Chemistry of photograhic fading
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!
Last edited by Marco B; 11-25-2007 at 10:19 AM. Click to view previous post history.
Yes, silver images not correctly processed will degrade to silver sulphide as in toning; the difference is that it's an uneven process that looks unpleasant because instead of an even tone, the print becomes mottled over time. This is why sulphide toning works - if the print is evenly toned it cannot degrade further.
Sulphur also damages the paper support.
Maybe not directly related but you might like to look at Mike Ware's website, particularly the section on conservation.
and the Stamford Albumen site;
also the Silverprint article on archival toners;
Last edited by kevs; 11-25-2007 at 09:37 AM. Click to view previous post history.
Thanks Kevin for the links, this is really useful. Other contributions welcome too.
See Wilheim's book, lots of references.
Can you be a bit more specific? Just "Wilheim" is a bit to generic for Google
Originally Posted by John Shriver
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I would not rely solely on Wilhelm's book. He is sponsored by a number of companies to predict image stability and there are a lot of factors that are not put into proper perspective in his work. See the thread on color image stability.
Downloadable from here: http://www.wilhelm-research.com/book_toc.html
Originally Posted by Marco B
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Last edited by Bob F.; 11-26-2007 at 11:00 AM. Click to view previous post history.
Thanks Bob for pointing out the right link. And PE: no, I won't rely on this as my sole source of information. That's why I ask for contributions from all of you on a forum like APUG...