Cleaning and Preservation of Coins - A Complete Guide, page 8
MECHANICAL CLEANING: How To Clean a Coin with a Toothpick
3) Cleaning Coins with a Toothpick
Circulated and unearthed coins having insoluble dirt on surface, in corners or recesses can be treated carefully with bone styli, a bamboo skewer or with small pieces of wood such as toothpicks.
Sometimes an attempt to brush the encrusted dirt off the coin fails, and the coin's appearance is still unattractive.
Before cleaning with a toothpick, the coins must be soaked in olive oil or unrefined sunflower oil for a few days. The oil's fat acids will soften the dirt thus making the following cleaning job easier (more info in Soaking Coins in Oils on page 6).
As the olive oil treatment and the brushing have failed, now it is time to try to remove the dirt carefully with a toothpick.
Begin scratching the dirt off the coin surface at any point and work your way all around the coin's relief.
