I only have experience of toning bronze, but an important factor must be common to silver,
namely that of temperature. I have a lamp that's on a lot, with a compact fluorescent lamp in it, mounted base down.
If you slide a penny that someone has 'cleaned' in between the glass coils, it's supported by its edges,
and sits at a nice temperature, and re-toning is surprisingly fast, and even on both sides of the coin.
Acetone first is a good idea.