MOVING IMAGE SPECIFICS:

There are many different varieties of motion picture film. However, motion picture films each have the same physical structure which consists of two primary parts. They are the base and the emulsion.

Emulsion – The thin layer of gelatin where the photographic image resides. The side of film with the emulsion is normally has a dull, tacky finish while the base side appears smooth and shiny. With color film where both sides of the film appear glossy, the emulsion side can be identified by holding the film to the light. The emulsion side is the side where the image appears raised.

Film Base – A transparent substrate that supports the photosensitive emulsion that lies on top of it. Even though there are many layers and coatings associated with the emulsion layer, the base normally accounts for the majority of the thickness of the film stock. Historically, there have been three major types of film bases used: nitrate, acetate, and polyester.

• Nitrate – A nitrate film base was the first transparent flexible plastic base made commercially available in the 1880′s. This film is extremely combustible and can not be extinguished once ignited. Nitrate film, once ignited, has to be allowed to burn itself out. This film is considered a hazardous material. Should nitrate film be in your collection, it should be kept as cool as possible, stored in a vented (not sealed) can, and separated from acetate reels. Nitrate film is known for its distinctive sweet odor, yet the best way to identify it is to unwind it to the picture area and examine the edges of the film. The introduction of safety film in the 1920′s lead to Kodak printing the words ‘Nitrate Film’ along the edges of many of its stocks. Laboratory tests are also performed in some cases to determine if a film is nitrate or not. Even though it does have chemical instability, studies have shown nitrate film can be preserved for an extended period of time when kept in proper storage conditions. It is best to store nitrate film in areas with cold temperatures. For further information regarding the shipping, storage, decomposition, and identification of nitrate film, please click here.
• Acetate – Almost all 8mm film and most 16mm film used by nonprofessional and independent filmmakers is acetate (also known as cellulose acetate propionate and cellulose triacetate). This type of film is often referred to as ‘safety film’ because it does not have the combustible components that nitrate film has. Acetate film is safe to use in homes or residences. All 16mm, 8mm, and 35 mm film (after 1951) made in the United States is safety film.Also, all motion picture camera negative are now shot on acetate film because it is safer than nitrate but not as strong as polyester bases which may damage the camera should a jam occur.Acetate film is subject to degradation over time. With exposure to heat, moisture attaches to long chains of cellulose in the film base which causes them to break their molecular bonds. Due to this, free acetic acid is released producing a smell characteristic of vinegar. This is known as vinegar syndrome which leads to film that becomes brittle and shrinks.
• Polyester – The most recent film base to have been developed, polyester film, was first used for specialized photography applications in 1955. In the 1990s, it became overwhelmingly popular for motion picture prints. It is highly preferred for post-production, exhibition, and archival purposes due to its flexibility, strength, and stability. However, its strength is sometimes seen as a disadvantage. Polyester-based films are so resistant to breakage that they are likely to break film equipment should a jam or extra tension occur. Film crews, therefore, do not use this base for shooting the original camera negative because it is more preferable and less costly for the film to break instead. Due to its heightened chemical stability, it is not subject to vinegar syndrome which gives it a typical lifespan at least ten times as long as acetate film. Polyester film can often be easily identified by the Kodak brand name ESTAR printed on the edge of the film. (1)

GUIDELINES FOR STORAGE:

Some forms of media (like CD-ROMS) are capable of tolerating a variety of storage conditions. However, most moving image media slowly and steadily decay without special environments that lead to a long, useful life. Each medium has its own special requirements. For example, dyes in color photographs fade spontaneously in a short period of time. Low-temperature storage is the only way to preserve them. The silver particles in black and white photographic images are very sensitive to high humidity and airborne contaminants.

Media preservation depends on our understanding of the vulnerabilities of each media type so that the proper storage conditions can be provided. When elements of a collection are older or already damaged, storage conditions that slow down the rate of deterioration are important. The presence of vinegar can be evaluated by the use of A-D testing strips. Further information about this process can be found here.

DECAY:

There are three categories of environmentally caused decay. They are biological, chemical, and mechanical.

• Biological Decay – This type of decay includes all living organisms that can harm media. Mold, insects, rodents, bacteria, and algae all have a strong dependence on relative humidity (RH) and temperature. Mildew and mold are serious dangers to media. Therefore, sustained high RH (above 70% or so for more that a few days) must be avoided.
• Chemical Decay – This decay takes place due to spontaneous chemical changes. Fading of color dyes in photographs and degradation of binder layers in magnetic tape are examples of decay caused by chemical reactions occurring in materials. The speed of these reactions depends primarily on temperature, however moisture also plays a role. Generally, the warmer the temperature of a storage area and the higher the RH, the faster the media collection will be affected by chemical decay. Chemical decay is a major threat to media that have color dyes, nitrate, or acetate plastic supports. Cold storage is recommended for these materials. Frozen storage is recommended when signs of deterioration are present.
• Mechanical Decay – Mechanical decay refers to changes in the shape and size of water-absorbing materials like cellulosic plastic films supports or the gelatin binder in photographic materials. The determining factor in how much water is absorbed into its collection objects is the RH. If RH is low (below 15%) for long periods of time, objects will lose moisture and shrink. An opposite effect happens when the RH remains high (above 70%). Expansion due to extreme dampness and contraction due to extreme dryness cause stresses among the layers of media objects, which can lead to permanent deformation and layer separation.

Dampness is a serious environmental threat to media collections because it contributes not only to mechanical decay but to biological and chemical decay as well. (3)

Sources Cited: 

(1) “Film Specifics: Stocks and Soundtracks,” The Home Film Preservation Guide, 19 May 2008

LEARN MORE

(2) “Videotape Preservation Fact Sheet,” The Association of Moving Image Archivists (AMIA), 19 May 2008

LEARN MORE

(3) Adelstein, Peter Z. (2004). ” IPI Media Storage Quick Reference,” Image Permanence Institute. 19 May 2008

LEARN MORE