Conservation problems

All analogue photographs are subject to a continuous process of decay from their date of manufacture. The nature, manner and speed with which the age-related deterioration processes take place are determined by the composition of the photographic materials, their processing and the storage conditions. Nitrate negatives become self-igniting as degradation increases, and the nitrous gases released with ageing react with humidity to form nitric acid, which can attack nearby images. Acetate negative films release acetic acid, which causes the film to shrink and the layer containing the image to decompose. In the case of glass negatives, silvering often occurs, and this can result in a separation of layers. With colour slides, some colours can bleach more quickly than others, depending on the production method, and this can cause colour distortion.

Timetable and stages

swisstopo plans to digitise the entire inventory of images and make them generally accessible in the medium term. But in view of the magnitude of this undertaking, the various sections will have to be processed in separate stages. For this purpose, prioritisation is based on the careful weighing up of criteria such as the current condition of the images, their historical and cultural value, the comprehensiveness of the products, and the clients’ needs. The processing of an inventory of aerial images dating from 1946 to 1972 and in grave danger of deterioration (cellulose acetate and nitrate negatives) was the most urgent requirement.

In a report prepared in summer 2007 on the conservation of inventories of images at swisstopo, a variety of potential acute and latent forms of damage were identified, and appropriate measures were proposed. For the conservation of acetate and nitrate negatives dating from the period from 1946 to 1972 that were found to be particularly susceptible to deterioration, the management of swisstopo approved the necessary funding in spring 2008 given the urgent need for action. For the other inventories, swisstopo submitted an application to the federal government for additional financing, which was approved in spring 2009. The action plan for the preservation of inventories at swisstopo can therefore be implemented on a step-by-step basis.