Deterioration of silver photographs

I thought that the following reference from the magazine Darkroom &
Creative Camera Techniques might be of interest to some:

    Chapman, Robert.  "Photochemistry: Agfa Sistan, Spotting."
    Darkroom & Creative Camera Techniques, July/August 1995, pgs
    4-6."

Chapman addresses a readers question about SISTAN, a proprietary
formula made by Agfa, which is described as a "post-processing
treatment .... for increasing the permanence of black-and-white
paper prints."  By way of his answer Chapman writes:

    Discoloration as we have defined it has, in the words of Agfa's
    Dr. Weyde, been termed a "civilization disease" because it first
    appeared about 25 years ago when it was recognized that the
    concentrations of airborne pollutants were increasing at an
    alarming rate due to automobile exhaust, industrial waste gases,
    chemical fumes, and so on. Putting two and two together,
    photographic detectives came up with the following mechanism for
    this pollutant-derived photographic defect.

Chapman goes on to give a brief description of the chemical
processes involved and some possible remedies.  BTW, he concludes
the article by stating, "Although I tried on repeated occasions to
obtain substantive answers to this question (e.g.  test data) [the
question being does SISTAN work as advertised? -gws] from Agfa's
technical personnel, I received only paltry answers."

For what it is worth I had a similar experience with FUJI when I
tried to find out if some of their films were as permanent as they
claimed in their advertising.

Greg Schmitz

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                  Conservation DistList Instance 9:15
                  Distributed: Friday, August 4, 1995
                        Message Id: cdl-9-15-006
                                  ***