Copper reacts readily with oxygen to create copper oxide, however silver sulfide is the most common corrosive film formed indoors on silver. The rate of corrosion depends on the concentration of atmospheric and environmental sulfur gases and humidity.
Audiophile silver cable manufacturers would have you to believe that silver tarnish is not a problem, but aerospace users of silver plated wire, terminals and contacts used in satellites and other military applications have extensively studied silver sulfide corrosion.
Silver sulfide (and/or silver oxide) are conductive in small thicknesses, but by the time you can visibly see the buildup as tarnish, it is usually thick enough to be quite a bit less conductive and can occur in as little as 1 to 6 months. Relative Humidity was found to be a major factor, as increasing RH from 5% to 75% resulted in a growth rate of 3 to 10 times.
Steve
Yup, I've built cables with air dielectric and cotton and after a few years the silver wire is completely black and the copper wire is a dull brown. UPOCC silver tarnishes much more slowly but after 5 years or so shows tarnishing but isn't black like the regular silver wire. This is in Colorado with relatively low humidity.
I don't understand how some cable companies sell cables that allow the wires to tarnish, any cotton or air dielectric type cable is going to do this and the sound is likely to change over time as a result.
I did build a cable with nitrogen dielectric, this is a good solution but expensive and just a little better vs regular teflon insulation....
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Also, I have SilClear, Mapleshade's contact enhancer and it changes the sound quite a bit, unfortunately it sounds like fairly low quality silver wire was added to the system. Some may like it but beware of this issue and never apply so much it can run, people have ruined gear this way too. Personally, I'd stick to Caig.
