The Canadian Council of Archives (CCA) basic guide to preservation offers general chapters for mixed and paper collections as well as various specific forms of photographic materials (C.C.A. 1990). The problems of foxing (yellow-brown aging stains) and mould growth are common to other paper items, but adhesion of gelatin layers and glass rot (at high humidity) and flaking emulsions (low humidity) are unique to photographic materials (Clark 1990: 41). Photographs consist of multiple layers, each of which reacts differently to humidity levels. Gelatin papers, for example, consist of these 4 layers: paper base, gelatin-barium sulfate, light-sensitive ‘emulsion’ containing the image, and a protective gelatin (Swan 1981: 280).
Photographic materials encode the image by reacting chemically with their environment and although this reaction is ‘fixed’ in the process, it is essential to understand that the materials continue to be sensitive to light and chemistry in their environment. Relative humidity (RH) is identified by Clark as “the most important environmental factor” (1990: 41): it should be 35-40% for a “mixed photographic collections” (Clark 1990: 41). Alice Swan similarly recommends RH control of about 40% in order to prevent fading of image silver (1981: 285) and ferreotyping (adhesion of mylar to print surface) in gelatin prints (1981: 284) as well as minimizing oxidation of silver (1981: 287). The visible signs of oxidization follow this order: shift in hue of the image highlights to yellow, spot formation in the midtones, shadow areas move towards red, and uniform fading (Swan 1981: 286).
The American National Standards Institute (ANSI) recommends 20° C and 30-40 percent relative humidity (RH) as base line environmental conditions (Wilson Part 1 1998: 4). Susie Clark recommends ideal temperatures of 5-8˚C for mixed photographic collections (up to 10-15˚C is acceptable) and 2˚C for colour film (Clark 1990). Swan warns against temperatures in excess of 22˚C to prevent oxidation (1981: 287). McCormick-Goodhart concludes that the optimum way to preserve photographic materials is to provide separate environments for storage and exhibition and to control the amount of time spent in each (1996: 19). He finds the optimum storage conditions to be those that increase chemical stability: sub-zero temperatures of -20˚C to -25˚C. (McCormick-Goodhart 1996: 19)
Susie Clark lists the air pollutants that are responsible for ‘silver mirroring or tarnish’ that can lead to the complete destruction on an image: acids, peroxides, ozone, sulphur-containing compounds and nitrogen oxide (Clark 1990: 42). The mirroring effect is the result of the transformation of image silver to silver sulfide, described by Alice Swan as “the natural end product of a variety of image deterioration processes” (1981: 288). Oxidation is a contributing factor to mirroring. In addition to controlling sources of active sulphur (e.g.: sulphur dioxide) through air-conditioning, storage materials without peroxide or active sulfur sources must be used. (Swan 1981: 288) Archival storage materials should be tested for these active pollutants because they are not commonly controlled in all ‘archival’ materials like pH levels. Therefore, all paper and board materials for negatives and prints must pass a Photo Activity Test (PAT) [*] to be truly archival photo storage material (Wilson Part 2 1998: 4).