Metallic silver forming the image in b&w prints is far from inert, and it undergoes chemical interactions with environmental pollutants not to mention moisture and air.
For a long time, inadequate fixing and washing was the accepted dogma for the cause of image deterioration. However, it has become very clear that (1) higher levels of residual thiosulfate than what was considered archival limit would not harm the life expectancy of the material (Kopperl and Hutteman 1988); (2) even the properly processed prints and microfilms are not very stable.
One of the leading threat[s] to the silver is the oxidative agents contained in the environment, which can cause a variety of change to the image, such as discoloration, fading, mirroring, etc. Peroxide is one such oxidizing agent most commonly encountered in film and print storage environments. Although abundantly present, peroxide is weaker than other oxidizing agents present in environment, such as nitric oxide and ozone. Therefore, if prints are given toning treatment for protection, complete protection from severe peroxide attacks should be the minimum practical goal of toning. A report from Image Permanence Institute (IPI 1991) indicates that almost complete protection against peroxide and some, incomplete protection against other more potent agents can be obtained through polysulfide treatment. The IPI previously found that selenium toning was inadequate for protecting microfilm from peroxide attacks. Although we shall review many types of archival toners in this page, if the goal is to provide maximum image protection, polysulfide toner is the toner of choice.
polysulfide toners:
-> kodak brown toner
-> agfa viradon new
non-toning protection:
-> agfa sistan
-> fujifilm ag guard