Early cellulose nitrate stock – in use from the medium’s earliest days until around the mid-1950s – was chemically unstable and prone to combustion, yet decay could be swift and absolute. Later acetate ‘safety’ stock was less combustible but it emerged that it was vulnerable to ‘vinegar syndrome’, in which the acetic acid is released with exposure to excessive heat or moisture, causing brittleness and shrinkage.

Perhaps surprisingly, nitrate seems to last longer but decomposition is very rapid once it starts and the result can be a congealed sludge. By comparison, acetate is relatively predictable. Unless kept cold and dry, it will fall victim to vinegar syndrome with all its attendant problems. Video tape meanwhile, can also suffer from physical deterioration but a bigger problem here is the obsolescence of recording formats. Once a format goes out of use in the industry, the players are no longer made, which can result in a tape becoming unreadable in a surprisingly short time.

Traditionally film was copied when it started to deteriorate. But such a strategy is not sustainable with very large collections: it is hugely expensive and time consuming. Today, the emphasis for film is on preventative measures: dry and cold storage environments delay or prevent the onset of deterioration. This also helps us to preserve materials with the best possible image quality, as repeated copying results in a degraded image. Materials that are deteriorating however, must ultimately be copied to preserve the content. Also, from time to time we must create new prints to make materials accessible.

With video, we maintain original recording and playback equipment for as long as possible. But ultimately it usually becomes necessary to copy (or ‘migrate’) the content onto new formats. So nearly all 2” Quadruplex video tape (the standard professional format well into the 1970s) has been migrated onto Digital Betacam – a still widely used broadcast standard. We are also now starting to preserve material in various digital file formats, held on disc or LTO data tape.