Here is what a highly respected numismatist says about removing crud on a coin. In my opinion, he is discussing conservation rather than cleaning. There is a significant difference!
Quote:"A simple but effective way to remove this crud employs an artist's brush and a solvent. I use Xylol (Xylene), which is an organic solvent available at most paint supply stores. Using a small artist's brush with a wood handle and relatively stiff bristles (equivalent to a baby's toothbrush), follow these steps:

Keep work area well-ventilated and follow solvent safeguards.

Paint one side of the coin with solvent.

Work crud out using tiny circular brush strokes. Don't drag brush across coin's surface.

Concentrate on cruddy areas; then quickly do rest of coin.

Blot off solvent while surface is still wet (rewet if necessary).

Repeat process for other side of coin and any areas where some crud remains.

Add a thin coat of Care, Blue Ribbon, or some similar protectant.

Brush away excess coating using standard "copper brush." These are jeweler's brushes with natural, cream colored bristles and a wooden handle. An old brush (well-used with a greenish tint on the bristles) is preferred."
from - The Die Varieties of United States Large Cents 1840 - 1857 by John R. Grellman, Jr. page v M & G Publications, 2001