the chemistry here is quite simple. One atom of sulfur combines
with two atoms of silver to form silver sulfide, which is the black
The chemistry of the process is not simple at all, and understanding
of this process contains clues to preventing tarnish. I was staying
away from this topic, but it is time to jump in.
The falsity is that sulphur can directly react with silver. No it
cannot. Sulphur has to be in form of H2S, or hydrogen sulfide. But
even this form cannot react with silver. What requires is presence of
moisture ( H2O ). In the presence of moisture H2S degrades to HS-
(negative ion), which directly react with silver forming Ag2S.
Another important point is that presence of Cl (chlorine) makes
reaction more aggressive.
To make things even more interesting, Ag2s has 3 crystal forms. The
form that is found as silver tarnish called acanthite and
crystallizes in monoclinic form. The other two are varieties of
argentite, both are cubic with one having body centered structure (
atom of sulphur in the center of the cube ), and another is face
centered ( atom of sulphur in the center of each cube face ). A
metallurgist should see parallel with forms of austenite. For some
reason, argentite was never found to be a part of silver tarnish. One
theory is that argentite, like austenite, can exist only at elevated
temperatures, which is not the case with silverware stored at room
To prevent silver from tarnishing is to control moisture. If
cleaning is required, do not use tap water because of chlorine and
always dry items completely. Storing silver with moisture absorbing
packets will keep it shining for years. Remember, no moisture, no
reaction. That brings another interesting question. Are there
gemstones which can keep silver from tarnishing ? Tiger's Eye is such
a gemstone. Also some Turquoises.
Tiger's Eye forms when some of crocidolite is replaced by silica. The
same silica replacement happens with Turquoises ( not all, but some.
I have a write-up on my website ) Silica is what is inside the
moisture absorbing packages, so mechanism is the very much the same.
This also has been known for centuries. Traditionally turquoise was
always mounted in silver, and so was Tiger's Eye. And now we know
that it wasn't just the fashion statement.