Although lot's of threads, discussions and articles on APUG and the internet in general mention photographic fading of silver based images, most fail to go into the chemistry or processes behind this.
To kick of some discussion:
- Assuming silver does not magically "evaporate" of paper, any fading must involve conversion to some other substance of the metallic silver. E.g. metallic silver (Ag) to some sort of silver salt (Ag+X-) where X unknown anion.
- The resulting substance must be a light or at least almost colourless substance, e.g. something like the silverferrocyanide / silverbromide complexes, as formed during bleaching in a bleach / redevelop sepiatoner.
- Many articles refer to sulphurdioxide (SO2) from the air and light as major causes of fading, probably in combination with water vapour and the formation of sulphuric acid, however, what it does to silver to cause fading is a little unclear. For example, if the sulphur reacted with the silver to form Ag2S, the image would not fade, but be sepia toned! ???
So what processes and chemistry is going on in fading photographic images
E.g. I could actually imagine sepia toned images to be MORE vulnerable to fading than untoned images:
- Let's assume inadequate washing out of the bleach and toning liquids took place. Now remaining bleach in the form of ferricyanide might react with with the formed Ag2S, maybe in combination with breakdown by SO2 in the air, to form light coloured silverferrocyanide (Ag+Fe(CN)64-).
- Also, we tend to think of "clean" or pure reaction products in photographs, but I have seen articles that actually show that even properly processed examples of such a sepia toned picture not only contains Ag2S, but actually can have a rather complex chemistry, with traces of bromide (Br-), chloride (Cl-) and all in the emulsion. So maybe part of the silver reacts with the halides to form light coloured AgBr/Cl substances???
Any insights or references to articles explaining fading would be appreciated!