If you are interested in this method in more detasil it is described an discussed in the article Ponting,
Bucher and I wrote and which is available as a
Forum resource article on
Otho denarii. There is a picture of the extreme use of the method in my
Otho Forum of an Alexandrian teradrakhm. The core appears as "pure" copper. Those coins are a special case, as they contained as much silver as an Imperial
denarius although they weighed much more, so the surface depletion
had to be more pronounced.
s.
Jyrki Muona
