acid reacts with silver:
2HNO3 + 2Ag -> AgNO3 + AgNO2 + H2O
The action of nitric acid on silver is greatly accelerated by the presence of nitrous acid, which is formed as shown above.
The order in which the film components are attacked by nitrogen dioxide is: the silver or dye first, the gelatin of the emulsion second, and the acetate base last. The rate of attack increases with increase in relative humidity. It is concluded that safety films should never be stored in the same can with nitrate films and preferably not in the same room. 1
Cellulose nitrate contains chemically combined oxygen, sufficient in amount so that it can partially burn or decompose without the presence of air. The gasses formed during burning or decomposition are both toxic and flammable and can be produced so rapidly as to create dangerous pressures in building structures and severe hazard to life. Free burning results in the production of less toxic gases, but due to the rapid burning, such fires are intense and still represent a serious life hazard. The actual heat of combustion of cellulose nitrate film is 6000 Btu/lb to 8000 Btu/lb (13944 kJ/kg to 18900 kj/kg), compared with 7000 Btu/lb to 8000 Btu/lb for wood.
However, the rate of combustion is about fifteen times that of wood in similar form.
For more information on the safe storage of Cellulose Nitrate film refer to the National Fire Protection Association ANSI/NFPA 40 ‘The Standard for the Storage and Handling of Cellulose Nitrate Motion Picture Film’, 1994 Edition
1 Society of Motion Picture and Television Engineers (SMPTE) http://www.smpte.org/