2 PRESENT KNOWLEDGE OF SILVER MIRRORING

Since the first accounts of silver mirroring on bromide negatives and prints, the origin of this type of degradation was proposed to be connected with the reaction of atmospheric gases such as hydrogen sulfide with silver compounds in the emulsion (British Journal of Photography 1901). Deeper investigations into the nature of the gases and into the mechanism leading to silver mirroring were carried out in the 1960s (Henn and Wiest 1963). These studies were spurred by the discovery of red spots on negative microfilms correlated to the presence of silver mirroring. Peroxides or atmospheric oxygen in combination with hydrogen sulfide were indicated as gases responsible for silver mirroring. Later, in the 1980s, Hendriks (1989) collected their results, together with the model proposed by research studies at Fuji Laboratories (Torigoe et al. 1984) and the evidence provided by transmission electron microscopy (TEM) images of mirrored emulsions taken in his laboratory, and proposed a three-step mechanism for the formation of silver mirroring:

1. oxidation of developed metallic silver grains by oxidizing gases and production of silver ions
2. diffusion of silver ions away from the original grain
3. reduction of the silver ions to small colloidal silver particles at the emulsion surface and formation of silver mirroring

Evidence for this model was provided by the experimental studies of Nielsen and Lavedrine (1993), who investigated with TEM cross sections of historical and artificially reproduced silver-mirrored emulsions. On a microscopic scale, silver mirroring consists of a layer of particles with dimensions on the order of a hundred nanometers (1 nm = 10-9 m) located at the top surface of the emulsion.

The three-step model proposed by Hendriks fails to explain the detailed mechanism of ion migration and reduction at the emulsion surface and therefore does not provide the possibility of predicting the rate of silver-mirroring formation. Nevertheless, it is widely accepted in the community of photographic conservation, and we believe the model to be substantially correct.