Silver mirroring is a natural chemical process that affects photographic materials containing silver over time (Chen 2001). It results in a metallic sheen over the surface of the photograph, typically affecting the darker areas of a photograph most (Gawain Weaver 2011).
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Silver mirroring occurs when the silver in a photograph degrades. As the silver breaks down, some silver ions travel to the surface of the photograph and reduce to become silver sulfide, which produces a mirror-like deposit on the photograph (Gann et al. 2010). Silver mirroring can sometimes cause certain features of the photograph to become obscured (Chen 2001). This condition can be exacerbated by poor storage conditions, such as storing photographs in high-humidity areas (Gann et al. 2010).
Chen, Jiuan-jiuan. "Documenting Photographs: A Sample Book." Senior Specialization Project, Buffalo State University, 2001. http://paulmessier.com/pm/pdf/papers/documenting_photographs_chen.pdf
Gann, Lyzanne, et al. "Strategies for Treatment of Silver Mirroring." March 2010. Accessed April 20, 2014. http://www.conservation-wiki.com/wiki/PMG_Silver_Mirroring
Gawain Weaver Arts Conservation. "Conservation of a gelatin silver print by August Sander." September 1, 2011. Accessed April 23, 2014. http://gawainweaver.com/news/News-conservation-gelatin-silver-print-august-sander/