Nitrate

The first successful, flexible and transparent film base. First made in 1889, nitrate (CNO3) was very satisfactory as a base for cinema film because of its toughness, and remained in use for 35 mm cinema film until the early 1950s. Nitrate sheet films (4×5, 8×10, etc.) were used until early 1940s.

Cellulose nitrate became the first film support because it was the oldest known and best understood plastic; it was also relatively easy to make and well researched due to its use in smokeless gunpowder. The fire hazards in the handling of this material were, and still are, considerable. It has now been replaced by various types of low inflammability safety base 1. These have included cellulose diacetate, triacetate, and polyester.

The problems with nitrate

The problems with nitrate film base were twofold: extreme flammability, and chemical instability. It was primarily the former and not the latter that provided the impulse for change in photographic film base. Cellulose nitrate is manufactured by the nitration of cotton linters with mixtures of nitric and sulfuric acids. By adjusting the ratio of nitric and sulfuric acids products of different degrees of nitration can be obtained.

Complete nitration of the cellulose produces gun cotton, which is explosive. For film base, a lower degree of nitration is used. Included in the base composition was about 10 per cent camphor as a plasticizer. While not explosive like gun cotton, the nitrate film base was very highly flammable and required strict safety handling regulations, including the restriction that the film be used only in approved projection booths. The well known explosive flammability of cellulose nitrate film is a consequence of the fact that oxygen from the nitri side groups (ONO2) is readily available to sustain combustion.

Attempts began as early as 1900 to find a less explosive plastic support, and the cellulose acetates seemed the most logical choice. But the intense mechanical stresses of cinema apparatus required a very tough plastic. Among the acetates, only the triacetates could stand up to 35mm professional use, but it was not commercially feasible because solvents used in its manufacture were too costly. As a result, nitrate remained in use for 35mm professional motion picture films until the 1950s.

As nitrate degrades, it shrinks, becomes brittle, and emits furiously corrosive fumes that rust metal cans, soften gelatin emulsions and aggressively fades silver images. Often it is the stickiness of softened gelatin that renders a film unusable in practice.

In 1988, the NFPA, published NFPA 40, the Standard for Storage and Handling of Cellulose Nitrate Motion Picture Film. This code doesn’t cover x-ray film. The NFPA has produced documents for nitrocellulose film since adoption of the first standard in 1919. Because manufacture of nitrate film was stopped in 1951, it was debated whether to suspend regulation on such film in 1979. It was decided that because of the large quantities of cellulose film stored in archival collections, the standard was still relevant. NFPA 40 also addresses projection room requirements for handling of nitrate film, and also specifies construction and arrangement requirements for buildings.

Flammability

Cellulose nitrate film is highly unstable and presents a very serious fire risk. It burns quickly with an intense flame. The rate of combustion for cellulose nitrate film is about 15 times that of wood. In the early days of the motion picture industry, movie houses and even film studios had devastating fires with many fatalities. Dry cellulose nitrate can explode when subjected to heat or shock. While decomposing motion picture film has been known to self-combust, still-camera negatives.has not. Cellulose nitrate contains chemically combined oxygen in sufficient amounts to allow burning and decomposition without the presence of air.

Toxic and flammable gases formed during burning or decomposition may be produced so rapidly that dangerous pressures may occur in building structures. The burning of cellulose nitrate film releases highly toxic gases, including nitrogen oxides and carbon monoxide. In one fire, these toxic gases were responsible for the deaths of 125 people not directly exposed to the fire. Since 1951, motion picture film has been produced with a 'safety’ base of cellulose acetate or other slow burning esters or polyesters. The fire hazards of these materials are similar to those of thick paper. When these films burn, there is no release of toxic nitrogen oxides.

Lower levels of heat will result in damage to 'safety’ film than to paper records, therefore special protection is needed to prevent fire damage.

Health risks

Cellulose nitrate film decomposes over time to release toxic nitrogen oxides. These toxic gases are skin, eye and respiratory irritants, and have poor odor warning properties. Chronic exposure to the decomposition products of nitrate film, especially nitrogen dioxide, can lead to headaches, blurred vision, loss of appetite, emphysema and other systemic damage. In the presence of water, these gases can form corrosive acid gases. These acid gases can lead to damage of other photographs and metal corrosion.

Recommendations

The decomposition and flammability of cellulose nitrate film makes the immediate identification, and proper disposal of this film necessary, while also preserving the materials stored with nitrate film. Although some still photographic negatives have the word 'nitrate’ printed along one edge, many nitrocellulose film negatives are not marked. Nitrate film should be stored in accordance with NFPA 40 — Standard for the Storage and Handling of Cellulose Nitrate Motion Picture Film. Below will be a summary of the NFPA’s reccomendations.

Construction requirements

The NFPA gives specific requirements on the type of construction required for buildings and laboratories that handle and store nitrate film. Buildings must be of Type I construction (see NFPA 220, Standard on Types of Building Construction). All rooms in which nitrate film is stored or handled shall be separated from each other by partitions that are continuous and securely anchored, and openings through these must be fire doors that meet the standards of NFPA 80. There must be two exits and adequate aisle space. Projection rooms, rewind rooms and rooms in which the stored amount of nitrate film is less than 20 rolls, are exempt from specific explosion venting requirements.

The NFPA requires at least 35 sq ft of floor area for each worker in inspection rooms, and no more than 15 people may work in room where nitrate film is handled. Tables, racks, electrical equipment, heating equipment and duct systems should comply to the specifications detailed in the standard.

Fire protection

Except for motion picture projection room of booths or rewind rooms, every room in which nitrate film is handling quantities greater than 50 lbs should be protected by an approved automatic sprinkler system. Other areas than film cabinets and vaults may use either an automatic sprinkler system or a deluge system that has fixed nozzles or open sprinklers. There are regulations on the water supplies for sprinklers. Every room where nitrate film is used or stored should have portable fire extinguishers with water or water solutions.

Nitrate film storage

The NFPA standard specifies the allowable amounts of nitrate film that can be stored. 25 lb (5 standard rolls) to 750 lb (150 standard rolls should be stored in approved cabinets or vaults. Amounts over 750 lb must be stored in vaults. Archival film should be stored in archival cabinets or archival vaults.

Film cabinets, film vaults, archival vaults, and archival cabinets are restricted in terms of ventilation, construction, materials, sprinklers, and dimension in accordance with the standard.

Handling of nitrate film also requires particular care. The NFPA specifies that all nitrate film must be kept in closed containers unless being worked on or examined. These containers may be individual metal cans for each roll. Those containers approved by the U.S. Department of Transportation (DOT) for shipment are particularly suitable. Film should not be placed in such a way, that sprinkler discharge would be hindered; for example underneath benches or tables.

Scrap film must be kept separate from paper waste, scrapped safety film and other rubbish. Nitrate film to be discarded should be collected daily from the work area, and removed to a unused room. Safe storage of the scrap film includes storage under water in steel drums or tightly closing metal containers. A storage vault can also be used. Scrap nitrate film should be disposed of frequently. Nitrate film should never be baled or burned. The DOT regulates transportation of nitrate film.

Projection rooms and special processes

Motion picture projection rooms are especially regulated, with specifications for fire resistance, enclosures, exits, openings, all shelves, furniture and fixtures, ventilation, and lighting. Processes like splicing, cleaning, repairing, cataloging and marking may be done in common work areas, but not in rooms where other operation are performed. The authority having jurisdiction shall be consulted to determine the necessary protection for safeguards against hazards involved with doing special processes. These are the types of activities that archivists, conservators, and museum personnel are often involved in. Here are some general health and safety guidelines:

  1. All cellulose nitrate motion picture film and decomposing film should be copied disposed of immediately according to the local fire code, and NFPA regulation. Never incinerate nitrocellulose film. A sign of decomposition is when the emulsion becomes soft and tacky.
  2. Only negatives in good condition should be kept. Nitrate film in good condition can be stored in acid free, buffered paper. Duplicate cellulose nitrate negatives and properly dispose of the original negatives according to the local fire code, NFPA and the DOT. Duplication of nitrate film can be done in two ways. The first option is to strip the emulsion from the base and transfer it to a new and stable support. This method is only useful for negatives whose decomposition has not affected the gelatin emulsion layer. Alternatively, copy the negative onto duplicating film and provide negatives from which prints can be made. In both cases, before the original film is discarded, be sure to inspect the duplicate carefully and approve its quality.
  3. Store cellulose nitrate film in a cold room where temperature and relative humidity can be carefully controlled. A suggested relative humidity is 35 per cent. Ventilation must conform to NFPA regulations.
  4. When handling nitrocellulose film, wear neoprene or buna-N gloves. If there are signs of decomposition, wear goggles as well and provide local exhaust ventilation consistent with NFPA regulation. The hazardous decomposition by-products of nitrate film have poor odor warning thresholds.
  5. Because all the hazards associated with nitrocellulose film increase with age, it is imperative to address safe storage and conservation before deterioration prevents conservation 2.

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References

1 1969, The Focal Encyclopedia of Film and Television Techniques, Focal Press, London, New York

2 Image Permanence Institute, 1991, Preservation of Safety Film: Final Report, Image Permanence Institute, Rochester Institute of Technology, Rochester, New York