Storage materials and storage conditions are key elements in the preservation of archival material. Materials should be stored in archival-quality containers constructed of permanent paper; this paper is often mistakenly called acid-free paper. To be truly archival-quality, the paper should not be acid-free or neutral, but low lignin, with pH of 7.5-9.5 and an alkaline reserve of 2%. Photographs should be stored in chemically neutral plastic enclosures made of either polyester, polypropylene, or polyethylene (brand names include DuPont Mylar D and ICI Melinex #516). Enclosures of polyvinylchloride (PVC) or albums with adhesive material are not archival.
Heat accelerates deterioration: most deterioration doubles with each increase of 18 degrees F. This does not take into account deterioration caused by light, pollutants and other factors.
High humidity provides the moisture for harmful chemical reactions. High temperature and high humidity encourage mold growth and insect activity. Water is a critical factor in acid formation: the higher the moisture level, the more acid formation and the faster the deterioration. For maximum preservation of materials, temperature should be between 35-65 degrees F.
Library and archival materials absorb and release moisture, expanding and contracting with changes in relative humidity. Dimensional changes in the paper accelerate deterioration and acid formation, leading to visible damage such as cockling paper, flaking ink, warped covers and cracked emulsion on photographs.
Maintaining stable temperatures is very important: temperature and relative humidity should remain as constant as possible 365 days a year and twenty-four hours a day. The cooler and closer to moderate the better: a good rule of thumb is 30-50% RH and temperature between 60-70 degrees F. Combined stack and user areas should be no higher than 70 degrees F and stacks should be no higher than 68 degrees F.
Photograph and film materials require lower temperatures and a bit lower RH. The emulsion of film (including microfilm) and gelatins of nineteenth-century prints are particularly subject to crackling and flaking if they are not stored properly. Improper storage contributes to the migration of film silver particles, making the film unviewable after a certain period of time.
Pollutants include gases and particulates. Gaseous contaminants include sulfur dioxide, nitrogen oxides, peroxides and ozone: these gases catalyze the chemical reactions that lead to acid formation, particularly in paper and leather. Paper becomes discolored and brittle, while leather becomes weak and powdery. Particulates - especially soot - abrade, soil and disfigure materials.
Light accelerates deterioration by acting as a catalyst in oxidation. Light damage is cumulative and irreversible: it weakens and embrittles paper fibers, causes paper to bleach, yellow or darken, and causes media and dyes to fade or change in color, altering the legibility and/or appearance of documents, photographs, art works and bindings. UV radiation is particularly damaging because of its high energy level.
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