Decomposition
Seite 1 von 4
Technical Glossary of Common Audiovisual Terms
Decomposition
Description
The break down of film material.
Explanation
As nitrate films accumilated 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
Nitrate base material begins to decompose from the moment of its manufacture.
This proceeds slowly however, according to our present knowledge, irresistably. 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.
http://screensound.gov.au/glossary.nsf/Pages/Decomposition?OpenDocument
28.02.2007
Decomposition
Seite 2 von 4
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:
fading of the silver image, and a brownish discolouration of the emulsion; the film
also gives off a strong odour (sickly sweet). In metal cans the strong acids
released cause the can to become rusty, and the film's edges become tinged
with red dust.
the base becomes sticky,
The film becomes soft in parts (formation of honey) and blisters, the exudation of
a gooey liquid onto the top of the roll.
http://screensound.gov.au/glossary.nsf/Pages/Decomposition?OpenDocument
28.02.2007
Decomposition
Seite 3 von 4
the film congeals to a solid mass, (sometimes called a "hockey puck" when the
film is stuck together into an inseparable mass)
http://screensound.gov.au/glossary.nsf/Pages/Decomposition?OpenDocument
28.02.2007
Decomposition
Seite 4 von 4
the base disintegrates to a pungent brown dust (brown cellulose). (4)
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 NO2,
a strong oxidising agent that attacks and fades the silver image and embrittles
the plastic. Oxygen and water then react with NO2 to form nitric acid (HNO3),
which is a very strong acid capable of vigorously catalyzing further hydrolysis.
The NO2 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.
http://screensound.gov.au/glossary.nsf/Pages/Decomposition?OpenDocument
28.02.2007