Coins with soil encrustation can be soaked in detergent water, or stronger kitchen cleansers, then brushed, but when dealing with large numbers of coins, a rock tumbler is most efficient. These can be had inexpensively from lapidary supply shops or Edmunds Scientific. Tumble with wood shavings (try a pet shop) or, for stubborn cases, wet-tumble them with sand and water, but check frequently or they can quickly become pale and over-cleaned looking. A narrow brush with stiff brass bristles can be very effective on soil, tars, and other adhesions, while leaving light green patinas underneath unharmed. If you are reluctant to take such a harsh instrument to your coin, remember that even harsher tools were used at the mint to remove the flashing left from the casting process itself. Never use a wire brush on a struck coin, though. A more gentle product which is still effective is a "grill scrubber," rough plastic fibres with a plastic handle used to clean barbecue grilles.
For heavy encrustation nothing short of acid cleaning will bring the characters to full visibility, especially if small calligraphy details are important, or you want to make a rubbing from the coin. Obtain muriatic acid (commercial grade of Hydrochloric acid), found at hardware stores. Mix about 1:1 or 1:2 acid to water. A large number can be immersed at one time and each batch of acid will serve several batches of cash. When bubbling action has ceased (usually 30-120 seconds) pour off acid and rinse coins. You can experiment with acid strength and immersion time to leave some patina, but usually it is the crusty blobs that go last. To neutralize, soak the coins for at least 24 hours in a solution of baking soda and water. Use a brush or Grill Scrub to remove the orange baking soda residue. To prevent rapid, ugly re-patination rub on mineral oil, Vaseline or other grease. Dellar's Darkener, a product made of Vaseline and sulfur powder, will seal the surface, and re-tone brightened copper coins at the same time, though the retoning action on brass is very slow.
I have seen cash coins with an unattractive gray color caused by cleaning with some other type of acid, perhaps nitric or sulfuric. A 1960's hoard of Song cash discovered in a shipwreck off the Thai coast was cleaned this way, and specimens of this hoard today show no ill effects other than the color. I have seen lots on eBay with this color.
I am not an expert on cleaning. Your feedback would be valued.