Articolo: La patuina sulle antiche monete di bronzo

[Vel Saties]

https://coinweek.com/patina-on-ancient-bronze-coins/

Patina on Ancient Bronze Coins

 

By Tyler Rossi

August 28, 2023

By Tyler Rossi for CoinWeek …..
 

Unlike modern coins, ancient coins are not held to the same minute condition requirements of the 70-point Sheldon Grading Scale. What matters instead is eye appeal, and one of the main factors that creates pleasing eye appeal is patina. So, what actually is patina?

Basically, true patina is any surface oxidation that occurs on the metal of a coin immediately after striking. In modern copper coins, this is the difference between the Red, Red Brown, and Brown color designations. However, in the common parlance of ancient coin collectors, a patina is any coating of foreign material created over the many years the piece was buried in the ground. This oxidization comes in many forms, and is highly reliant on two factors: the environment of the soil the coin was buried in and the type of alloy from which the coin was struck.

There are a number of different types of patina, most of which fall into two categories: Green and Red.

The most common patina type, green, is formed by copper acetate Cu(CH₃CO₂)₂. This copper oxide is the natural result of a transformation between the coin’s copper metal and acetic acid present in the atmosphere. As can be seen on the two examples below, a group of small Greek bronze coins and a 25.5mm AE of Roman emperor Severus Alexander, this green coloration can come in a variety of tones, ranging from almost black to a lighter green.

Lot of Greek Bronzes. Image: VAAuctions. Roman Provincial Bronze of Severus Alexander. Image: CNG.

A subset of the standard green patina is what collectors have termed the “desert patina”. This is actually a combined effect. It is generally characterized by a sandy light brown coloration set off by a darker sub-layer of green or brown patina. Desert patina often acts as a spotlight helping to highlight a coin’s details. True desert patinas are generally formed in dry climates where the dirt is generally dustier and lighter in color. Effectively, it is sand or dust that has become chemically attached or glued to the surface of a coin that already has a patina. The dramatic highlighting effect is then created by a collector who removes a portion of the encrustation to reveal the sub-patina.

Rabbathmoba bronze of Septimius Severus. Image: Heritage Auctions.

It should be noted that a desert patina can be easily faked by removing the original surface encrustation, revealing the brownish surface of the coin and then gluing sandy dirt to the coin. Luckily, depending on the adhesive used, it can be relatively easy to remove, either with water or a soft brush.

The second main type of patina is comparatively rare and can either be highly appealing or quite unattractive. Formed by either cuprous oxide (Cu₂O) or copper(I) oxide when the cupric oxide (CuO) or copper(II) oxide in the coin and surrounding soil react, this can appear as bright or even dark red. It is rare, however, to find a coin with only a red patina, and, as with all patinas, it occurs on top of the natural browning of copper as described at the start of this piece. This can be seen in the first example below, a sestertius of Severus Alexander minted in Rome in 231 CE. The relatively visually unappealing splotchy red encrustations are sitting on top of a thick reddish-brown patina.

Roman Sestertius of Severus Alexander. Image: Goldberg Auctioneers.

This second example, a bronze Roman Republican coin struck by Spurius Afranius in 150 BCE displays a more even red and brown patina. While eye appeal is definitely subjective, this kind of patina is more objectively beautiful than the example above and definitely more stable.

Roman Republican Bronze of Spurius Afranius. Image: Davissons, Ltd.

Brown coloration, another common type of patination, is either formed by a very thin film of copper oxide deposits formed naturally through contact with oxygen or a simple mix of green and red patinas. The former would have occurred directly after striking, before the coin was buried or lost.

Ionia Greek Bronze (380-360 BCE). Image: CNG.

Blue patina is extremely rare and more often than not it is actually “bronze disease”. “BD” is when a bronze artifact undergoes a chemical reaction and produces acid internally. As a result, the piece will display powdery blue-green growths on the outside and begin to disintegrate. On this example below, the green-blue spots of bronze disease can be seen highlighted by the green-brown patina. Unlike patina, BD is a subtractive process, and if it is brushed off the result will be a pockmarked surface, whereas a patina is an added layer of material.

Roman Bronze of Nero. Image: CNG.

Interestingly, BD was originally thought to be a bacterial infection, hence the term “disease”. Many collectors and scientists have weighed in on how BD is caused, and what can be done to stop its spread; I won’t be discussing that here, as it is a complex topic worth of an article on its own.

On the other hand, while often delicate, a true blue patina is not bronze disease. For example, most bronze statues that are not treated or sealed will form a light greenish-blue patina. This coin below has is a spectacular example of the delicate light blue patina formed by some variety of copper oxide.

Gordian III Sestertius. Image: Bertolami Fine Arts.

On the other hand, blue-green patinas can also appear waxy and be very solid. This AE double unit struck by the Italian mercenaries known as the Mamertines in Sicily between 278 and 270 BCE is one such example.

Sicilian Bronze (c. 278-270 BCE). Image: CNG.

Next is the famous “river patina”. Often called the Tiber Patina, this is a type of surface corrosion formed in anaerobic conditions, such as at the bottom of a river or in waterlogged clay-like soil. These coins are often characterized by a subtle brass color, with little to no actual brown patina. Additionally, they usually have a light pitting spread evenly across the entire surface. This is due to the clay preventing oxygen from reaching the coin, and potentially reversing oxidation. As a result, the coin will appear to be in the same condition it was in when buried.

A Tiber Patina in not simply a coin with a heavier patina stripped off. For example, this bronze dupondius of Augustus and Agrippa struck between 10 and 14 CE has had its patina stripped and was harshly cleaned. As a result, this coin hammered for $100 USD in 2015.

A different example with similar circulation wear and strike but with original patina sold for nearly $400 in 2013.

Dupondis of Augustus and Agrippa. Image: Auctiones GMBH.

On the other hand, a true Tiber Patina will present a more freshly struck appearance. This coin, for example, shows no sign of tooling or of the removal of a patina. And while there is no documentation to prove where it was dug, it was likely found in the anaerobic soil of a peat bog or riverbed. For example, bronze coins found during archeological digs in the boggy soil around the fortress of Vindolanda in Britain often display these same surfaces.

Bronze sestertius of Vespasian. Image: Heritage Auctions.

This example from the Roman emperor Vespasian is unusual in its state of preservation. More commonly, coins with a Tiber Patina have more pitting and had experienced more circulation wear before being deposited. For example, this sestertius struck under Titus is, as ROMA Numismatics describes, “light[ly]-corroded”. The coin is, however, an exceedingly rare type and sold for 8,000 GBP in 2016 (about $10,841 USD when adjusted for inflation).

Bronze sestertius of Titus. Image: Roma Numismatics Ltd.

So, if patina is technically corrosion, then why don’t all buried coins simply rot away to nothing?

First off, some do. It is not uncommon to find an ancient bronze coin so corroded from acidic soil that it is basically a blank disc or slug.

However, those that don’t corrode away to nothing are saved either because of an absence of soluble salts (which in turn ensures a relatively pH-neutral soil) or because the corrosion on the coin has reached a chemical equilibrium with the surrounding dirt.

These patinas, if visually appealing, are able to smooth over serious condition issues on the host coin and may increase the value significantly. As a result, it is not uncommon for people to try and fake a patina. Using a number of different chemical treatments, an unscrupulous individual can create a relatively convincing fake patina.

Luckily, it is usually not too difficult to tell the difference between a fake and the genuine article–especially since most genuine patinas that have been deposited on the coin over potentially thousands of years will have hardened to such an extent that they almost become part of the metal and are extremely difficult to remove without damaging the underlying coin. Nevertheless, this is not a hard and fast rule, and some patinas (like the rare blue-green type) will be soft and easily wiped off.

Overall, it is important for a collector to carefully examine any potential purchase to ensure that it has not been tampered with.

Happy collecting!

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Sources

https://www.metaldetectingworld.com/cleaning_coin_p3_patina.shtml

https://www.cambridge.org/core/journals/powder-diffraction/article/abs/nondestructive-evaluation-of-roman-coin-patinas-from-the-3rd-and-4th-century/EB04FC26C4E9913EE9A8068F10D0BEE1

Caridi et al., 2014 – An investigation on the patina of ancient bronze coins

Sandu et al., 2006 – Authentication of old bronze coins I. Study on archaeological patina