ModernCoinMart proudly presents an in-depth article on coin toning. This article will cover what causes coin toning, precious metals that are the most prone to toning, and more!
Toning is the visual evidence of a chemical interaction process. The metal in the coin interacts with ambient chemicals, often oxygen or sulfur, in the environment. Toned coins are typically composed of silver and copper and, to a lesser extent, nickel, and gold.
Copper is a very reactive metal, and copper is alloyed in just about every coin that the U.S. Mint has struck. Pre-1933 classic circulating 90% gold issues and circulating 90% silver were both alloyed with reactive copper. Likewise, every nickel that has been struck in the history of the United States has been alloyed with at least some degree of copper. Copper and bronze cents are very reactive and change colors as they are exposed to heat, moisture, ambient chemicals, and naturally changing environmental conditions. The same is true of gold and silver alloyed coins.
Toning and tarnish, and patina are essentially all the same process. All coins will tone, tarnish, or patina over decades or centuries. Toning is considered a natural and expected process that will happen sooner to coins with more variables in their environment and more slowly to coins stored in more controlled conditions. The terms "toning" and "patina" will tend to be used to describe attractive colorations on the surface of a coin, while "tarnish" will tend to be used to describe unattractive or dark coloration of a coin's surface.
Silver that has been cleaned or polished—like sterling silver (92.5% silver, 7.5% copper) table wear—will tend to tone back darker in the future after being polished. A great numismatic example of this is the U.S. Trade Dollar. These 90% silver coins were often subjected to harsh cleaning because they had been initially damaged by extreme conditions such as saltwater and high humidity. If you look today, many examples of this series have darker surfaces due to the harsh cleaning, polishing, and subsequent natural re-toning over the last 150 years. This dark toning tends to be dull and is often considered unattractive.
Natural Toning simply happens to a coin over the years. As stated above, toning is essentially a chemical reaction between the precious metal of a coin's surface and an element such as sulfur or oxygen. The reaction causes a compound to form, which changes the color of the metal over time. Some of the most vibrant examples of toning occur over a long period.
The degree of toning that can occur is often dependent upon how the coin is stored and if archival materials are used or not. Sometimes harmful chemicals can be found in older, cheap coin holders and flips. PVC is a major culprit here. Other factors contributing to natural toning are temperature (and temperature fluctuations) and moisture. Proper storage may slow toning; however, the natural color progression associated with toning is just a normal part of a coin's lifecycle. Each type of precious metal has different toning characteristics. For example, a toned copper coin will not look the same as a toned silver coin.
Copper coins, which are the most prone to toning due to the reactive nature of copper, typically begin their lives as brilliantly red, and they tend to darken over time due to oxidation. As a result, there are a variety of designations cooper coins can receive including Red, "RD," which means they retain most of their original red color, Red Brown, "RB," which means they have a red-brown color, and so on. However, coins with a prominent cooper content will eventually darken to the point that surface and design details are hard to discern. This process may take centuries.
Nickel itself is not a very reactive metal. The reactive nature of copper is the primary reason they will tone, as typically, nickel coins are alloyed with some degree of copper to strengthen their composition. Alloy problems can also contribute to toning or spotting.
Gold is one of the least reactive precious metals. Many gold coins are often alloyed with copper, which causes toning. When gold oxidizes, it tends to be orange and seldom seen, red or purple.
Artificial Toning attempts to replicate natural toning through the deliberate introduction of chemicals to the coin's surface. Popular methods include treating the surface of a coin with liver of sulfur or inserting coins, typically silver coins, into a potato and baking it. These methods are often discernable because they result in dramatic toning with little to no transitions between colors. Unfortunately, the unscrupulous can use artificial toning to cover up a coin's shortcomings or profit from collectors of toned coins. In addition, mild to moderate damage may not be apparent to an unskilled collector because the toning may obscure evidence of an old cleaning or other surface issues.
Rainbow toning occurs when atmospheric chemicals, especially sulfur, interact with the surface of a coin over a long period. Such toning is more frequently seen on silver coins. In addition, certain environments, such as paper envelopes or canvas bags, especially when used in warm and damp conditions, can result in lovely toning.
Rainbow toning is a natural coloration that will include perhaps a bluish color and progress to orange or red, or pink over either part of or the entire surface of the coin. This colorization happens naturally without intervention or "acceleration techniques" that are easily detected artificial methods. As a result, some collectors covet and even go out of their way to collect appealing Rainbow Toned coins.
Silver does not rust. Pure silver typically won't tone either. The copper content is the fundamental contributor to toning. There can be "target toning" on a pure silver coin like an American Silver Eagle when the coin is stored in a book with high sulfur or acid content in the paper, especially if there is ambient moisture. Target toning is a coloration that is primarily limited to the edges and rim area of the coin.
The classic example of naturally toned coins would be Morgan Silver Dollars stored in canvas mint bags in dark vaults for decades. These can be vibrantly beautiful with subtle changes in coloration or be vibrant. They are often popular with specialist collectors and can be truly one-of-a-kind coins. The most attractive toned coins can be a welcome difference in a collection of blast white coins. Like many things in life, beauty is in the eye of the beholder.
NGC has an "Artificial Toning" designation. This designation is used on coins that exhibit a range of colors inconsistent with natural toning. For example, PCGS will note questionable color on some slabbed coins.
In the end, PCGS sums it up when they say that "Toning can be positive, neutral or negative" and that, in the end, everything is dependent upon eye appeal.