6 DISCUSSION

All observations on edge pattern formation in stacks that were obtained in this study, from both naturally formed and artificially created silver mirroring, indicate that relative edge-width variations of silver mirroring on single plates are directly related to the local distance between two adjacent plates. This finding confirms the theory that the diffusion and subsequent reaction of gases cause the formation of silver mirroring.

Fig. 14.

Silver mirroring artificially created by exposing a single plate in the incubation chamber with continuous ventilation. The silver mirroring is homogeneously distributed on the plate.

The pattern calculated from the theoretical gas absorption rate formula is in good agreement with the patterns found on the real plates. The silver-mirroring extension is underestimated at the plate corners, where diffusion takes place simultaneously from two sides, a more complicated situation than the one-dimensional model here presented.

A comparison of edge patterns from different plates shows that both artificially created patterns and naturally formed patterns on nonprocessed plates are narrower, sharper, and more intense than the naturally formed patterns found on processed plates.

This difference can be explained as a result of different reaction speeds. In the case of the artificially produced patterns, the reactivity is comparatively high, owing to the presence of hydrogen peroxide, a more powerful oxidizing gas than the natural organic peroxides released by cardboard and paper. In the case of nonprocessed plates, the reactivity is high compared to exposed plates, owing to the presence in the emulsion of silver halide crystals instead of metallic silver.

With the exception of the naturally formed silver mirroring on processed plates (figs. 4 and 5), the silver-mirroring edge patterns on all other plates (figs. 9 and 12) show a relatively even distribution of silver mirroring with a remarkably sharp boundary. Although these visual observations can serve only as a rough indication for the mass distribution of silver mirroring, the actual patterns observed indicate the presence of nonexponential mass distribution profiles. It can be concluded that for these plates the amount of gas reacting is not proportional to the gas concentration. The simple exponential model that was discussed in the theory section cannot describe the phenomenon of a sharp boundary.

The results for the single plates that were freely exposed in the incubation chamber indicate that in the absence of ventilation, an edge pattern is formed. When the air around a plate is agitated, no such edge pattern is formed. These results show that a simple diffusion-and-reaction mechanism can give rise to edge patterns on photographs freely exposed to the surrounding atmosphere. As suggested by Nishimura (2000), the same mechanism could play a role in the edge yellowing of photographs freely exposed to the environment.