Silver mirroring on silver gelatin glass negatives

Silver mirroring is a bluish metallic sheen appearing on the surface of silver based photographs as result of ageing. One of the photographic processes most affected by silver mirroring is that of silver gelatin glass negatives, the most common photographic negative process between the 1880s and the 1920s when they were slowly replaced by nitrate and acetate negatives.
The present research was initiated by the findings of plates that, beside the usual silver mirroring along the negative edges, had mirroring stains at the centre of the plate whose shape matched the creases of the glassine envelope in which the plates were stored. An informal inquiry among photographic conservators revealed that patterns connected to the enclosure material are rather common and they are not necessarily related to the poor quality of the material.
Although silver mirroring has been observed since the early years of silver gelatin photography and it has been investigated again and again in the course of the XX century, confusion is still present on its chemical composition, on the compounds responsible for its formation and on the reasons for the specific patterns.
The aim of this work is to better understand the mechanisms of both local and pattern formation of silver mirroring in order to set the choice of best suited enclosure materials and storage conditions on a more rational basis. This work is focused on silver gelatin glass negatives but the results and models here presented can be easily applied to other photographic processes exhibiting silver mirroring. The first chapter is a gallery of possible patterns found on silver gelatin glass negatives. This is first of all a visual definition of silver mirroring. Moreover, as patterns do not arise by coincidence but they are the result of simple physical processes, the visual features of silver mirroring suggest the causes for its formation.
The models about silver mirroring developed in the course of the XX century are reviewed in the second chapter. The outcome is the definition of the open questions on silver mirroring: the detailed microscopic processes leading to its local formation on one side and the macroscopic processes leading to the pattern formation on the other side.
In order to answer the first question new experiments on the chemical composition and the physical structure of the silver mirroring layer were performed. Based on these results some improvements to the well-established oxidation-migration-re-aggregation model of local silver mirroring formation are proposed in the third chapter.
The reasons for the arising of the usual silver mirroring edge patterns are investigated in the fourth chapter. A mathematical model based on the diffusion and reaction of gases explaining the formation of both historically and artificially produced edge patterns is presented.
The fifth chapter deals with the formation of inner patterns of silver mirroring. In this case, it was not possible to propose a unique model explaining the formation of the many different inner patterns that are likely to be found on silver gelatin glass negatives. The case of negatives with mirroring stains resembling the wrinkles of glassine envelopes is examined in details and a mechanism of formation is proposed.
Finally, in the conclusions, it is stated which are the contributions of this dissertation to the development of strategies to prevent or mitigate silver mirroring and which are the questions on which more research is needed.