Question regarding bronze coin surface adhesions

[swampaggie]

A few months back I viewed a Claudius sestertius with odd surfaces that I haven’t seen before on an ancient Roman bronze coin (sorry, no photo).  There were several raised spots of metal on both the obverse and reverse.  The metal bumps had fresh shine and did not share the patina that was on the rest of the coin’s surface.  Today while looking at acsearch I came across this Antonia dupondius that has similar surfaces (interestingly both coins have NCAPR countermarks).  Are these raised metal bumps natural surface adhesions?  Do coins with these surfaces typically turn up in a particular region or soil type?  Thanks in advance for sharing your knowledge.

[SC]

These "pimples" are a kind of corrosion.  They are not an adhesion in that it is not an outside material that is sticking to the coin, instead the actual metal of the coin is corroding and expanding.  You can see that there were several larger "blisters" on the same coin that either burst naturally or were chopped away in cleaning. 

Shawn

[swampaggie]

Thank you for your reply, Shawn.  Is this form and extent of surface corrosion common on such bronze coins?  Admittedly I have never worked with uncleaned coins but this is the first time I've seen a coin exhibiting such surfaces.

[Joe Sermarini]

It is unusual.  I have never had a coin that looked quite like that.

[SC]

Not usual but I have a few.  Mine all seem to be first century and mostly copper - not bronze or orichalcum/brass.  Might be something in the copper of that period that made such reactions occur from time to time.  I don't recall right off what Claudian sestertii are made of.

Shawn

[swampaggie]

It is unusual.  I have never had a coin that looked quite like that.

Thank you for sharing, Joe.  That helps put the frequency of such surfaces in perspective given the large number of coins you've handled.

Not usual but I have a few.  Mine all seem to be first century and mostly copper - not bronze or orichalcum/brass.  Might be something in the copper of that period that made such reactions occur from time to time.  I don't recall right off what Claudian sestertii are made of.

Shawn

Shawn, your last comment prompted me to pull out my copy of Sear.  Both coins in question are brass (not bronze as I inaccurately stated at the beginning of this thread - sorry).  Sestertii and dupondii issued under Claudius are brass, while as and quadrans are copper.  The Antonia dupondius is brass. 

[dougsmit]

It would seem that the next step in this question would be to scrape off some of the bubbles and have them analyzed for content.  Perhaps they are surplus zinc not alloyed properly or brought up by fire???   The problem with such an investigation is that it would be easy to spend several times the value of the coin and gain no knowledge that would improve it. 

[Joe Sermarini]

I also suspect fire may have had a role.

[lordmarcovan]

Fire was certainly the first thing that crossed my mind.

[Romanorum]

These "pimples" are a kind of corrosion.  They are not an adhesion in that it is not an outside material that is sticking to the coin, instead the actual metal of the coin is corroding and expanding.  You can see that there were several larger "blisters" on the same coin that either burst naturally or were chopped away in cleaning. 

I agree, although I posit that the "blister" might be a reaction between some impurities in the metal itself which slowly erupt and then integrate with whatever minerals on the soil. Hence the "mineral" appearance of the blisters on the Antonia coin. The example below, Philip II, is not quite the same type or blisters, but you can see a few different variants of blisters that might have followed a similar process.