A few years ago I left a dozen or so common silver halves, Franklins and a Kennedy, in flips, that didn't sell at a flea market, in a black leather satchel for almost a year. In that time the coins started to tone to a nice grayish blue around the edges.
I have since tried other coins, including a 1835 CBH that has been dipped. No luck yet in over a year.
Maybe I should buy a new black leather satchel.