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Patinations are the natural surface coatings that occur on ancient coins over time. For the most part, they are the results of chemical changes to the coin's surface, but in some cases are actually deposits on the surface of a coin from external sources. Unfortunately this is a very complex subject, and we cannot deal with it in depth here, but will just touch on the basics.
The vast majority of ancient coins are rather ugly when they are first found (or dug up) and they require professional cleaning before most collectors would want them. For the most part, this discussion is about the patination that remains after the coins have been cleaned. Silver, gold and bronze coins all patinate in different ways, and will be discussed separately.
GOLD COINS
Unless buried in a very corrosive or abrasive environment, very little changes occur on gold coins and if properly cleaned they come out looking pretty much the way they did when they entered the ground. Most collectors of gold coins consider any form of patination a detraction which reduces the value, and should be mentioned when describing the coin.
SILVER COINS
For the most part, silver coin hoards have rather ugly patination when first dug up, often in the form of silver sulfide (hornsilver) coatings, and have to be cleaned before they are attractive and salable. Only on rare occasions will the original hoard's patination be attractive and desirable. The cleaning process leaves most silver coins with very bright metal, but if cleaned properly they will re-tone over time and look very natural.
There are two schools of thought about toning. Most experienced collectors prefer toned silver coins and will pay a premium for attractively toned examples, but some collectors, especially inexperienced ones, prefer bright and shiny silver coins. One should remember that silver coins naturally tone over time and will require periodic re-cleaning in order to keep them bright. Since each cleaning will have at least a small detrimental effect, and a patina offers a protective layer for the coins, we feel it is better to just let them tone. Then again, one should not turn down an otherwise desirable coin just because it is still bright.
This silver denarius came to us in a totally un-cleaned state and, while somewhat worse than most, it shares many of the problems of silver coins fresh from hoards. There is a thick layer of silver chloride, including a number of raised bumps and slightly rough areas. We think you will agree that it has very low eye-appeal and is of very low value the way it is. To make it marketable, it has to be cleaned, but the risk is high, as each bump could hide a corrosion pit, and the rough areas could be just in the patination, or actually in the silver. The only way to find out was to clean it, but doing so still resulted in a very ugly coin.
Coins with a subdued natural silvery appearance are the norm and a coin should be assumed to be like that if no comments are made. Attractive natural toning can increase a coin's value, so toning may be mentioned. Very bright silvery coins should always be described as such, as they have a slightly reduced value. One must also be aware of artificial (chemically-induced) toning, which seldom looks natural, and the chemicals used may have long term detrimental effects. Such coins should be discounted. Unfortunately, only experience can teach one the difference between natural and artificial toning.
BRONZE COINS
Under this heading we are lumping together patination and encrustations. Patinations are the chemical changes that occur to the metal surfaces, while encrustations are deposits of foreign matter (dirt, etc.) cemented to the surfaces and which do not just simply wash off. The two are very closely related because the cementing process that holds the encrustations to the surface is often (but not always) part of the patination process. Here things get very complex and we can only discuss the basics in the space available.
The vast majority of ancient bronze coins come out of the ground in pretty rough shape, with very ugly patination, and for the most part there is nothing that can be done with them. These are known in the trade as "Floor sweepings" with little or no value beyond being tourist curiosities and examples on which to experiment while learning cleaning techniques. Note how some of these coins have traces of designs and one might even be able to identify some of them, but a careful examination shows that most are very worn with corroded surfaces. As a rule, if the surfaces are corroded, no amount of conservation or cleaning is going to make the coin into a nice example.
There are several types of natural patinations but the vast majority will be either of copper oxide (mostly brown or black) or copper carbonate (usually green and sometimes blue), and more often than not the two types will appear together with the green overlaying the browns. There are no firm rules, but we have found that green patinations are more popular with European collectors and browns with North American collectors.
Tiber Patina is a third type of bronze preservation, referring to brass coins (the conditions that create them normally destroy copper and bronze) which are well preserved with no patination, but rather a very lightly pitted brassy surface. True Tiber patination is normally seen on coins recovered from river bottoms with oxygen free conditions, but there are some soil conditions that can cause this as well. The term is also loosely (and incorrectly) applied to coins found patinated but with their patinations stripped away to give a very similar appearance. Tiber Patina coins can be collectible though normally they are not as highly prized or valued as coins with other natural patinas.
A very small percentage of bronze coins come from the ground in high grade, looking very pretty, and they go to the market as is. These have the ultimate in natural patinations and are prized by experienced collectors who can recognize them. However, usually the nicer coins are found in a rather ugly state, with both heavy patinations and encrustations, and they must be cleaned. A skilled cleaner working under a microscope with small tools can often remove the encrustations and ugly parts a bit at a time, retaining the more attractive solid natural patination that often lies below. If done expertly, the coin can look attractive with no evidence it was ever cleaned. Unfortunately, this is a very slow process and not economically viable for inexpensive coins, although many collectors clean coins for their own collections in this way, without concern of the economics.
At this point chemical cleaning comes into play, which generally means using solvents and corrosive substances to strip away much or all of the existing patination. In some cases this can be done selectively leaving some natural patination, but often the coins are cleaned down to bare metal (normally exhibiting a light pitting) which is then artificially patinated.
Artificial patination is a specialized subject that cannot be dealt with in depth here. At its worst, it can be a simple layer of paint which can be hiding heavy pitting or other problems. There are also chemically induced artificial patinations, some of which are very ugly, and others that look very natural (the chemistry that creates them can simply be an accelerated form of what happens naturally in the ground). The best artificial patinations look so natural that they often go unrecognized as such. The effect of artificial patination on value depends very much on the eye appeal, and how natural it looks. The worst ones can render a coin almost worthless, and the best have little effect on value.
As with other coin characteristics, it is all about eye-appeal. The more attractive one finds the patination, the more highly one should value the coin, but one should always keep in mind that some chemicals used to clean and artificially patinate coins, if not properly rinsed and neutralized, can remain active with unexpected and possibly very serious results many years later. Also, ancient bronze coins with a genuine natural patination pretty much have to be genuine, while an artificial patination could be hiding a fake. It is well worth the time and effort it takes to learn the difference.
As a rule, artificial patinations are worth less than attractive genuine patinations and should always be commented on. Really attractive natural patinations can increase a coin's value and may be commented on.
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