Toning is an alteration of the chemical makeup and color of a coin's surface.  It takes place naturally over time as the metal reacts with chemicals in its environment, typically sulfur based compounds.  Natural toning takes place more quickly in a warmer and more humid environment.  Toning is a thin film of sulfide that is sometimes found on the surface of reactive coin metals such as copper and silver coins.  It is what can cause a coin to have beautiful hues ranging from bright blue to deep magenta to bright orange and light gold.  However, heavy toning can cause the coin to turn to an ugly brown or even black.  Therefore, a coin's toning can either be attractive and add to its overall grade and value or negative and bring down a coin's final grade and value.



Silver coins as a whole, tone much more beautifully than those made of other metals.  Silver, exposed to small amounts of sulfur in the right conditions and environmental influences can naturally turn subtle or brilliant shades of yellow, blue, red, magenta, turquoise and other colors of the rainbow.  Often, coins placed in certain albums, envelopes, mint canvas bags, paper rolls and other older type holders, were subjected to small amounts of sulfur found in the albums, envelopes, mint bags, etc.  This sulfur, over time, combined with the silver to create these beautifully toned coins.  This was exactly what happened with certain Morgan Dollars known as the Battle Creek Collection.  Morgan dollars had been left for over 120 years in their original mint bags in the vault of a bank.  Those dollars touching the canvas of the mint bags were subjected to trace amounts of sulfur from the bags and over time, created beautifully toned Morgan dollars.  If coins are in contact with too much sulfur and time, the coins will eventually turn black.

Though toning on silver is most often caused by sulfur, the word TONING is sometimes used to describe other coloration on the surface of the coin, even stains or dirt.  Silver can react with other substances such as chlorides found in water to create Silver Chloride which typically appears as an unattractive black, gray or dark brown stain that is actually on the surface of the coin and can often be smeared like grease.  You may have seen this grease like appearance on some silver coins.

The toning of silver coins is partly a factor of the other metals in the silver.  Most silver is alloyed with copper.  Ninety percent silver coins from the US such as Morgan Dollars, tone differently than sterling silver coins found in Britain or 99.999% silver such as American Silver Eagles.  Ancient silver coins and most world silver also had differing amounts of silver which determine their reactivity with their environment and the colors created.  Many of the world coins and ancient coins had a higher content of copper which results in a Green Verdigris color often seen in coins such as the Ancient Roman coins.

Copper is the most chemically reactive numismatic metal used in the US.  Copper and it's alloys such as bronze made from copper and tin OR brass made from copper and zinc usually turn from red to dark brown over time.  But, they can also turn brilliant shades of blue, green, orange, red, purple, green and yellow during their course of changing colors. 



Toning on copper and its alloys is often called Patina.  Brown or black patina on copper is caused by copper oxide, cupric oxide, or cuprous oxide.  Green is caused by copper sulfate or copper sulfide and Blue is created by copper carbonate. 

Nickel generally tones only slightly, typically becoming a hazy gray though sometimes a light golden, pale champagne or pale blue.  Wild rainbow toning found on nickel is almost always artificial.

The color of the toning on any coin is a factor of time and how thick the film of toning is that covers the surface.  For example, early toning on silver coins is yellow with colors progressing to orange, magenta or purplish red to blue, and then finally to green or cyan.  If given enough time and enough reactive materials such as sulfur, the coin will turn to black.

Different coin types also tone in different ways.  For example, Morgan Dollars tone more beautifully than Peace Dollars because the planchet of the Peace Dollar was given a more concentrated acid bath at the Mint.  Walking Liberty Half Dollars tend to acquire an unbalanced toning as a result of their assymetrical design and many early commemoratives of the 1930s have tab toning from their original cardboard holders which contained varying amounts of sulfur.



If sealed in an airtight environment, the surfaces of a coin will deplete the sulfur and other chemicals around it and stop toning.  Coin Holders used by grading companies are designed to allow for the depletion of those chemicals which stabilizes the color if stored in the proper moisture free environment.  Thus, you don't have to worry about your graded coins changing colors too radically over time if properly stored.

I hope you have found this chemistry lesson on the toning of coins informative and interesting.  Please don't forget to vote to let me know.