The safest approach is to soak the coin in distilled water for several days or even weeks, changing the water regularly to help loosen surface dirt. After soaking, only loose debris should be gently removed using a wooden toothpick or a very soft brush, preferably under magnification.
Harsh methods such as acids, vinegar, baking soda, metal tools, electrolysis, or commercial coin cleaners should never be used, as they can strip the patina and expose the bronze to corrosion.
