I'd never heard that term so I've done a bit of looking into it today.
I've always assumed the black sooty deposit found in conjuction
with copper sometimes, is copper sulphide but putting "black wire
corrosion"
and "copper sulphide" in search engines turned up nothing.

Some years I asked a chemist what this black and insulating material
was likely to be that I often find on 20 or 30 year old switches that
because it insulates and unless the wiping type contacts break through
it,
stops
the switch action. He said likely Copper Sulphide, in my instances the
suphur from air borne polution and in BWC presumably from battery
sulphuric
acid
vapours. Any thoughts ?

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/


or even spelt Copper Sulfide and BWC no references



The black tarnish that you find on old switch contacts is most likely silver
sulfide, since many switches were (and still are) silver plated. It's the
same stuff that gets on your good silverware after having eggs over easy for
breakfast, and then letting the egg residue stay on the silverware for a
while. Eggs are heavy in sulfer content (Remember the rotten egg odor? It's
hydrogen disulfide).
--
Dave M
MasonDG44 at comcast dot net (Just subsitute the appropriate characters in
the address)

Never take a laxative and a sleeping pill at the same time!!