Silver is a common component of most historical photographic processes. Silver mirroring is a natural deterioration, inherent within silver-containing photographic material. Silver is affected by oxidizing agents and silver ions are produced. Once these ions are produced, they can migrate downward through the gelatin layer, towards the support . Hendriks (1991) found that silver ions can also migrate to the surface, and through reduction process at the surface, transform to silver sulfide.
Silver Mirroring is a result of a physical alteration of the colloidial surface of a photograph caused by aging. Over time, the image bearing colloid layer shrinks and conforms to the underlying structure of the substrate or image particles. These physical alterations cause a change in the optical properties and, therefore, in the appearance of the photograph.
Silver mirroring appears as a bluish-metallic deposit or sheen. It can appear iridescent, which may change in reflective light. If not severe, silver mirroring may not be evident in certain lighting conditions. When very severe, silver mirroring can appear bronze in color. On negatives seen in transmitted light, affected areas appear more dense or yellowed depending on the degree of damage
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