1 INTRODUCTION

Silver mirroring is an iridescent, often bluish, metallic sheen that appears on the surface of silver-based photographic materials as a result of aging processes. Silver mirroring is generally confined to some local areas of the photograph, and it is present in various distinctive distribution patterns that seem to be connected to the environmental conditions in which the photograph is stored.

Silver gelatin glass plates are particularly prone to silver mirroring. They were the most common photographic negatives during the first decades of the 20th century. They consist of glass plates covered by a gelatin emulsion layer containing silver halide crystals that turn to metallic silver after processing.

In some cases, silver mirroring patterns on silver gelatin glass plates are related to the features of the enclosure. Patterns sometimes coincide with wrinkling patterns (fig. 1) or with folds and edges (fig. 2) of the glassine envelopes. In other cases, isolated spots occur that are possibly connected with particles contained in the paper envelope (fig. 3). However, on most historical plates the silver mirroring sheen is confined to the plate edges, thus forming an edge pattern. The extension of the pattern can be either reduced (fig. 4) or enhanced (fig. 5) at the plate corners. Silver-mirroring edge patterns have been found on both processed and nonprocessed glass negatives.

The edge patterns observed on historical plates appear to be related to the fact that they are usually stored in stacks. However, in this study we have found that edge patterns can also arise on single freely exposed plates as a result of hydrogen peroxide and hydrogen sulfide exposure experiments aimed at artificially reproducing silver mirroring.

Fig. 1.

Silver gelatin glass negative (Schweizerische Landesausstellung 1912, 13x18 cm, State Archive, Bern) with silver-mirroring pattern resembling the creasing pattern of the glassine enclosure

Fig. 2.

Silver gelatin glass plate (13x18 cm, ca. 1915) showing a silver-mirroring line following the fold of the glassine enclosure

Fig. 3.

Silver gelatin glass plate (13x18 cm, 1941) showing silver-mirroring spots. The spots seem to be caused by particles in the envelope in which the plate was stored, since each of them repeats regularly three times, probably in connection with three different positions of the envelope.

Fig. 4.

Silver gelatin glass plate (Adolf Cueni, 9x12 cm, ca. 1915, University of Basel) showing silver-mirroring edge pattern. Note that the extension of the silver mirroring is reduced at the corners.

Patterns do not arise by coincidence; they are due to specific physical and chemical mechanisms. The observation and investigation of silver-mirroring patterns thus yield information on the process of silver-mirroring formation and on the conditions under which the degradation has taken place. This article is confined to the most common pattern of silver mirroring, the edge pattern. For the moment, we believe that the development of a general theory to describe all the possible silver-mirroring patterns is still too big a step.

Fig. 5.

Silver gelatin glass plate (Adolf Cueni, 9x12 cm, ca. 1915, University of Basel) showing silver-mirroring edge pattern. Note that the extension of the silver mirroring is enhanced at the corners.