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Photographic Film - Acetate Negative Film Digitisation

Acetate Film Base

Beginning in the mid-1920s, highly flammable nitrate film was slowly replaced with cellulose acetate film base (cellulose diacetate, cellulose acetate propiarate, cellulose acetate butyrate and cellulose triacetate). 

It became known as "Safety" film. Despite this name, cellulose acetates do have stability problems. Like cellulose nitrate, the deterioration of cellulose acetate is autocatalytic: once deterioration has begun, the degradation products induce further deterioration. It affects the plastic support of acetate film, causing it to become acidic, to shrink, and to give off of acetic acid producing a vinegary odor. 

A useful tool in helping determine the amount of acid vapor present, and gain an overview of the condition of acid vapors in an entire collection are "A-D Strips" (acid-detecting strips). They are acid-base indicator papers, which turn from blue to green to yellow in the presence of acid, and measure the extent of the acetate base support deterioration. 

As with nitrate negatives, deteriorated acetate negatives are easy to identify, but in good condition, they are almost undistinguishable from other types of plastic films. There are four ways to identify acetate film base negatives. 

1. Edge printing

Some cellulose acetate film base materials have the word "Safety" printed in the border. Edge printing may also include the name of the manufacturer, manufacturing code data, and notch codes.

Again, note that acetate negatives may have been copied at some point and the edge printing from the original will appear on the copy. Therefore, just because you see the words acetate or safety does not guarantee your item is acetate. See the section on testing to be sure. 

2. Dating information  

Types of Cellulose Acetate
 Type of CelluloseSheet FilmRoll FilmManufactures
Cellulose diacetate
1925–1950
1920s–1935Cellulose diacetate – Agfa, Ansco, Dupont, Defender, Koda
Cellulose acetate propionate1930–19451920–1945Cellulose acetate propionate: Kodak
Cellulose acetate butynate1935–present_Cellulose acetate butynate: Kodak
Cellulose triacetate1945–present1945–presentCellulose triacetate: almost every film manufacture

 
3. Acetate film base deterioration

 Deterioration is generally catealogued in six progressive stages


Level 1
No deterioration.

 

 


Level 2
The negatives begin to curl and they can turn red or blue.

 
Level 3 (not shown)
The onset of acetic acid (vinegar smell); also shrinkage and brittleness.


Level 4
Warping can begin. 

 


Level 5
The formation of bubbles and crystals in the film. 

 



Level 6
The formation of channeling in the film

When acetate base film is stored in a poor environment at high heat and humidity—or exposed to acidic vapors from nearby degrading film—it undergoes chemical reactions within the plastic support to form acetic acid. This acid causes the support to become acidic, brittle, and shrink. In turn, the acid spreads into the gelatin emulsion or into the air creating a harsh, acidic odor. It is a slow form of chemical deterioration known as "Vinegar Syndrome." It places acetate film at risk, and then deterioration may place otherwise stable photographic materials at risk as well. 

Deterioration is generally catalogued in six progressive stages:


CARE

Environment

The deterioration of both cellulose nitrate and cellulose acetate negatives is highly dependent on temperature and relative humidity. Ideally, to minimize decomposition, negatives should be stored in a freezer. At freezing temperatures, the natural decomposition of cellulose nitrate and acetate is slowed down. While the cold storage of small numbers of negatives is sometimes feasible, the cost and inconvenience of freezing a large collection can be prohibitive. However, cold storage is predicted to extend the life of acetate negatives by a factor of ten or more.

In general, a large commercial freezer (which should defrost automatically) is a relatively inexpensive cold storage unit. A combination of chemically stable boxes placed in polypropylene bags and then sealed with humidity control cards is a design that works well in preserving photographic materials in cold storage. This allows the stored items to warm at room temperature safely (8–12 hours should be sufficient) and can be easily accessed. 

A low cost set up for storage would provide a controlled environment with the constant temperature at 68° F (20° C), and relative humidity between 20% and 30%. Rapid changes in temperature and humidity will hasten deterioration. A dark and well-ventilated area around the negatives allows gases to dissipate. 

STORAGE

Because they present a great potential hazard to other materials due to their flammability and the strong acid formed from gases that the negatives release, cellulose nitrate negatives should always be stored separately from other negatives in a collection. 

Three layers of protection are recommended for the storage of film base photographic materials. Negatives should be placed in sleeves, the sleeves placed in a box or drawer, and these boxes or drawers on shelves or in a cabinet. Motion picture film and microfilm should be stored in unsealed containers in cabinets or on shelves. 

Negatives should be stored in individual, seamless, high alpha cellulose content paper enclosures. These enclosures are recommended to allow for the dissipation of harmful gases. Acid-free paper will resist deterioration caused by the formation of acids. Enclosures that have been used to store negatives must never be reused. These enclosures retain acids from previous materials, and anything placed in them will be damaged. Disposal of used enclosures is recommended in order to avoid reuse.

REFORMATTING/DIGITISATION

Negative collections should be inspected regularly for signs of deterioration. Any negatives that show signs of deterioration should be digitised as soon as possible. The more advanced the stage of deterioration, the sooner the negative should be reformatted to minimize the amount of image detail lost. Other factors that need to be considered are size and use of collection, space available for storage, and financial resources. 

Nitrate and safety negatives that need to be handled but are in good condition should still generally be digitized. The digital image can then be used while the original remains in cold storage. This minimizes the potential of damage to, or loss of, the original negative. 

Digital image collections should be created in standard file formats, such as TIFF, with sufficient image resolution to capture a high level of detail. Metadata describing the object and the technical specifications used in the creation of the digital object should accompany each file. Imaging photographic negatives requires expertise (or dependence on a company with that expertise) and data will have to be migrated over time as the hardware and software become obsolete. 

A plan for redundant storage will help ensure long-term access to materials, but once files are placed on a storage device, they must be checked regularly to guard against device failure and data loss. 

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