C.C. PONTA
IRASM Radiation Processing Center,
Horia Hulubei National Institute for Physics and Nuclear Engineering, Magurele, Romania
Email: cponta2013@gmail.com
13.1. INTRODUCTION
At the end of 1990, the long neglected National Film Archive of Romania began a modernization effort. A new building was built, with a controlled atmosphere and a modern storage system. Before relocation to the new building, the film reels showing fungal traces (Fig. 13.1) were cleaned using a special machine. The procedure included brushing and washing with a detergent.
FIG. 13.1. A film reel showing serious effects of fungal attack (courtesy of National Film Archive, Romania).
The structure of film favours fungal attack. Photographic film is made of a transparent support on which is applied a gelatin emulsion that includes silver atoms (black and white film) or organic dyes (colour film). Gelatin is a hydrophilic protein that remains dry in normal humidity, but can take water from the air if the relative humidity is high. Gelatin emulsion is an excellent source of food for fungi. As a consequence of contamination, part of the gelatin disappears.
Instead, fungal metabolic by-products appear. They can chemically interact with the dyes or the support. Of course, the degradation is in direct proportion to the development of fungal attack. In a dry environment, fungi do not develop further and films can be considered stabilized.
It was believed that the fungal attack on film reels in the National Film Archive was not active and that the traces of previous attacks would be washed away by the cleaning action. It was thought that there was no need for emergency action. The discovery of several hundred reels where contamination was active and had developed disastrously was a very unpleasant surprise (Fig. 13.2).
The signs of a potential disaster were obvious:
— The presence of very aggressive contamination on a large number of reels;
— The imminent destruction of the films if the biological attack was not stopped immediately;
— The impossibility to act by the usual means, because the treatment capacity of the cleaning equipment was far below that needed for the intervention.
Radiation decontamination was chosen. In Romania, this was the only available method for the decontamination of large volumes in a short time. The literature did not indicate any antecedent, and there was no information regarding irradiation side effects on films. Given these circumstances, the action was preceded by a programme of exploratory tests.
13.2. EXPERIMENTAL PROCEDURES
The intensity of the fungal attack prompted the use of the sterilization dose of 25 kGy for decontamination, applied by a tote box irradiator. In these conditions, Dmin is 25 kGy and Dmax is assumed to be 50 kGy.
The microbiological tests evaluating the effectiveness of decontamination were performed on spores of the identified fungi, and on the infected film samples, after treatment at 25 kGy. All tests related to the assessment of side effects were performed at radiation doses of 25 and 50 kGy.
Mechanical degradation refers to the degradation of the film support made of plastic. This makes it impossible to use the film. It consists of changes in the distance between the perforations, and/or mechanical strength parameters.
Both lead to film breakage during screening. Tests searched for irradiation induced modifications of the distance between perforations, tensile strength and elongation at break. These tests were also performed on samples of new motion picture film produced by Azomures (a Romanian company that produced Fuji type film) and the National Film Archive. Test equipment and procedures belonging to these two institutions were used in this research. The plastic support of the films was made of polyester.
The degradation of the visual information contained on the film is a result of the emulsion losing its coherence or even disappearing. These phenomena occur because the gelatin is used as a nutrient by fungi. In colour films, even dyes (organic substances) can be sources of food for fungi. To assess the degradation of visual information, the changes in the basic colours (yellow, magenta, cyan) were measured in a sensitogram1. Colour tests were performed on Kodak film, by a Kodak laboratory approved by the National Film Archive, using a sensitogram
1 A sensitogram is a quantitative tool for the calibration of colour films. It consists of a piece of film containing film frames in the same colour but having different intensities.
for both negative and positive films. Densitometry values were measured before and after irradiation for each colour layer.
Theoretically, after irradiation, subtle changes may appear in the structure of the dyes that are not detectable by tests performed immediately after irradiation, but that could lead to an increased rate of ageing of the film. Ageing manifests itself by weakening the image. Unfortunately, there is no standardized test for artificial ageing of motion picture films. In order to evaluate these effects, a novel artificial ageing test was used. At the same time, free radicals trapped in the film were measured. A thermal procedure used for accelerated ageing of paper was adopted as a test for films. Temperatures up to 75°C and exposure times up to 6 hours were used. Sensitograms were performed on negative and positive, and irradiated and non-irradiated test samples. An EPR spectrometer built at IRASM, IFIN-HH was used to identify free radicals in the film. Measurement was made one day after irradiation.
13.3. RESULTS AND DISCUSSION
All tests — mechanical, colour and artificial ageing tests — indicated irradiation induced modifications of no more than 6% for the highest irradiation dose. No trapped radicals were identified, which is not surprising. The cages where free radicals are trapped are the microcrystalline regions of the polymer.
The polyesters used as film support have no microcrystalline component.
Detailed information about the project, the irradiation facility, the testing programme and the decontamination were published in Ref. [13.1].
13.4. CONCLUSION
The results justified the radiation decontamination of the infected film reels. The tests lasted for two months and the treatment of several hundred reels lasted two days. Thus, biodegradation was quickly and effectively stopped and the films were out of danger while they waited for complete cleaning. After the cleaning they were stored in the modern building (Fig. 13.3).
ACKNOWLEDGEMENTS TO CHAPTER 13
Thanks go to the management of the National Film Archive of Romania for the collaboration and support, and for their trust in science and their openness to new approaches that the nuclear domain can offer to society.
REFERENCE TO CHAPTER 13
[13.1] MITRAN, A., PONTA, C.C., DANIS, A., “Traitement antimicrobien des films cinématographiques au moyen du rayonnement gamma”, La conservation à l’ère du numérique, Actes des quatrièmes journées internationales d’études de l’ARSAG, Paris, Groupe Lienhart Press, Paris (2002) 235–248.
FIG. 13.3. Motion picture film reels stored in modernized building (courtesy of National Film Archive, Romania).