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Handling and Fluctuation in RH and Temperature

Handling of photographs should always be done in a clean environment, using cotton gloves and kept away from tape, glue, rubber-bands, thumb-tacks or paper clips. Original photographs should not be exposed to direct sunlight or fluorescent light without filters since the ultraviolet (UV) range causes fading and discoloration, especially in colour prints. (Zanish-Belcher 1997: 5; Wilson Part 1 1998: 4) Copy photographs are always preferable to an original for permanent or long-term display (Wilson Part 1 1998: 4). Canadian Council of Archives recommends total darkness for photographic prints whenever possible, 50 lux [†] for exhibition of original prints and 100 lux or higher for copies (C.C.A. 1990: 77).

Susie Clark recommends that RH should not fluctuate more than 5% since this would cause the layers of photographs to separate (1990: 41). Although the physical damage resulting from fluctuating RH and temperature has been difficult to quantify, damage in the form of cracks, flaking, warping and curl results from exposure to fluctuations outside of an allowable range (McCormick-Goodhart 1996: 8). Although modern HVAC systems can control temperature within ±1˚C and ±2% RH, such narrow tolerances raise the issue of ‘frequency of access versus the physical well-being of the collection’ (McCormick-Goodhart 1996: 8). In order to maximize access while ensuring physical well being, McCormick-Goodhart carries out research to measure exact ‘yield points’ of materials (gelatin prints) at which they still contract due to these fluctuations “in a completely reversible manner.” (1996: 8)

Mylar sleeves should be avoided if the access environments include humidity levels above 65 %, since the sleeves trap excessive moisture and invite mold on gelatin and albumen prints (Swan 1981: 285). Materials have to be warmed slowly to avoid condensation and handled carefully since they are inelastic and brittle when cold. The safe limits on access environments are between 35% - 65% humidity at 25˚C. (McCormick-Goodhart 1996: 19) Temperature and humidity must be adjusted simultaneously (reducing RH by 3-4% for 10˚C drop) “to compensate for the moisture absorption capacity of the gelatin” (McCormick-Goodhart 1996: 19). Alternatively, cold storage should not be used for items that are accessed frequently, due to possibility for formation of condensation (Clark 1990: 41).

[†] the International System of Units unit of illuminance (luminous flux per unit area). It is equal to equal to one lumen per square metre (1 lx = 1 lm/m²), moonlight represents about 1 lx. (<http://en.wikipedia.org/wiki/Lux> visited 20 Nov. 2005 )