silver degradation

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
Silver gelatin glass negatives, also called gelatin dry plates, were the most common negative process in the years between the 1880s and the 1920s when they were slowly replaced by nitrate and acetate negatives. They have glass plates of different dimensions and of thickness of the order of 1-2 mm as a base. The emulsion, of thickness of the order of 50 micrometers, is applied
on one side of the plate. It is made of gelatin and, in most cases, of silver bromide grains which turn to metallic silver after chemical development.

The possible shapes of silver mirroring degradation on silver gelatin glass plates will be divided in two main groups based on the location of the stain on the negative: edge patterns and inner patterns. Edge patterns include all the cases in which the mirroring stain is distributed at the four edges of the plate. The features of the stain can vary in width, detailed shape, sharpness and colour but it can always be identified as a stripe all along the plate edges. Edge patterns are the most usual silver mirroring patterns. Inner patterns include all the cases in which the silver mirroring stain is located either at the centre of the negative or, when present at the edges, has a shape not falling under the definition of edge patterns. Spots, lines, irregular shapes of silver mirroring are categorised as inner patterns.

Silver mirroring edge patterns are so common on silver gelatin glass negatives that they are sometimes used to distinguish this photographic process from other types of glass negatives.Although there is a large variety in their features, they always seem to bear a relation with theway historical negatives were normally stored, i.e. in stacks.In some cases the mirroring extension is constant all along the four sides but very often it variessteadily between the centre of the sides and the corners, usually being smaller at the cornersthan at the centre of the sides.Among the constant extension cases, examples of both narrowstain (Fig. 1) and of wide stain partially obscuring the image (Fig. 2) were found. In the mostadvanced cases silver mirroring can cover almost the entire plate surface.

Silver gelatin glass negative. study collection (~1910).
The mirroring sheen is narrow and blunt, just visible at the plate edges.
Silver gelatin glass negative. study collection (~1910).
The mirroring stain iswide, partially obscuring the image.

Silver gelatin glass negative. study collection (~1910).
Spot of silver mirroring connected with a black spot on the storage glassine envelope. For purpose of showing this effect, the plate is shifted to the top.



Silver gelatin glass negative. ~1915.
Stripes of silver mirroring (a) similar to the creases of the glassine envelope (b)in which the plate was stored.
private collection