ABSTRACT
Photographic negatives are unique camera images used as masters for the production of multiple positive prints (Valverde 2005). A black & white negative image is made up of three generic components: a support, which could be paper, glass, or a plastic film; an imageforming material which is fine silver particles, and a binder, gelatine being the most common material since the beginning of the 20th century. The last two components are the main materials of a single coat, which is called the emulsion (Abrusci et al. 2005). Several materials have been used to manufacture supports, namely, cellulose nitrate (from 1889 to 1950), cellulose acetates (mainly from 1924 to 2000) and polyester (from the 1950s to the present), so that cellulose triacetate constitutes the bulk of most of the film collections (Abrusci et al. 2005). The image stability depends on both intrinsic factors, namely the nature of the polymer base, and extrinsic factors, as environmental conditions. Although the problem of flammability observed in cellulose nitrate film was eliminated with acetate film, this material is also unstable and subject to deterioration in ordinary room conditions. Acetate’s deterioration process, known as vinegar syndrome, is autocatalytic and moisture and temperature dependent, causing the material to become acidic, to shrink, and to give off an odour of acetic acid (vinegar). Vinegar syndrome affects only cellulose acetate plastic materials. Under the influence of heat, moisture, or exposure to acidic vapours from nearby degrading film, cellulose acetate undergoes chemical reactions that form acetic acid. This acid is generated within the plastic support, then diffuses into the gelatine emulsion and may evaporate into the air, creating a sharp, acidic odour. Other important deterioration process is biodeterioration. In fact, microbial deterioration of photographic materials is a frequent problem and constitutes an important cause of damage of historical visual documents (Valverde 2005, Abrusci et al. 2005, Poole 1999). Fungi cause chromatic alterations such as stains of different colours, tonalities, and textures due to mycelial growth and pigment production. Bacteria play a more limited role than fungi in the biodeterioration process since they require higher humidity conditions (Borrego et al. 2010).
