How can their lifetime be extended?
Control of environment is viewed as the primary means of extending the life of audio-visual materials. Because hydrolysis is the principal route to deterioration of a photographic film it is extremely difficult to inhibit by chemical means within the film matrix i.e. inhibition of moisture uptake by films has already been optimised by manufacturers. Since moisture is the species which brings about the hydrolysis reaction a dry atmosphere will prevent degradation.
Unfortunately, the gelatin of the film emulsion needs some moisture to prevent its embrittlement. When sufficient moisture is present, an increase in temperature will serve to accelerate the hydrolysis reaction. At higher temperatures, other reactions (free-radical processes) are facilitated which are normally insignificant at ambient temperatures.
Because of this latter feature, care must be exercised when carrying out accelerated ageing tests as a means to predict film lifetime: if the mechanism of degradation changes with increasing temperature then the relationship between time to a given property change and temperature will not be a linear one. Fortunately current standards relating to film storage have been derived from information compiled from accelerated ageing tests and from direct archival experience. This having been said the standards are still subject to debate, probably because they are a compromise between good intention, economics and what is attainable practically.
A more ingenious way to use the available information pertaining to storage has been provided by the Image Permanence Institute's (IPI) Archival Storage Guides. Data taken from IPI*s Archival Storage Wheel is given in Table 3.
The data give an indication of the relative lifetime of either a new film (or a degrading one) when kept under given conditions of temperature and relative humidity. information on the effect of time out of storage on film lifetime is also provided by the guides. Even if storage conditions recommended by standards are not within reach, this valuable data provides the archivist with an idea of the limits imposed on film life by the actual conditions implemented.
Standards relating to the keeping of magnetic materials are not as clearly laid down as those for photographic film, ostensibly due to the poorer understanding of relevant factors associated with their breakdown. In the case of magnetic materials because oxidation precedes hydrolysis there are several ways in which breakdown could be inhibited, and current research is concentrating on these.
We have already said that degradation is exacerbated by confinement of the acids evolved. Though the auto-catalytic reaction can be prevented by removing acid which is initially formed, it is not sufficient to place holes in film containers through which the acid vapours may escape since they may infect neighbouring films which are in good condition. The acid may be reduced by aerating the film periodically, though rewinding film is a labour intensive process.
What is needed is an inert scavenger of the acid, and this has recently been provided by Kodak's Molecular Sieves. The sieves reduce the build up of acids in the film can preventing secondary reactions taking place (refer to the work of Tulsi Ram) The film can itself also plays an important role in film stability. Recent studies show that aluminium cans are superior to tin-plated metal cans. Tin-plated cans are subject to rusting by evolved acids and may themselves contribute to oxidative breakdown of films, especially for nitrate film.
Although peroxides in cellulose acetate film are considered to be stable below 160ºC, their decomposition may be catalysed by metal ions at ambient temperatures. Aluminium cans do not appear to facilitate such processes.
Plastic cans commercially produced are either polyethylene or polypropylene or copolymers of these two. These plastics contain inherently greater levels of peroxides than photographic films themselves. The peroxides in plastic cans are frequently less stable than those in photographic film and their decomposition may instigate degradation reactions in the films they are in contact with. The plastic can may absorb some of the acids emanating from the film it holds, but this does not necessarily offer any beneficial effects. More research into suitable film containers is still needed.
Control of environment will never be enough and break down of film is inevitable. It follows that along with appropriate environmental control, a test is needed to identify the end of the useful life of a film. Herein lies the crux of the problem: How do we define the useful life of a film? It has been proposed that if acidity levels build up so that pH reaches <4, then the film should be copied. So
How can we test for degradation?
