Oh yes I forgot to mention that IPI (Image Permanence Institute) developed a method for detecting the stage of deterioration of acetate based film. I dont see why this cant be used with audio tapes as well. If you contact them (www.imagepermanenceinstitute.org) they will give you the necessary information. There is a kit including 250 A-D (acid detection) strips), instruction guide and a colour reference pencil. They also sell a Storage guide for Acetate Film publication. The process involves putting a little blue strip into each tape package -box, envelope whatever, leaving it overnight -they tell you how long depending on the storage temperature- and then comparing the colour of the strip to the colour bands on the pencil. The amount of colour change indicates the level of degradation and therfore whether existing storage conditions are adequate for preservation. As to why some more than others according to brand, its hard to say, as many of the manufacturers are quite proprietory about their formulations. So, other than the history of storage conditions and handling...??? And for film from the 50s: that is most likely NITRATE, much more scarey. Especially moving picture film - by the nature of which it is rolled tightly against itself, previously in non ventilated metal tins, and those tins stored close together in small rooms, again without ventilation..the chemical breakdown creates heat...its literally an explosive situation. Nitrate is not a stable material, it breaks down into nitric acid, &/or nitrogen gas! If someone were to drop a tin of deteriorated nitrate film in that storage situation, the result could be an explosion, instantaneous fire igniting the rest of the collection. Thats not to say all nitrate film is bad- but it needs to be kept under carefully controlled and monitored conditions, and copied to a more stable base before deterioration sets in. Thats why the big repositories of film in the US have built literally underground bomb shelter type storage facilities. You probably know this: the first moving picture film is on nitrate and even though they stopped producing it when acetate was introduced, it was still around of course and filmmakers used it. Nitrate has a beautiful asthetic quality they felt they could not get in other types of film-of course its all black and white. Still photographic negatives have the same history but dont pose quite the threat because they are small and usually each is separated by its own enclosure.
Also I neglected to tell you who I am. I am not a specialist in film, but had to find out about all these media to advise the archives I work for on proper storage for their collections. You can find out more from the Canadian national archives, now called Library Archives Canada, and large repositories in the US such as Library of Congress and research institutions such as IPI. They all have web sites with Preservation pages.
Beverley Lambert, Conservator Provincial Archives at The Rooms St. John's, Newfoundland & Labrador, Canada 709-757-8057 Fax: 709-757-8031