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Patina is that beautiful and brilliant kind of time-created varnish, of a green or brownish color, which covers the surface of some ancient brass medals. It prevents them from deteriorating, and is regarded as an evidence of antiquity.
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 endeavoured 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 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 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.