I wonder if we combined our Caracalla coins if we could get a good portraiture series showing his aging? Here's my two again: Imperial Rome Caracalla, r. 198-217 A.D. (205 A.D.) Rome Mint, AR Denarius, 17mm x 3.3. grams Obv.: ANTONINVS PIVS AVG, Laureate bust right, draped Rev.: LIBERALITAS AVGG V, Liberalitus standing left, holding abacus and cornucopia Ref.: RIC IVi, 136b (?) Imperial Rome Caracalla, r. 198-217 A.D. (211 A.D.) Rome Mint, AR Denarius, 17.5mm x 2.8 grams Obv.: ANTONINVS PIVS AVG BRIT, Head of Caracalla, laureate, right Rev.: P M TR P XIIII COS III P P; Pax, draped, advancing left, holding branch in right hand and sceptre in left hand Ref.: RIC IV Caracalla 184 (?)
Just to change it up a bit, here's an eastern mint portrait. Struck AD 298, so when he was about 10? CARACALLA AR Denarius 3.44g, 18.8mm Laodicea ad Mare, AD 298 RIC 337d; RSC 168e O: IMP C M AVR ANTON AVG P TR P, laureate, draped and cuirassed bust right. R: MONETA AVG, Moneta standing left, holding scales and cornucopia.
There are enough dated types that you could get a set by year with few exceptions. I can think of many worse gameplans. I see deposits and toning but don't think it has to do with alloy. I can be wrong (ask my wife).
My OP coin looks very similar to yours up close, Z - not deposits, but the actual color of copper in the metal, hence my conjecture about the pickling process. Doug seems to think otherwise, but like I said, I'm not glued to the idea.
I find the colour a little closer to gold than copper, but it's an interesting question... I think I have some other Severan denarii with similar colouring and have seen Gordian III period ants that look positively yellow. Definitely worth a closer look.
I'd prefer you not do this but the question could be answered by cleaning the coin. If the color is toning, it will be removed. If it is excess copper, it will become more red. Either way the coin looks worse so I'd prefer the mystery remain in place.