Mint state ancient bronze and copper coins!

Exceptional examples Al.

 

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Great coin, Randy.

 

I don't recall seeing many ancients that I believe to be mint state but the Magnentius below has toned to an even black on smooth surfaces so is close.

 

I'm late to the thread as usual but I wanted to add a little to the chemistry side of things. It's incorrect to state that it's impossible for copper to remain untarnished since ancient times. It is possible, just unlikely. The most critical steps actually happened in the first few days after minting and to what extent the coin eventually deteriorated is in many cases due to what happened before it was buried.

The tarnish on modern copper-based coinage is due to chemical reactions precipitated by sweat. This is an irreversible process that often leads to unpredictable secondary reactions. Since it's all but a foregone conclusion that ancient coins were being constantly handled as they were being made and transported not even "mint state" coppers buried in pots shortly after being made would have escaped at least some degree of corrosion.

Oxygen actually has only a secondary role in the chemistry of ancient coin corrosion. Copper is fairly unreactive with oxygen directly at room temperature. This you can prove to yourself by taking a bit of the freshly cut metal and leaving it out in the open. It will stay coppery for years without discoloration so long, again, as it wasn't in contact with skin. It will eventually turn black but will take decades to occur. More on this in a bit.

Once in the ground, the patina on ancient coins most often formed due to percolating water which carries with it the acids of decomposing organic matter of the soil above. Depending on the precise cocktail of chemicals dissolved in that water you will get different types of corrosion byproducts.

Black patinas are the least destructive as the reaction is primarily the result of copper oxide which tends to be self-limiting. Cu2O forms as a dusty red film but this molecule is unstable and breaks down to the monoxide CuO which is black and impermeable both to water and oxygen itself (thus creating a barrier arresting further damage). Black ancient coins are thus typically those found at the center of clumps where there was little direct contact with soil or water.

Blue-green patinas, while often attractive, spare less of the coin and can often cut right through to its center effectively leaving behind a "fossilized" shell of a former coin. It is caused by the action of sulfuric acid on the black oxide to form copper sulfate, which is blue, which then decomposes to the much more familiar green of copper nitrate. Apparently the reaction is as follows CuO + H2SO4 → CuSO4 + H2O (yes, I had to look that up. I'm not that smart lol). The reason that it's more destructive is that as you can see the reaction releases water which immediately dissolves more of the surrounding acids and starts the cycle anew until either no more copper is available or the moisture runs out.

A second major problem to consider is that we forget that ancient copper was rarely anything approaching the pure elemental state. The effects of inflation savaged the purity of post-Tetrarchic copper coinage to as low as 50% of the coin with the rest being filler metals like lead and tin. The effects of their corrosion on the integrity of the coin are impossible to predict with so many variables but the fact you rarely find a bona fide EF AE4, let alone cartwheel luster, leaves little doubt that the odds are stacked against those poor things escaping intact into modern times.

However, as the Titus sestertius from earlier shows, it does happen however rarely. I vaguely remember a video of another pulled out by an archeologist that was absolutely gem GU. Not that so-called "Tiber patina" bs. It was flashy in a way you think of a Sacajewea or whatever those vending machine dollars are called that nobody uses. The chemistry of the sestertius, that orichalcum we've all heard of before, is significantly hardier than your average AE. This is because the presence of zinc confers a fair degree of corrosion resistance. If one such coin fell in riverbed clay, for example, there's a reasonably good chance very little corrosion would take place since this is an environment that is both anoxic and relatively inert.

Oh, and in case any y'all are still torturing your uncleaneds with olive oil please stop. You're hurting them.

Rasiel