Observation 1: American "junk" silver circulation coins don't tarnish much, e.g. pre-1964 quarters, dimes, etc. These coins are 90% silver, 10% copper. While they aren't completely immune, they don't tarnish as much as pure silver bullion, or Sterling. Observation 2: Sterling silver is notorious for its propensity to tarnish. Sterling wares have to be laboriously maintained, polished, etc. People buy all sorts of products to fight its propensity to tarnish. Strangely, Sterling's composition is very similar to coin silver: 92.5% silver, 7.5% copper. Observation 3: Pure 999 silver bullion readily tarnishes. If I leave it in a random flip in a sock drawer for a couple of months, tarnish. I don't know how pure silver's propensity to tarnish compares to Sterling's. My hunch is that Sterling is the worst of the three. Anyone know why 90% coin silver tarnishes less than both Sterling and 999? It's not the linear relationship I expected. 90% barely tarnishes, 92.5% badly tarnishes, but 999 tarnishes more than the 90%? That doesn't make sense to me. Is there something metallurgically special about the 90/10 silver/copper recipe? Is this why it was chosen for coins? Why does Sterling behave so differently, given its very similar composition? Is there a magic silver/copper recipe that never tarnishes? The tarnish-resistant silver alloys I've seen lately tend to add other metals like germanium, silicon, etc. (Argentium Silver is a good example – great stuff, wish they made bullion from it.) Thanks for your help.