Historical photographs may be daguerreotypes, using copper sheets and mercury vapours, or albumen prints made on very thin papers with an emulsion derived from egg whites. Early photographs were also made with supports of glass and metal and collodion emulsions. In the late 1880s glass negatives were replaced by a clear plastic film made from cellulose nitrate. This was superseded in the 1930s by 'safety film' on cellulose diacetate base and again in the 1950s by cellulose triacetate. Triacetate is still used as a film base today in conjunction with polyester film which was introduced in 1960.
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 just because of their properties, and that other materials deteriorate as a result of particular environmental conditions. A photographic image is produced because of the reactive nature of the chemicals present in the emulsion and the processing. This reactivity makes the photographic image very susceptible to deterioration from poor quality raw materials, packaging, processing, poor storage environment or pollution. High humidity may promote mould growth, particularly in gelatine emulsions which provide an excellent nutrient for mould, while low humidity can cause the emulsion to crack and the support to become brittle.