There is no formula.
Patina is a result of several factors:
- moisture in soil, sometimes this even varies across one object if
part was more often wet, this is
rare to see in coins but can be seen in larger
antiquities- minerals in the soil, combined with moisture levels,
- proximity to other objects when buried and their metallic content,
- isotopic content of the metal itself, which varies across the coins and metals did not mix perfectly in the ancient world.
SC