In general, there are three types of film bases commonly used for microfilm: cellulose nitrate, cellulose acetate, and polyester. Fortunately, we do not believe that any of our microfilm are the flammable nitrate type, but it is likely that a fair number – especially our older microfilm – are of acetate which deteriorates over time.
During your visual inspections of the film, please try to identify the film base and any signs of deterioration.
Most of the information below can be found on the NEDCC website: http://www.nedcc.org/resources/leaflets/5Photographs/01ShortGuide.php andhttp://www.nedcc.org/resources/leaflets/6Reformatting/01MicrofilmAndMicrofiche.php
NITRATE – used from ca. 1890-1950s. Some signs:
- Sometimes it can be identified if manufacturers stamped the information along an edge
- It curls into very tight rolls as it deteriorates. Further signs of deterioration described below.
- When viewed through the polarizing filters, nothing will show (the same is true of acetate but polyester-based film will seem iridescent).
From NEDCC website (Monique Fischer):
“Cellulose nitrate decomposition can be very rapid. Deterioration is generally categorized in six progressive stages:
|Level 1||No deterioration.|
|Level 2||The negatives begin to yellow and mirror.|
|Level 3||The film becomes sticky and emits a strong noxious odor (nitric acid).|
|Level 4||The film can become an amber color and the image begins to fade.|
|Level 5||The film is soft and can weld to adjacent negatives, enclosures and photographs.|
|Level 6||The film can degenerate into a brownish acid powder.|
Most negatives will retain legible photographic detail into the third stage of decomposition. These negatives may become brittle, but with careful handling can be duplicated. Negatives in the fourth, fifth, and sixth stages of decomposition generally have no legible image and should be either placed in cold storage or duplicated.”
IF YOU BELIEVE THAT YOU HAVE FOUND A NITRATE-BASED FILM, PLEASE SEE DEB OR SARAH.
ACETATE (aka SAFETY FILM) – most commonly used from 1920s-1970s, some stock still available through 1980s. Some signs:
- Sometimes it can be identified if manufacturers stamped the information along an edge (might be labelled “Safety” instead of “acetate”)
- When you hold a tightly wound roll up to the light and look through the side of the film, it is likely acetate if it appears opaque.
- When viewed through the polarizing filters, it appears opaque (the same is true of nitrate but polyester-based film will seem iridescent).
- A-D (Acid-Detecting) strips can detect acetic acid that is a sign of deteriorating acetate film base. After period of exposure to the film in an enclosed environment, the color of the strip will indicate the extent of deterioration. The strips should show no signs of deterioration with polyester-based film, which is chemically more stable than acetate.
- It smells like vinegar during some states of its deterioration.
- It sometimes curls across the width of the film. Further signs of deterioration described below.
From NEDCC website (Monique Fischer): “Deterioration is generally catalogued in six progressive stages:
|Level 1||No deterioration.|
|Level 2||The negatives begin to curl and they can turn red or blue.|
|Level 3||The onset of acetic acid (vinegar smell); also shrinkage and brittleness.|
|Level 4||The warping can begin.|
|Level 5||The formation of bubbles and crystals in the film.|
|Level 6||The formation of channeling in the film.|
- When viewed through the polarizing filters, it appears iridescent (acetate and nitrate will appear opaque).
- When you hold a tightly wound roll up to the light and look through the side of the film, polyester-based film will be slightly translucent and allow light through.
- Because it is chemically stable, the A-D strips should not change color.