Standard sterling is simply 92.5% silver and 7.5% copper. As Noel
points out, both copper and silver may react with sulfur compounds,
especially hydrogen sulfide, which is naturally in the air we breathe
at 0.1 ppb levels on average. When silver reacts with hydrogen
sulfide, it forms silver sulfide, which is dark grey color. When
copper reacts with hydrogen sulfide, it form copper sulfide, which is
blue. Initially, the color of tarnish appears yellow, because the
film is so thin (less than 10 nm). As the film grows, the color
darkens. When Argentium tarnishes, the films are usually much thinner
and therefore appear yellow. The advantage to this is that the yellow
can easily be wiped off and the tarnish resistance grows over time,
due to the growth of protective germanium oxide film.
Exposure to sweat (or perspiration as we train engineers say) is a
bit different. Sweat composition and pH is heavily influenced by body
chemistry, which is, in turn, influenced by diet, medical condition,
physical activity level, amino acids and so on. For some people
(appears to be a very small % of the population), their sweat is
heavily acidic (low pH) and contains corrosive salts (sodium
chloride, magnesium chloride, potassium chloride, etc…). At low pH,
the salts react with the copper to form a copper chloride/copper
hydroxide complex which appears green. Under oxidizing conditions,
the copper may precipitate out as cupric oxide which is black. Pure
silver is largely immune to these types of corrosion reactions.
With that said, Argentium® 93% silver alloy contains copper and
for some people or conditions may react to the environment. Argentium
93% has the highest tarnish resistance of any sterling alloy I’ve
tested, but the sweat resistance is about on par with standard
sterling. There is a premium version of Argentium with 97% silver and
less copper, which has much, much better sweat resistance. We sell it
in primarily casting grain and the item number is G6597.
Samuel A. Davis, P.E.
Sr. Process Engineer