The break down of film material.
As nitrate films accumulated in film vaults and collections, the problem of chemical instability began to manifest itself. Symptoms included pungent smells, shrinkage, brittleness, softening of the emulsion, and severe oxidation attack on the image silver.
The decomposition of nitrate and acetate behave differently when decomposing. In the early years of modern safety film it was believed that triacetate would not decompose and was therefore an archival medium unsurpassed.
Unfortunately triacetate can decompose and quite badly but little is yet known about how the decomposition comes about.
Nitrate base material begins to decompose from the moment of its manufacture.
This proceeds slowly however, according to our present knowledge, irresistibly. In the process it releases nitrous gases, mainly nitrogen dioxide which combine with the water in the gelatin and in the air to form nitrous acid and nitric acid . These acids bleach the silver image or the colour in the emulsion and accelerates the decomposition of the base to the point of total destruction. The nitrous gases exert the same fatal influence on all the cinematographic film stored in the same area, irrespective of its age and whether it is nitrate or acetate, black and white or colour. Under favourable conditions the process of decomposition is very slow extending over many years.
For a long time the film shows no visible change; only in the final stage, which may last only a few months before total disintegration, are the following outward changes visible in this order:
Please note that in the early stages of the above (1,2,&3) it is possible, and indeed likely, that only a part of a reel of film is affected. By the time stage 4 is noted in part of the film then it is almost certain that the rest of the film will be in stages 1 or 2.
Although all the cellulosic plastics release acids upon hydrolysis, some are stronger or weaker than others. The hydrolysis of cellulose nitrate liberates NO 2 , a strong oxidising agent that attacks and fades the silver image and embrittles the plastic. Oxygen and water then react with NO 2 to form nitric acid (HNO 3 ), which is a very strong acid capable of vigorously catalyzing further hydrolysis. The NO 2 may also escape as a gas, attacking enclosures and nearby film. The gelatin emulsion becomes soft and sticky as well, itself affected by the combination of strong acid and strong oxident.
With a few exceptions, the decomposition of nitrate was only a minor impediment to the photographic and movie industries during the era when nitrate film was actively produced. The problem was known, but the full extent of the losses that would result from it were not imagined.
Acetate base material may be separated into two distinct types, diacetate and triacetate. In acetate films, the weaker acid hydrolysis products such as acetic acid (hence the name ‘vinegar syndrome’), butyric acid (which has a ‘rancid butter smell’) and propionic acid do not attack silver images or liquefy gelatin. However, the indirect consequences of hydrolysis is massive shrinkage, where the plastic support shrinks up to 10%, while the gelatin emulsion does not. The bond between emulsion and base gives way, and the emulsion buckles up, rendering the film unprintable. Far less than 10% shrinkage is necessary to cause motion picture film to become unprojectionable or copiable without expensive special effects. Embrittlement also occurs because of chemical change in the polymer.At the same time, hydrolys causes the fire-retardant additives to become incompatable with the plastic. they are normally held in place by chemical association with the side groups, and thus become free when the side groups are hydrolyzed. Plasticizers appear on the surface of the deteriorating film as needle-like crystals or liquid filled bubbles in the gelatin.
Diacetate base film was used between the early 1920’s and 1951. Triacetate base had become a commercial proposition after World War II. Diacetate was based on a chemical constituent of camphor, and when this material degenerates it gives off a smell characteristic of camphor or napthalene. Another indication of its degeneration is brittleness. This type of base had a relatively unstable plasticizer (that which gives a film its flexibility) which evaporates from the film in time.
Because of the length of time since diacetate was last manufactured most of these types of film are now relatively brittle, though not all. There always seem to be exceptions to every rule, even nitrate.
Triacetate base’s first indication of decomposition is a rather strong smell of vinegar. Commonly called vinegar syndrome. The next stage will exhibit a grey/white crystalline deposit. This deposit may be removed but only temporarily, at the shortest a matter of seconds.
After this stage the film becomes brittle, has a tendency to arch and the adhesion between emulsion and base is lost. The emulsion peels from the base in small flakes quite easily, most easily when winding on a bench. Ultimately parts or the whole of film will become so brittle as to be untouchable without causing damage. Some films have entirely disintegrated to a fine white powder.
FIAF, 1986, Preservation and Restoration of Moving Images and Sound, Federation Internationale des Archives du Film, FIAF, Brussels, Belgium