How Coins Tone

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Coin toning is a topic that sparks much intrigue and debate among numismatists. Many collectors, myself included, are fascinated with the way toning forms and what factors cause it to develop. But first, I want to define coin toning. A lot of collectors associate toning with the pleasant vibrant colors that some coins display, but it actually refers more broadly to any patina, discoloration, or tarnish that forms on a coin’s surface. Some toning is attractive and will raise the value of a coin, but most is not. Toning, especially when it’s deemed unattractive, can hide and dull the natural mint luster of a coin, covering a bit of its personality in a sense. Toning in these cases can hinder the coins’ value as they become less eye appealing. 

This article focuses on the process of toning with an emphasis on the types of toning that are more sought after by collectors. How a coin gets toned has five main considerations: 

• The coin’s metal composition
• The way the coin was prepared/minted
• The presence of reactive chemicals
• The environment in which the coin is stored
• The method of storage

Composition

The metal composition of a coin matters because different metals have different properties that react in different ways to external factors, which results in some distinct and recognizable toning patterns for certain materials. For instance, silver is more easily able to tone in a spectrum of colors, or rainbow tone, than copper. The hardest metal to tone is gold. Due to the chemical stability of the material, it simply refuses to react to factors that would normally result in toning on other metals.

Minting Process

How coins are minted also plays a role. Differences in the way planchets are prepared can affect how toning appears. Proof planchets are heavily polished and prepared with the presence of other trace chemicals, which is why toned proof examples often have a color and pattern distinctively different from their business-strike counterparts. Furthermore, coins with designs that allow for more mint luster bring out color more vividly. For example, a Morgan dollar and a Peace dollar that have the same toning will look different, with the Peace dollar appearing duller than the Morgan.

Chemical Reactions

Coins are toned as a result of a process called oxidation. Oxidation describes the reactions that take place between the metal of the coin and outside substances mixed with oxygen. Oxygen plays a supporting role, however, with sulfur being the compound most associated with toning. Sulfur contact with a coin’s surface, along with other environmental factors, will cause a thin layer of the sulfur to stick to the coin. It’s this layer of sulfur that we see as toning. Subsequent colors depend on the thickness of sulfur, ranging from light to more intense coloring, eventually becoming a very dark brown. So the most desirable examples are those with rich, deep colors, as opposed to overly toned coins that have turned dark brown.

Environment

The environment coins are stored in heavily contributes to how they get toned, as it dictates the rate of toning. Factors like moisture, temperature, and air quality all play a role in this regard. Moisture accelerates the oxidation process, with warmer environments increasing the speed of chemical reactions. The water acts as a medium, allowing sulfur and the coin to interact more efficiently. However, too much moisture can be detrimental, as this can result in the formation of black spots on the coin’s surface. Heat also accelerates the process of toning—the hotter it is, the more excited and reactive the chemical particles are. This is why coins stored in cooler environments tend to stay brilliant for longer. Air quality plays a role as the presence of other compounds in the air can also interfere with the toning process. If the air quality is poor and filled with various other materials, it may lead to corrosion rather than attractive toning.

Storage Methods

The method of storage determines the amount of compounds a coin gets exposed to. Many old ways of storing coins, such as Wayte Raymond folders, manila envelopes, and canvas bank bags all contain a trace but notable amount of sulfur. Many coins housed in those materials, and combined with the other factors associated with coin toning, have resulted in some wildly toned examples. How airtight and stable of a storage solution a coin is in also determines how much environmental exposure it gets. So if a collector wants a coin to stay brilliant, an airtight capsule or box is the best option.

Conclusion

Toning is a complex and multifaceted process where any number of factors can greatly impact the color outcome. As such, the result is unpredictable and incredibly difficult to control, which is part of why toned coins are so valued. Above all, toning does not happen overnight—it takes time, sometimes a very long time. As a result, and paired with their beauty, toned coins are becoming increasingly sought after. Many examples boast eye-watering sold prices, which only brings more intrigue and draws in more collectors. A marvelously toned example versus a non-toned example of the same coin in the same grade could mean a price difference in the thousands. All this is what makes toned coins so compelling to study, collect, and admire.