Preserving film sound in the cold at the British Film Institute: new evidence for archives

At the British Film Institute, picture film has long been stored at -5 °C to slow chemical decay, while magnetic film sound elements have remained outside sub-zero vaults because their response to cold conditions was uncertain. A collaboration with PSI now shows no evidence of damage in the studied samples and gives archivists an important first body of evidence that sub-zero storage may be compatible with magnetic film sound elements, with lower temperatures also helping to stabilise the recorded signal.

In analogue film production, picture and sound are created, edited and preserved as separate elements: the image is carried on picture film, while the soundtrack is assembled from multiple separately recorded and mixed audio elements often on magnetic film. At the BFI National Archive, picture film has long been stored in the Master Film Store at -5 °C to slow chemical decay. Magnetic film sound elements, however, were kept out of sub-zero storage because archivists lacked clear evidence about how low temperatures might affect the binders, coatings and lubricants of magnetic media.

The question sparked discussions between PSI postdoctoral researcher Jack Harrison and Charles Fairall of the BFI at the FIAT-IFTA World Conference in Locarno. The study was carried out as part of the BRIDGE Discovery project 'Play It Again' led by Sebastian Gliga, funded by the Swiss National Science Foundation and Innosuisse. 

For the study, BFI Conservation Care specialist Robert Ewart selected representative 16 mm and 35 mm magnetic sound tracks and prepared matched samples: one set was kept at room temperature, one was stored in the cold for a week, and a third for a month. At PSI, the samples were examined using infrared spectroscopy, scanning electron microscopy and vibrating-sample magnetometry. Across the studied samples, the measurements found no evidence that cold storage altered the chemistry of the tracks, damaged their surface morphology or degraded their magnetic properties when compared with control samples kept at room temperature. No signs of lubricant crystallisation were observed.

The magnetic measurements also confirmed something familiar from magnetism but highly relevant for archival preservation: at lower temperatures, the recorded state becomes more resistant to unintended erasure and print-through. For archivists, that matters because it means the colder environment is not only compatible with the studied materials, but also supports signal retention.  

The work is therefore significant beyond a storage decision at the BFI. It points to a broader direction in archive conservation: the long-term conservation of complex audiovisual materials increasingly depends on measurement-based scientific investigation. At PSI, this approach already extends toward large-scale facilities: the researchers are also developing synchrotron X-ray methods at the Swiss Light Source SLS to read magnetic recordings non-destructively and to probe degraded media in even greater detail. Together, lab-based techniques and research at large-scale facilities are becoming essential for understanding how these materials degrade and how they can best be conserved.