Managing Records: Storage & Preservation
by Tyler Selle
Archives Technical Information Series #15
2003
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Summary
Many local governments and state agencies now face the problem of deteriorating acetate microfilm because they used film with an acetate base. Since acetate was the most common film base from the mid-1920s into the early 1980s, if your organization holds microfilm produced during this time period it is almost certainly acetate-based. Poor environmental storage conditions, combined with acetate’s intrinsic susceptibility to decay, frequently accelerate the film’s deterioration. Polyester-based film, used after the early 1980s, does not have the same risks of deterioration as acetate film. The two major risks posed by acetate microfilm, information loss and off-gassing, are correctable, but only planning and quick action will help minimize damage. This publication describes acetate-based film, the problems associated with it, and mitigation strategies.
Background Information
Acetate was the most common film base from the mid-1920's into the early 1980's, so it is probable that microfilm produced during this time period is acetate-based. Poor environmental conditions combined with acetate's intrinsic susceptibility to decay accelerate the deterioration of acetate microfilm. Polyester-based film, used after the 1980's, does not have the same risks as acetate film. These risks include information loss and off-gassing, which are both correctable, but it is only with planning and quick action that these risks can be minimized.
Microfilm is composed of layers, the most basic being the base and the emulsion layers. The base is the thickest layer, providing rigidity to the film and a foundation for other parts of the film. The emulsion layer contains, or holds, the image and is chemically fixed to the base layer during the developing process. The two layers make microfilm useful and durable.
Microfilm has historically been made from a support or base material such as cellulose nitrate, acetate (including cellulose diacetate and cellulose triacetate), or polyester. Acetate is not used in the image creation process and is not present in the image layer of the film, creating an acetate film problem before the filming of any images occurs.
Acetate film base has an inherent tendency to degrade. The cellulose acetate composition changes and acetic acid is formed, which increases the rate of deterioration and emits a vinegar odor. The chemical deterioration also causes the base layer to shrink while the emulsion layer maintains normal size. In extreme cases of deterioration, warping, curling, buckling, embrittlement, and blisters can appear on the film.
Film that has greatly deteriorated is unreadable by microfilm devices and may result in expensive reproduction costs or total information loss. Loss of information is a critical concern for organizations that hold the only copy of certain permanent records. It is usually easy to identify acetate film in a highly degraded state, but microfilm that does not readily show visible signs of deterioration is also at risk. The acetic acid and associated gasses that are released as by-products of the chemical reaction is also a catalyst in the further decay of the film. High temperature and humidity levels combine with the acetic acid causing accelerated decay. The presence of these substances can contaminate surrounding material, damaging film and other material not originally in danger. This problem can be compounded by tight enclosures, such as boxes, drawers, and cabinets.
There are several ways to detect acetate-based microfilm. The date of production can be a good place to begin, as film produced before 1980 most likely has an acetate base. Simple and inexpensive methods to determine if film is acetate-based include holding a wound roll of microfilm up to a light and checking for opacity; unwinding several inches of film and looking for curling across the width of the film; attempting to tear a small portion of the leader of the film; and smelling the film for a vinegar scent. More scientific evaluations include a polarization test or using acid-detection (A-D) test strips.
After determining the level of deterioration, decide on a mitigation strategy. An immediate response would be to separate microfilm by base type. Store film in a cool (below 72° F) or cold (below 50° F) environment, with moderate humidity (20% to 50%). Long-term strategies include refilming original documents, if available, onto a polyester-based film. The most realistic long-term solution to an acetate film problem is refilming, especially if the film has not decayed significantly. Imaging provides another option by producing electronic copies if the original paper documents are still available, or by using an electronic film scanner to create digital images from a roll of microfilm. Another option is to have a professional conservation lab remove the emulsion layer from the deteriorating acetate base and placing the image on a new, stable polyester base.
To minimize the risks from the deterioration of acetate microfilm, identify the quantity of acetate film, assess its level of deterioration, control the storage environment, segregate acetate-based material, decide on an immediate response and long-term strategies, and conduct periodic surveys of microfilm. Following these steps will minimize loss of information, health risks and possibilities for damage to other records.
