Because it has an extremely long life expectancy, it is also used as a master storage copy. The emulsion is easily scratched, so it is not ideal as a working copy. Diazo is used by the Archives to make working copies from the silver halide original. Diazo film is scratch-resistant and durable, although it has a shorter life expectancy than silver halide. It is not used by the Archives.
Vesicular copies reverse the polarity of the original, so they have positive-appearing images.
Film Longevity Documents with permanent retention periods must be reformatted on the most stable medium possible if they are to be used into the indefinite future. Silver halide film can last more than 500 years if it is manufactured, processed, and stored properly . When choosing a microfilm system, it is important to decide ahead of time if the film must be of archival quality.