Sulfiding of silver images

There has been a question in the back of my mind for some time regarding
sulfiding of the image silver in photographs...

Why does "natural" sulfiding of the image silver (from atmospheric
contamination, excess hypo, etc.) result in *fading* of the image when
the deliberate *toning* of the silver (in toners such as IPI SilverLock
or Kodak Sepia Toner) results in a good-contrast image that is protected
from further oxidation of the silver image?

I *do* have a notion about why, but I don't have any scientific back-up
for this.  This question recently came up again in regard to my
recommendation of IPI SilverLock formula polysulfide toner in a
conference on the WELL.  A sharp guy pointed out that if toning was
supposed to protect the image from sulfiding, why intentionally sulfide
it?  Additionally, he proposed, why wouldn't gold toner be a better
choice than even selenium for toning the image?

Any and all comments welcome.

Yours,

Loren C. Pigniolo
Photographic Preservation Services

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                   Conservation DistList Instance 7:7
                   Distributed: Sunday, June 27, 1993
                        Message Id: cdl-7-7-004
                                  ***