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Patina is the oxidized metal that coats many ancient bronze and copper coins. It is usually green, but can be many colors including blue and red. The main reason you want to preserve it is that it is usually attractive and the coin under the patina usually isn't as nice. A coin with an attractive patina is often worth more than a coin that does not have a patina at all. A common coin, like a common Constantine dynasty AE3, that has been stripped of its patina down to bare shinny metal is almost worthless. See the extended definition below.
A chocolate patina, lightly worn on the high-points highlighting the design...wow!
Sometimes a coin can be improved by removing an uneven patina...not usually...not this one.
A very attractive lime green.
A touch of red. Some have said this is artificial and the coin is tooled. It is from the same hoard as the Agrippa as the coin above. Most of the high-grade Agrippa ases in collections today are likely from this same hoard. It is not tooled and the patina is natural. Red is most often an indicator of harmful corrosion but in this case it is very thin red toning or patina on copper similar to that of highly collectible red U.S. pennies.
Notice some encrustation remains, but this coin is perfect the way it is!
Like all the coins on this page, this one is worth far more with this beautiful patina than it would be without it.
Near black highlighted by a red earthen fill (that is red dirt). Would you clean it? I hope not.
This blue-green must at least double the value.
Hopefully, you have an idea what a patina is now and why it shouldn't be removed.
Please add updates or make corrections to the NumisWiki text version as appropriate.
The agreeable appearance of this splendid rust, having rendered it particularly desirable to the taste of the Italians, they gave it the name of Patina verde, as counterfeiting the emerald. The French numismatists introduced the expression into their own language by calling it Patine.
Because it is desirable patina may be imitated to improve the appearance of coins. Patina does not, however, readily attach itself to brass and copper: this depends much on the state of the soil in which the medals have lain for ages. The fabricators of false coins have endeavored to imitate it with salammoniac, vinegar, and other artificial compounds; but a coating of this kind is easily removed, and it is by no means difficult to detect the fraud. On the other hand, the genuine patina becomes so inherent to the metal that it would be impossible to scrape it off without injuring the medal which it covers.
False patina is in general black, coarse, and glossy or the color of verdigris, tender to the point of any sharp instrument. The genuine patina or antique encrustation is most often extremely brilliant and hard as the metal itself.
Some more sophisticated false patinas are made by applying ground genuine patina material in binding medium or using other more advanced chemical methods. Sometimes, but rarely for an experienced collector, the result is convincing. Even the most convincing false patina is more likely than the genuine to wear or peel over time.
Any coin with a false patina must be examined carefully to ensure the coin itself is genuine and to detect any tooling.
The novice, unfamiliar with genuine patina must be cautious before condemning patina as artificial. Genuine patina can be softer, particularly if it is blue. A black patina is very often genuine and formed naturally in the desert soils of the Middle East.
Powdery light green deposits, particularly in pits in the coin, are not patina but are an indicator of harmful oxidation, commonly referred to as bronze disease. Such coins must be treated to prevent further damage.
Silver coins do not develop a patina. Silver darkens with age, which is referred to as toning. Thick black silver oxides, similar to patina, can develop in areas on a coin or cover it entirely. This is called horn silver. Horn silver is exceedingly difficult to remove and under the protruding oxide, the silver is severely damaged because the oxide is made up mostly of the silver of the coin.