Is it possible for a coin like that to achieve deep iridescent toning naturally?
Coins can achieve remarkable tonings. One of the most popular coins in my collection is this double-struck
Furia denarius, which has mellowed over time - it was a uniform light-grey when acquired, now it's resplendent in blues, golds and reds.
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I cannot comment on whether Doug's
Antonia denarius reached its state naturally, but as my
Furia is pretty typical of many coins in my collection, I assume there must also be outliers. I can comment on factors conducive to
toning: mint-lustre surfaces, whether on an EF+ coin or on protected surfaces at devices' edges, seem to spark the best
iridescent blues and golds; in contrast, cleaned coins often struggle ever again to achieve a nice
toning, because those minty lustrous surfaces get abraded, even if only to a tiny degree (this is a reason I'm wary of cleaning silver - the damage can be irrecoverable). So on Doug's coin I would expect blue
toning between
palm and
arm, and between
palm and horses, and around the legs and
chariot, and between lettering and
head, because these are all protected areas, and that is what we see. The general blue-gold
toning to the wider exposed fields in Doug's coin strikes me as odd, but I don't also know whether that's partially an artifact of photography that may have boosted the gold's and blues. So I'm agnostic.