Hello, someone could help me with a question: In gold coins from the 15th and 16th centuries, the formation of patina (oxidation of the alloys that make up gold, usually silver and copper), normally this patina forms from the center of the coin to edges or from the edges to the center, or is there no pattern? 
The example below me looks like it forms a patina from the edge to the center of the coin, without reaching the planet.

France, Duchy of Bar, Robert I, Florin d'or

Thank you!