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Beyond Paper: Cellulose Nitrate and Cellulose Acetate Film in Collections

Bonnie Parr, Abraham Lincoln Presidential Library and Museum
 
Historical collections that include film-based photographic materials – negatives in sheet or roll form, motion picture film, and microfilm – are very likely to contain cellulose nitrate and cellulose acetate plastic films. These types of films are known to be unstable materials, and they present many preservation challenges for collecting institutions. This article is an introduction to how to identify these films, their characteristics of deterioration, tips on handling and storage, an overview of options for reformatting, and sources for further reading.

Identification

Dating Information

  • Cellulose nitrate film – ca. 1889 – ca. 1950’s (this date range relates to Kodak products; date ranges for other manufacturers are not well known)
  • Cellulose acetate film – ca. 1925 – Present (though largely supplanted by polyester-based films by the 1990’s)
  • Even though production ceased by certain years, older film stocks were still available and likely used until depleted

Clues to dating also may be found through:

  • Photographer’s notes or other descriptive information accompanying the film
  • Subject matter of the images
  • Clothing fashions, vehicles, equipment, or other scenic details in the images that give a clue as to the era that the photograph was taken

Edge Printing and Notches

  • Cellulose nitrate – the word “NITRATE” printed on the edge; a “V” notch (first notch in from the edge, looking at the film with the emulsion side up)
  • Cellulose acetate – the word “SAFETY” or “SAFETY FILM” printed on the edge; a “U” notch (also first notch in the from the edge with the emulsion side up)
  • Not all manufacturers used edge printing or notch codes
  • Original edge printing also shows up on copies made on later film bases

Non-destructive Tests

Indicating whether films are polyester-based or cellulose nitrate/cellulose acetate-based

  • Polarization test – in front of a light source, view the film between two crossed polarizing filters (or the crossed lenses of 2 polarized sunglasses); polyester film will appear to have a pattern of rainbow colors like soap bubbles; cellulose nitrate and cellulose acetate films will not show any color
  • For motion picture film – hold the reel of film up to the light; polyester film will appear transparent; light will not show through cellulose nitrate and cellulose acetate films

Destructive Tests

It is strongly recommended to not undertake destructive tests (briefly described below) because they pose serious safety risks and require proper safety precautions, facilities, and training.

  • Burn test – cellulose nitrate burns fast, hot, with a bright yellow flame, and not extinguishable until sample totally consumed; cellulose acetate is slow to ignite and burn, film melts, burns without a flame
  • Float test – floating a sample in trichloroethylene (chemical is toxic and a carcinogen); cellulose nitrate film will sink; cellulose acetate film will stay near the top
  • Diphenylamine and sulfuric acid (a hazardous chemical) test – cellulose nitrate turns deep blue when solution is applied to a sample; cellulose acetate (and polyester-based film) will not show color

Characteristics of Deterioration

Stages of Deterioration for Cellulose Nitrate Film

  1. Film support shows silver mirroring and turns yellow, rust ring may form inside metal film cans – image still legible
  2. Film turns sticky and gives off a noxious odor of nitric acid (some descriptions mention a sickening camphor smell) – image still legible
  3. Film turns amber in color and image starts to fade, noxious odor – image partially legible
  4. Film is soft, contains gas bubbles, welds to adjacent materials, noxious odor – no legible image
  5. Film turns into a brownish acid powder – no legible image

Cellulose nitrate film, as a highly flammable material, has a reputation for spontaneous combustion. However, for this to occur these conditions must be met simultaneously: 

  • Stored near sources of heat or flame 
  • Stored in conditions with prolonged high temperature (100 degrees Fahrenheit or greater) and high humidity
  • Storage area has reduced ventilation
  • Deterioration to the stage that produces heat in an enclosed environment (particularly of concern for movie film stored in tightly closed film cans)

To reduce this hazard:

  • Isolate all cellulose nitrate film from other collections
  • Separate deteriorated cellulose nitrate from film in good condition
  • Increase ventilation in storage areas
  • For movie film, replace metal film cans with ventilated polypropylene cans (slits in the lid provide air circulation)
  • Prioritize duplication onto a more stable format

Stages of Deterioration for Cellulose Acetate Film

  1. Film begins to shrink and become brittle, gives off odor of acetic acid (vinegar smell) – image still legible
  2. Film begins to curl, may show pink or blue stains (from dyes used in an anti-curl layer), vinegar odor – image still legible
  3. Film base noticeably warped, vinegar odor – Image still legible
  4. Bubbles and crystalline deposits form between layers, strong vinegar odor – image may or may not be legible
  5. Channels form on both sides of the film as the base shrinks, strong vinegar odor – image may or may not be legible

Cellulose acetate film does not pose as much of a fire hazard as cellulose nitrate, but its outgassing vapors aggressively attack the condition of nearby photographic material (especially cellulose-based films) and hasten the degradation of those materials.  When “vinegar syndrome” is detected:

  • Isolate affected acetate film from film in good condition
  • Increase ventilation in the storage area
  • Prioritize duplication onto a more stable format

Handling and Storage

Protect Your Health

Take these precautions when handling cellulose nitrate and cellulose acetate films:

  • Wear disposable plastic gloves (neoprene gloves if handling deteriorated film), goggles, and a mask (preferably a respirator type that filters out acid and organic vapors) to prevent the noxious odors produced by degrading film from irritating the skin, eyes, or respiratory system
  • Work in a well-ventilated area
  • Limit exposure times

To Mitigate the Effects of Outgassing Products

Produced by deteriorating cellulose nitrate and cellulose acetate films (which are a catalyst to accelerating the degradation of nearby collections in storage, especially photographic materials):

  • Ensure or increase adequate ventilation in storage areas
  • Store cellulose nitrate and cellulose acetate film away from other collection materials
  • Separate cellulose nitrate film from cellulose acetate film in storage
  • Replace old enclosures with acid free envelopes and boxes for sheet film, ventilated polyethylene cans for roll or movie film
  • Monitor cellulose nitrate and cellulose acetate film for evidence of deterioration

Provide a Proper Environment

The rate of deterioration of cellulose nitrate and cellulose acetate films is dependent on temperature and relative humidity:

  • A proper storage environment is one that is stable, cool or cold, and dry
  • These conditions will slow deterioration rates and extend the life of these films – particularly those in good condition
  • Cold storage – at or near freezing, ideally with low relative humidity levels – provides the best long-term preservation environment
  • Storage options, depending on the quantity of film to be housed in cold storage include frost-free freezers, large commercial freezers, cold storage vaults, or renting off-site cold storage space with a film storage vendor 

To Prepare Film for Cold Storage

  • Package film in layers of protective enclosures – this will provide a buffer to fluctuating humidity in cold storage freezers
  • Protective enclosures must pass the Photographic Activity Test (P.A.T.) for housing of photographic material

For sheet film:

  • House negatives individually in acid-free envelopes
  • Pack the enveloped negatives upright in acid-free boxes (do not pack too tightly)
  • Double-bag each box with zip-sealed polyethylene bags, making sure to press out as much air as possible before placing in a freezer

For roll film and motion picture film on reels:

  • If possible (and if the film is flexible enough to allow rewinding), replace the reel with a core
  • House in ventilated polypropylene cans
  • Double-bag the film can with polyethylene bags, pressing out as much air as possible
  • If the bags are not the zip-seal type, seal closed with heavy-duty packing tape

Reformatting

Factors to Consider

When deciding to reformat and whether to retain the original cellulose nitrate and cellulose acetate films:

  • Condition of the film – prioritize duplication of materials that are showing signs of degradation, outgassing, and image loss
  • Cellulose nitrate film should be placed at the top of the priority list because of the hazard it presents due to its flammable nature
  • Quantity and material types in the collection
  • How the collection is used and how accessible it is for use
  • The artifactual, informational, and monetary value of the collection; if showing no signs of deterioration, can the film materials be retained and proper storage be provided for its long-term preservation?  Are the same materials available elsewhere?
  • Storage space and ability to provide appropriate storage for preservation
  • Financial resources 

Duplication onto Polyester Film 

  • For collections that are still used in a film format, such as microfilm
  • Polyester-based film is a stable plastic and is a good material for long-term preservation in a physical format
  • Disadvantage is the equipment needed (and ability to maintain in operable order) to project the film for viewing
  • Finding a vendor that will copy film onto film is increasingly rare

Digitization

  • Currently the most common practice for duplication and storing of photographic images
  • Quality has improved in recent years, but it is still important that digital surrogates are of the highest quality possible for preservation of the images
  • Follow FADGI guidelines when specifying the digital product
  • Redundant storage – backing up the digital copies onto multiple storage devices will provide increased accessibility to the collection and insurance against data loss due to device failure
  • The major disadvantage is the constant change in technology that renders older systems obsolete
  • Plans must be in place and part of collection management to check the digital files periodically for readability and to migrate them to new formats as technology and hardware changes 

Information Resources

General Information

Cellulose Nitrate Film

Kodak – Storage & Handling of Processed Nitrate Film | Kodak
National Archives – Film Preservation 101: Is Nitrate Film Really Dangerous? – The Unwritten Record
National Film Preservation Foundation – National Film Preservation Foundation: Nitrate Degradation
National Park Service - Conserve O Gram 14/8: Caring For Cellulose Nitrate Film (U.S. National Park Service)

Cellulose Acetate Film

Library and Archives Canada – Six stages of acetate film base deterioration - Canada.ca
New York State Archives – New York State Archives - Publication #15 - Acetate Microfilm

Storage

Image Permanence Institute – “Media Storage Quick Reference” and “Storage Guide for Acetate Film”
National Park Service – Conserve O Gram Volume 14 Issue 10: Cold Storage for Photograph Collections, An Overview

 

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