Vesicular film uses diazonium salts but the image is produced by tiny bubbles formed in a polymer layer when the film is exposed to ultraviolet light. The film is heat-processed making it popular for quick and easy client copies. The resolution quality is less than diazo and silver halide, the image colour is in the pink to purple range, and all formats are available. It is normally used for reference copies or material for short-term retention, as the inherent instability of this film makes it unsuitable for archival filming. The minimum acceptable requirement for archival purposes is one master and one reference copy.
All materials degrade over time.
We cannot control this inevitable deterioration, but we can control how fast it happens. It is useful to know that certain materials are susceptible to deterioration in particular ways because of their properties, and that other materials deteriorate as a result of particular environmental conditions. Microforms are photographic transparencies or negatives and have the same problems associated with that type of material. Some vesicular films produced in the 1960s and 70s were extremely unstable, resulting in severe image loss and liberating chlorine gas which can cause degradation of other records stored nearby. Diazo and vesicular films can be particularly sensitive to high temperatures, affecting image quality.