Coin cleaning is currently in disfavor; there are many who say it should not be done. Because cast coins are more porous than struck, their metal is even more vulnerable to attack by cleaning chemicals than a struck coin. Cleaned pieces I have seen in 1920's-30s collections often have ugly (but removable) new light-green or powdery patina, but I have not noticed acid damage. For many Banliang, Wuzhu, Kaiyuan, Northern Song, some Qing and early Vietnamese, you will not be able to attribute by calligraphy variations without cleaning. A properly cleaned, neutralized, and retoned coin will look much as it did during its period of use.|
Coins with soil encrustation can be soaked in detergent water, or stronger kitchen cleansers, then brushed. I've seen good results from a few days' soak in vinegar but have not tried this myself. When dealing with large numbers of cash, 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. This method is used for cash destined for jewelry or other manufacturing uses, not so much for the collector trade.
A narrow brush with stiff brass or steel 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 acid to water. Several coins can be immersed at one time, but do not reuse dirty acid as it can redeposit particles on fresh coins. When bubbling action has ceased (usually 30-120 seconds) pour off acid and rinse coins. Using a plastic or metal brush on each coin under running water may be helpful. To neutralize, soak the coins in a solution of baking soda and water, at least a few hours to be safe. A brush or grill scrubber can remove any 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 post-1400 coins can be very brassy and much slower to retone. Unfortunately this product is no longer made. Substitutes I have seen mentioned online include sulfur powder (or ground match heads) in vaseline, shoe polish, and dandruff shampoo!
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.
One Chinese coin seller also offers small grinding bits for a Dremel-type tool, but I would be reluctant to try this. I have seen ugly scrapes in the fields of coins from small chisels or etching tools.
I am not an expert on cleaning. Your feedback is welcome.