Interesting problem.

>Elemental surface analysis showed the black surface material to be
>composed of Ag and Cl.  No Na, K, or S was detected.

The elemental analysis suggests that the coloring of the silver was
intentional.  The color suggests silver sulfide which has a high
probability of building up over time, but could start as intentional and
then add over time.  I had silver foil exposed at CAL for weeks, and
then for years; it turned brown in weeks and black in years; with areas
of very little tarnish (one a fingerprint).

I also did some analysis of silver tarnish on oriental paintings.  This
was presumable tarnish that built up over time.  Certainly, the visual
pattern on the paintings was that of sulfide tarnish over time.  I had
seen it time-and-again at the Morgan library (NYC) on illuminated
manuscripts in storage. Using the Microprobe (wavelength dispersive), Ag
and S were the primary components.

The other point that I wonder about is: silver chloride is white or pale
yellow in solution.  I would expect it to be a similar color (at least
light in tone) as a solid.  Silver sulfide is clearly a dark material.
I've seen it range from brown to warm brown, to black.  If the silver
chloride found on the material is black, I'd look for carbon pigment.
Try a close look in the SEM of a sample.  Application of a colorant may
be visible.  The "tarnish" would probably be amorphous.

Tim Vitale
408-684-2731