Magnetic Tapes
Audiovisual documents, the associated risks and solutions
At this point it is impossible elaborating all types of materials and specifics, which is why further reading and the acknowledgement of recognised norms should be provided. As just as well it is to abstain from a discussion with respect to the provided contents.
The following audiovisual materials are primarily found in the conventional archive:
- Cinematographic collections of photochemical film and magnetic film along with partial contents and derivatives on analogue and digital audio, video and IT data carriers;
- Sound archives from the beginnings on rollers and records/film through to digital magnetic tapes/magnetic tape cassettes and IT data carriers including derivatives;
- Videography, from local and authority deliverance frequently in consumer and semi-professional formats on analogue and digital magnetic tape cassettes as well as increasing amounts on IT data carriers;
- Multimedia contents, former slide-sound presentations on framed photographic slides, connected with audio productions and accompanying prints, recently as linked multimedia files on IT data carriers.
Most of these media can be externally reduced to two types:
The “discs” are the analogue records, the digital CD, DVD or BluRayDisc as well as diskettes and computer hard-drives.
Strips/tapes are photochemical and magnetic film, analogue or/and digitally played sound, video or data magnetic tape; on winding cores, spools or in cassettes.
Despite these different types of packing, lots of material types can be compared because they are made up of several, connected layers.
The carrier layer hereby guarantees the stability of the shape and therefore the availability for recording and reproduction technology. The loss of structure of the carriers means a danger or total loss of the medium.
Endogenous effects to the carrier are spontaneous self-combustion (nitro-film!), aging, out-gassing of components (softeners), autocatalytic decomposition/hydrolysis (nitro-film; acetate = "security film", also for magnetic film/sound tape). The results are shrinking, warping ("cavities"), waviness, brittleness or sticking, loss of the solid consistency and discolouration/cloudiness, and this can also cause the disintegration of adhesives.
Exogenic effects are produced, mainly including mechanical destruction (cracks/ruptures, scratching, treads, wrinkles, stretching or tension splits), as well as burnt images, fingerprints, oiling and deformities caused by heat.
Fixed onto the carrier, the recording layer prepares the actual medium. The audiovisual recordings are to be differentiated according to the active principles:
- Mechanical recording (sound rollers and records, industrially produced CD/ DVD/ BluRay-Discs);
- Optical recordings (photochemical film, "burnt" CD/ DVD/ BluRay-Discs);
- Magnetic recordings, analogue and digital (magnetic layers = magnetic sound tape/cassette, magnetic film, magnetic track on sound film, video tape/cassette, computer diskette/hard-drive and data tape cassette).
In a general approach, it can be expected that a recording can be destroyed by exactly the type of effect with which it has been produced.
Mechanical records are comparatively stable but become increasingly unusable due to wear and tear, scratches and deformation.
Energy-rich light such as UV rays bleach the colour-photographic layers just as much as the colour layer of "burnt" disc media carrying information.
Magnetic fields impair or delete all types of magnetic recordings; magnetic alternating fields, high-energy fields and those at close distance have a particularly fatal effect.
Alongside the damage to the recording, the actual recording layer can also be destroyed e.g. with water (leaching, detachment and disintegration), abrasion, biological attacks (fungus or bacteria also in the case of modern disc media) and aggressive environmental air (oxidisation processes on photographic and magnetic layers, "silver layer"). Even surface changes, such as the loss of gliding properties, put the availability of recorded contents under doubt.
Manufacturing and processing errors, as well as residual chemicals, can impair the carriers, recording layer and the binding agent between the two in the same way. Detachment of the layer is frequently identified on photographic materials, on magnetic sound tracks on films and on the actual magnetic tapes.
The unavoidable disintegration of the materials can be delayed with suitable acclimatisation: equally the reduction of the temperature and humidity (!), cleanliness of the environmental air and maintenance of these forced parameters. Priorities are set by the particular weak points of the material: nitro-film should be separated first, whereby acetate carriers are also significantly more threatened than modern polyester carriers. Coloured layers must be retained with more heavily reduced parameters than black-white-silver layers. Colour nitro-film would therefore be the most problematic material. Film with an attached magnetic sound track does not tolerate any major reduction of the climatic parameters because there is the threat of detachment. Due to the expected stretching/contraction effects, all magnetic tape winders should only be negligibly retained in a normal climate.
Similar legal regulations apply to retaining and handling "nitro-film" as apply to explosives! Nitro-film ("celluloid"), consisting of cellulose nitrates, was produced from the beginning of cinematography up to the end of the 1940s, so it could have been frequently processed in the 1950s. For identification: the copied-in light signature at the edge of the film should refer to the type of material; the markings "Nitro" or "N", as well as a missing light signature, can refer to nitro-film. A float test in trichloroethylene p. A. (C2HCl3, poison, climate killer!) can serve as a test to verify nitro-film: nitro-film sinks while acetate and polyester films float. Nitro-film was normally not used in narrow film formats below 35 mm normal film.
The conservation of the supplied original materials has priority over use and security copies.
Electronic and digital conversions cannot replace a cinematographic original in the medium-term. Based on the model of the federal archive/film archive of germany, cinematographic security packages of image and sound are provided in double original format along with a usage-master (video suitable for the studio). The original work must be reworked by a specialist for preparation on a cinematographic security copy. Special processes such as wet-copies can contribute to visual suppressions of errors in the original.
However, if the contents of original sound archives are to be secured on secondary media, the transition to digital file formats and corresponding long-term IT memory solutions can no longer be avoided due to the obsolescence of conventional audio technology. High scanning rates (48 kHz) and uncompressed file formats (*.wave) should be maintained in the future if possible. Irreversible reworking should be avoided.
The life expectancy cycle of video cassettes is between 20 and 25 years so the contents should be copied onto high-quality master formats suitable for studio use in good time. In this case, compressed formats and recoding should also be avoided in anticipation. The transition to video file formats and long-term IT memories is foreseeable in the medium-term; however the extensive video files cause considerably more work than audio files.
Modern multimedia products are generally "digital born" and a return to studio technology formats seems to be impossible due to the associated functional loss. Maintenance of the original file package while using long-term IT memories is necessary. Unfortunately, longer periods of retention of "burnt" disc media has to be seen as uncertain.
All audiovisual documents generally do not tolerate any dirt due to the readability of machines. Packing in dust-tight containers is therefore gaining importance. In contrast, airtight packing tends to be rejected in order to permit the removal of decay products. Tins that are rust-free inside and only contain film in a lying position are to be provided for cinematography material. An upright position is appropriate for all tape winders. Disc media, particularly modern ones, should be stored uprights wherever possible and without contact to other media.