Patina or toning, is a thin layer of chemical compound on the surface of the coin which subtly affects the way the coin is viewed.  As the layer on the coin thickens with time, the effects diminish, and the coin will appear more and more tarnished.  It is analogous to the difference between a suntan and sunburn.

 A coin without patina is very much like a mirror.  All frequencies of light – excepting for those the color of the coin – are reflected back at the viewer.  This is why it is said that proof coins have mirror fields.  As patina forms, the coin’s function as a mirror will change.  Different colors are absorbed; different colors are reflected.  As well, light can be refracted by the surface of the coin, as happens with a prism.

 A tarnished coin is no longer capable of efficiently reflecting light in the way a mirror does.  Light hitting the surface is scattered.  Usually only the color not absorbed by the tarnish is reflected.  Thus when you see a heavily tarnished copper coin, you are no longer looking at copper.  Rather, you are looking at a whole lot of copper-oxide, which is green.

 One consideration to use in regards to patina is that what goes in greatly affects what comes out.  Different light sources produce light which varies in spectral composition.  Fluorescent and incandescent lights will make a coin look different than it will in sunlight.  Thus what is reflected back for you to see will differ.  Remember this when you are looking at the scan of a coin.  An HP scanner does not produce the same spectrum as the sun…

The compounds which comprise patina are ionic.  Oxides and sulfides are most common.  Chlorides, sulfates and carbonates are also to be found.  These are formed on the metal surface of the coin by the oxidation of the coin metal by gasses in the atmosphere.  Oxides are formed by the action of oxygen, often as allotropic ozone (Ozone is no more than a slightly more reactive form of oxygen).  Sulfides are formed by the action of the pollutant hydrogen sulfide.   Other common atmospheric villains like sulfur dioxide, hydrogen chloride and nitrogen dioxide can influence patina.

 It is important to remember that patina is comprised of a thin layer of molecules on the metal surface; otherwise the mirror effect would greatly diminish or lost.  Also, the molecules will not all be of the same kind.  There will be a bit of this, more of that, and some of another thing.  All of this complicates the expected result.  For example, in the case of copper, the oxide is green, the sulfide is black and the copper-chlorine complexes are red.   Thus a penny can vary in color greatly depending on the coat it wears, or not.

 The rate at which patina accumulated on a coin is proportional to the concentration of pollutants in the air.  The nasty, sulfur-laden air over a major city will tone a coin faster than the air in an isolated forest.  Local environmental factors also can speed the process.  Silverware, both sterling and plate, is famous for tarnishing rapidly in the presence of egg yolks, which have a significant amount of organically bound sulfur.  The black tarnish is nothing more than silver sulfide.

 Unfortunately, scoundrels have also learned their chemistry, and artificially toned coins, both real and counterfeit, are often offered to the unwitting as something they are not.  One takes a heavily tarnished coin and removes the tarnish by cleaning.  This greatly lowers the coin value, as the coin will obviously have been cleaned – a no-no.  Adding a little good looking patina will hide this, thus artificially increasing the price at which the coin may be sold.  One can also counterfeit a coin, as is often done in New Cathay, and then add a soupcon of toning.  Voila, a “legitimate” treasure for the trusting pigeon.

 Examination of such a coin under a strong lens will reveal wear patterns more consistent with cleaning than normal handling, such as circular scarring, obviously buffed areas or pitting caused by the chemical removal of the tarnish.