How do you make silver coins rust?

Silver (coins) do NOT rust.

Rust the iron when oxidised.

Silver will oxidise very slowly, to form silver oxide, which is black in colour, This oxidation process can take weeks/months to occur.

If you have any silverware, you will normally polish it once a week. this prevents oxidiation. However, if you leave it unpolished for say , 6 months, the silver will appear dull or black in places; this is silver oxide.