Well, these powder-soft layers you call
patina is no real
patina but an oxidation layer that has to be stabilized before starting any cleaning attempt.
Depends how hard the
patina layer on the coin is and how easy the dirt can be seperated. Sometimes this helps:
Soak the coin in alkaline
weapons oil for a couple of days, maybe with some gentle heating-cooling cycles.
Then start brushing (selective if possible) with soft nylon brushes, using detergents under running water jets. Observe carefully how the dirt and the surface change. Stop if there is damage at the
patina.
If you are lucky some dirt separates and the
patina remains nearly unchanged.
Then start picking off the dirt at the
sharp edges you created by the brushing procedures, using soft wooden sticks or a
sharp but flexible steel blade (eg. scalpel).
If you are running out of
sharp edges between dirt and
patina the probability of causing damage increases. So repeat the brushing or the whole procedure to create new starting edges for mechanical cleaning.
Sometimes the higher parts can be cleaned just by brushing and the dirt remains in the lower fields of he coin - creating a "sand
patina" like contrast.
If the
patina is hard enough you might use harder nylon brushes or even soft brass brushes.
Sometimes the dirt and the
patina can´t be separated this way cause they are completely mixed and grown together and there is no
border line between any more. But that´s a different procedure
