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An image on a black & white print is the result of microscopic metallic silver granules contained in the emulsion. If such a print is left in the sun, the dark areas will eventually fade due to "sun bleaching".

What physical process is occurring to the silver granules as a result of EM radiation flux, and what happens to reduce the darkness of the silver? Or, is "sun bleaching" a result of something unrelated to the direct effects of EM radiation, such as heating causing a chemical change (in which case this question belongs on Chemistry)?

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  • $\begingroup$ I have to say I see nothing but articles about how to bleach, how to enhance your photos by bleaching, the complete opposite of you are asking. They all seem to love bleaching. Imo, I would definitely consider chemistrySE, but first maybe someone here has an idea. Best of luck with it. $\endgroup$ – user108787 Sep 8 '16 at 7:24
  • $\begingroup$ gawainweaver.com/images/uploads/… suggests that light itself is not usually a problem, but heat can be. That said, your description of "bleaching" doesn't exactly match the common issues that it lists. $\endgroup$ – BowlOfRed Sep 8 '16 at 7:36
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The best description I found was from A Guide to Fiber-Base Gelatin Silver Print Condition and Deterioration by Gawain Weaver

There it mentions that the dark color in the photograph is due to the excellent absorption of light by small ($0.5 \mu \text{m}$) aggregations of silver. If these particles are modified in some way, the overall appearance changes.

...as prints decay, silver atoms can migrate away from the silver particles, gradually creating many smaller silver particles in place of the original filament. This causes a gradual warming of the image, as well as fading, since the smaller particles appear more yellow and sometimes cannot be seen at all.

and..

The color of silver also changes when it reacts with chemicals in its environment to form new compounds such as silver sulfide. This can occur intentionally during toning, or over time, as the silver image reacts with chemicals in the air, such as hydrogen sulfide. When this process occurs over time, the silver grains tend to disperse into smaller particles causing image fading.

The text further details multiple types of image damage and some of their effects. It does note that light by itself isn't normally a problem, but temperature is.

You may have noticed that light has not been included here as a primary factor in image stability. Exposure to high levels of light for extended periods may cause embrittlement of the paper support, and the associated heat may speed image silver deterioration. However, compared to the damage that light inflicts on most color prints or on resin-coated gelatin silver prints, fiber-base gelatin silver prints are quite stable with respect to light exposure.

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