domingo, 14 de janeiro de 2007

Silver mirroring on silver gelatin glass negatives

Giovanna Di Pietro, Silver mirroring on silver gelatin glass negatives, Tese de Doutoramento, Universität Basel, 2002

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.

Silver mirroring can appear in a variaty of patterns which can be grouped in two categories: edge patterns and inner patterns. Edge patterns, the most usual silver mirroring patterns, include all the cases in which the mirroring stain is distributed at the four edges of the plate. Inner patterns include all the cases as spots, lines, irregular shapes of the silver mirroring stains located at the centre of the negative. As patterns do not arise by coincidence but they are the result of simple physical processes, the investigation of silver mirroring patterns gives information on the process of silver mirroring formation and on the conditions under which the degradation has taken place. The knowledge of these processes is the basis to decide rationally on the conditions and on the materials best suited to prevent or mitigate silver mirroring degradation.

In this work both the local and the pattern formation of silver mirroring are investigated. As far as the local formation of silver mirroring, this work presents new experimental results which allows to propose some improvements to the established physical-chemical model of local silver mirroring formation (the so-called oxidation-migration-re-aggregation model or Henridks' model) (Chapter 3). As far as the formation of silver mirroring patterns, a model of diffusion and reaction of gases with the emulsion which explain the formation of edge patterns, both on historical plates and on freshly processed plates exposed to a polluting environment, was developed and tested with experiments (Chapter 4). Among the inner patterns, the particular case of stains with shape resembling the creases of the glassine envelopes in which the plates were stored in analysed and the creasing of glassine papers when exposed to humidity is investigated (Chapter 5).

Moreover, this work presents a large number of examples of silver mirroring patterns on silver gelatin glass plates and a literature review of the models dealing with the formation of silver mirroring.

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