I was curious about how humidity affects silver so I went down that interesting rabbit hole for a while and, now that I'm back, I'll share some of what I learned.
When silver is exposed to sulfur-containing gases in the air, it discolors and will then keep darkening as it reacts with the gas to form a surface layer of tarnish. On pure silver that tarnish is a compound called silver sulfide (Ag2S). Tarnish on sterling silver contains silver sulfide as well as copper sulfide (Cu2s).
Tarnishing is a process, so the color of tarnish on silver changes as tarnishing proceeds. This is due to a cool phenomenon called, thin film interference. Thin film interference is a natural phenomenon that reflects different layers of the silver and tarnish.
When you look at a coin with tarnish on it the light hits the thin film of tarnish and splits, with some of the light reflecting back from the different layers tarnish and some of it reflecting off the silver underneath. That's thin film interference and you've likely seen this same phenomenon in light reflected from soap bubbles, or in a parking lot when it's wet and you see the multiple colors of an oil spot on the water.
A few things that contain and release hydrogen sulfide are; Humidity, air pollution, rubber, wool, natural oils in our skin, acidity, and sweat from our skin can contain sulfur and contribute to tarnish.
Here is an extremely tarnished 1830 capped bust half dollar.
At nearly 200 years old it makes sense that this coin would be covered with silver sulfide and copper sulfide, as this coin is 89% silver and 11% copper.
That's all for today's lesson ;)
