The photo above shows coins I have recovered over the last few months - mostly from playgrounds. I have experimented with various cleaning strategies using my small National Geographic tumbler with aquarium gravel with varied success until now.
Since the tumbler has a timer that only works for DAYS - I built my own timer box (behind the tumbler) that lets me set 1,2,4 or 8 hours. I rarely use more than 4 hours.
Using white vinegar with a little salt leaves clad coins clean but dull and I was never satisfied with coppers, they just never came out really clean. Then I discovered Finch Products Magic Tumble Clean: This stuff is literally MAGIC! Pennies and clad come out shiny new - it's really impressive and works with all metals! I have cleaned jewelry and relics like crotal bells and they come shiny and near mint condition. Of course some items should not be cleaned since they lose value without the original patina. By for modern coins this is a great way to clean them up so they will run through the Coinstar machine.
I always clean copper and clad separately and use only about 1 tablespoon of this stuff per run. I typically run 20-30 coins at a time and could not be more impressed with this product. Here are some pennies before and after:
Pennies take up to 1 hour to clean, and clad takes 2 to 4 hours depending on how bad they are.