Weaver, Gawain. "Image Decay"
From George Eastman House : Notes On Photographs
return to the Lexicon for Photograph Deterioration
A black-and-white image would remain unchanged for a very long time if it were kept in a cool, moderately dry, and perfectly inert environment without exposure to any airborne pollutants. Sulfide toners can have a similar effect by converting the silver into the very stable compound silver sulfide. Selenium and gold toning provide some protection, but will not protect a print completely. In real life, there are no such perfect environments, and few prints are sulfur toned. For a typical gelatin silver print, image deterioration proceeds by exposure to pollutants in the presence of moisture and heat. The basic mechanism of silver image decay is the same in every case: the silver particles that form the typical black-and-white image (Figure 14) undergo changes in their shape and size, and may react with sulfur to form silver sulfide. The change in particle size and the reaction with sulfur can produce dramatic changes in the color and density of the silver image (Figure 15).
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Nearly all image decay begins with a single step: the oxidation of the image silver into silver ions. The developed silver image is composed of particles of silver metal (see Figure 1). When silver from these particles are oxidized, they become silver ions (Ag+). Unlike the silver metal (Ag0), these silver ions are able to move within the gelatin. Thus, small dense image particles become larger clouds of even smaller particles. This oxidation and migration of silver atoms is the first step in image decay (Figure 16).
Once the silver has been oxidized to silver ions and has travelled away from the silver image particle it has three possible fates:
The first occurs when the migrating silver particles are quickly reduced to metallic silver. These new and smaller particles of silver appear yellow/orange (Figure 17). This yellow/orange discoloration typically occurs around the edges (Figure 20) and in the midtones and may be the result of exposure to fresh oil-based paint or varnish. See Sidebar 4 for a story of how this commonly occurs (P1).
The second result occurs in the shadows where there is a large amount of image silver. If the mobile silver ions reach the surface of the print, they form a metallic blue sheen known as silver-mirroring (Figure 18). This will occur only in the shadows, and either along the edges (P2) or across larger areas of the print (P3). Silver-mirroring along the edges is often caused by air pollutants, while more overall mirroring may be the result of contact with poor-quality storage materials.
The third and most common result is when the mobile silver ions disperse into very small particles of silver or react with a sulfur compound to form yellow/brown silver sulfide (Figure 17). Depending on the size and composition of the resulting silver or silver sulfide particles, the image may fade or shift to more yellow or brown tones. This may occur at the edges (P4), overall (P5), or in localized areas across the print (P6). Exposure to pollutants in the presence of moisture and heat is the most com¬mon cause. The deteriorated Lewis Hine print (Figure 15) is a classic example of this deterioration. The silver has broken down into smaller particles and some have likely reacted with sulfur to form silver sulfide. As a result, the highlights have faded, and the highlights and midtones have turned yellow/brown. In this case, even the shadows have been affected and display a warm brown tonality.
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Occasionally, faulty processing is the cause of this deterioration, and for this there is a telltale sign—the non-image areas, where there shouldn’t be any silver at all, will have a yellow discoloration due to residual silver/fixer compounds (P7). But be careful in making this judgment! There is one other form of deterioration that can look deceivingly similar. It is the yellowing of the gelatin binder that results from contact with poor-quality storage materials (P7 and Sidebar 2). Also closely related to faulty processing are chemical stains and other localized image deterioration caused by contamination from chemicals or fingerprints (P8 and Figure 19).
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A parting note on gelatin silver print image decay: while it is useful to study the individual mechanisms of deterioration, it should be remembered that in real life, gelatin silver prints are a complex product of their manufacture, processing, and subsequent storage. Visible deterioration patterns will always be a complex combination of many factors, not all of which can be fully explained.[1]
- ↑ Gawain Weaver, "Image Decay" excerpt from A Guide to Fiber-Base Gelatin Silver Print Condition and Deterioration, electronic publication, April 2008.