Silver Vs Clad Coin Expose To Air

There is slightly different chemistry at work int he two coins. Silver "oxidation" is actually "sulfidation", because silver oxide does not form naturally, so you need some sulfur in the atmosphere or environment. Copper alloys like cupronickel will oxidize in just regular air, eventually, though will react and darken faster if sulfur is also present. Moisture in the air will also accelerate both oxidation and sulfidation. How quickly they tone/tarnish will depend to a large extent on the amount of sulfur present, which will vary depending on what exactly is happening in the room the window is. The window of a kitchen where lots of eggs, garlic and onions are cooked, for example, will have air filled with sulfur, which a lounge room window might not have. If your silver coin is darkening quicker than the cupronickel, I suspect it's been placed in a high sulfur environment.