Fun Varieties Spring 2022, Toned Coins, a Variety and The Questions

by: Dr. Ron Brown   |   Tuesday, January 11, 2022   |   0 Comments

Fun Varieties

Spring 2022

Toned Coins, a Variety and The Questions

I’m not sure when this fascination with toned coin came about but it reminds me of an incident I observed very early in my medical carrier. We were offering free prenatal vitamins to expectant mother in a clinic I rotated though – No one took them.  Then a senior nurse decided to offer them at a very low price, and they flew out of the clinic. I also recall a time when I wanted to sell a rare, engraved rifle and I offered it at the discount price as I needed the money. I received NO offers. So, I decided to offer it a price double its value and I received multiple offers. The dynamics at play in these examples are kind of like this – not preferred but then offered for a premium they become desirable.

Not often thought as one, but a toned coin is a variety coin. An ever-growing trend of the selling and collecting of toned coins raises questions in the minds of many. I was prompted in writing this article from my observations and comments like the following. In the August 31st letters section of Numismatic News, a reader said, “I have been collecting certified toned coins – so I don’t have to worry about artificial toning. With these coins in holders, I feel that the toning process is stopped thus color will remain great.” This opinion is no fault of the reader who made the comment. Rather, it lays at the feet of our hobby that’s in a frenzy to make money, has not been accurate in provided information to rightly inform the hobbyist/collector about toned coins. There would be no need to write about toned coins, if they were offered saying that the stability of the toning cannot be guaranteed, but I have not seen this here-to-date. Granted, if the coin is removed for the environment that caused the toning, it will be slowed but is it stopped? This article may make some collectors or sellers of toned coins unhappy, but then there are those who will support it. I only wish to stimulate thought and convey some data about the subject.


One of the most reactive of our coin metals is copper as evidenced by toning or color on the coin. Most red brown (RB) cents these days were likely red (RD) sometime in the past, slabbed or not. Toning is a chemical process where the surface of a metal such as silver, reacts with its adjacent surroundings, environment or atmosphere. This interaction can be described with the following formulas below for silver and copper.

Silver:  4Ag + 2H2S + O2 = 2Ag2S + 2H2O

Copper:  2CuO + CO2 + H2O → Cu2CO3(OH)2 ?

There are also oxidation formulas for Nickle and Gold coin

I recall around 2005 a senior executive from a large firm selling toned coins to say,” To create such a myriad of vibrant and diverse tones required ideal storage conditions.” Wrong! In an “ideal” storage condition, no toning would have occurred. For toning to occur, there had to be poor storage conditions and/or contaminates that caused the toning and who knows what was found in the bag unfit for selling that you never heard about.  Maybe ideal for toning but not ideal for storage or coin preservation. If you want “right, you need white”. White meaning, cartwheel shinning silver coins”.

Coins, being made of various metals, are composed of molecules, atoms and their ions just like everything else. And the atoms of these molecules have an affinity to interact with other molecules and their atoms. In this case, almost all this activity occurs on the coins metal surface. This of course is a process that is influenced by the various alloys of our coins and the reactive oxidation potentials that each metal in the alloys are susceptible to.

There is no way to describe this interaction other than to call it oxidation, or patina, or corrosion. Strictly speaking, they are all correct! And while “patina” sounds appealing, “corrosion” undoubtedly does not. Maybe that’s why collectors can have such strong opinions about this one way or the other when it comes to toning. The main difference between corrosion and oxidation is that corrosion happens chiefly on metal surfaces whereas oxidation can happen with almost anything.

On silver coins, those that seem to have the most brilliant toning possibilities; though I have seen some bold coloring on copper coins and to a lesser extent on nickels; this interaction resulting on it as a thin layer of silver sulfide or Ag2S on a silver coin, which gradually forms on the coin surface from the interaction of hydrogen sulfide. The hydrogen sulfide or H2S is present in the atmosphere around us in small quantities and H2S is the major producer of silver corrosion in atmosphere that produces the Ag2S on the coins. This explains why silver will always tone if left exposed to the air with the substances of the surrounding environment determining the location of toning on the coin and the rate of change. Heat and humidity will accelerate the process. The formation of a discontinuous film of Ag2S, is the principal constituent of the corrosion product. By the way, oxidation come from the word oxygen, which is among the many known oxidizers. Ask yourself, how do you protect your coins from oxygen? If you think slabbed, it has trapped oxygen in it, and if your coin is already toned, the oxidants that caused the toning in the first place are still present on the coin and in the slab, substantiated by its colors and are still interacting with the coins surface, SLABBED or NOT.


The silver sulfide itself is black and can often be seen on tarnished table silverware. However, the thickening layer of silver sulfide will produce various colors through what is known as the “Thin-Film Interference Effect” if applied correctly, (which generally means slowly).

The Thin-Film Interference Effect is familiar to anyone who has ever seen the bright colors refracted off a blown soap bubble. It appears like this.  A beam of light strikes the surface of a coin containing a thin layer of silver sulfide Ag2S, at an angle. The beam is split into two beams, or light waves, one is bent or refracted immediately and the other penetrates the upper layer and then is refracted. At any rate, metal ions on the surface of a coin react with chemicals in the environment to create a thin layer of a new substance on top of the coin. The interaction of light as it reflects off of both the new material and the original coin surface results in the colorful patterns, we now are calling coin toning. See illustration below

The vibrant colors of "toning" that coin collectors seem to appreciate are caused by thin film interference, where the "thin film" in question is a layer of corrosion by-products sitting on the surface of the metal. The thinnest films develop and then it moves down through the rainbow: yellow, orange, green blue. Once you get past blue, the corrosion layer becomes too thick for light to pass through it, and it becomes opaque - which we see as black. At this point, people usually start to call it "tarnish" rather than "toning" and it suddenly becomes undesirable. But I say, this whole process is undesirable!


Light striking a thin film is partially reflected (ray 1) at the top surface. The refracted ray is partially reflected at the bottom surface and emerges as ray 2. These rays will interfere in a way that depends on the thickness of the film and the properties of refraction of the various metals the thin film is on. A is thin film surface and B is surface of coin that the film in on.

In this case, the light that goes through the layer is affected based upon the type of metal and the thickness of the layer it is passing through. This part of the beam is out of sync with the other beam and now is in a different wavelength. The wavelength shift results in two light wavelengths, producing a different color to the naked eye in each. Light that is visible to the human eye covers a spectrum of wavelengths, ranging from about 380 nanometers (nm) for violet to about 700 nm for deep red. Every color in the spectrum has a corresponding wavelength of light.

The wavelengths of visible light by the human eye are:

    Violet: 380–450 nm

    Blue: 450–495 nm

    Green: 495–570 nm

    Yellow: 570–590 nm

    Orange: 590–620 nm

Red: 620–750 nm

Thin Film Thickness

-      Color


Light Yellow




Cobal Blue












Light Blue





The phase shift to cancel blue light is roughly 220 nm, which requires a toning layer of about half of 110 nm. Without blue, you have red and green, which usually combines into yellow. This is the reason why the first signs of toning will always be yellow on a silver coin. However, it is possible for “artificial” (hereafter we will call it “accelerated”), attempts at toning a coin for the process to occur so quickly that the toning layer will be so thick that the natural progression of colors is completely bypassed. As I like to say, “Do it Fast and you get Black.” You may then end up with a silver black coin in its final state. The table below gives the natural progression of toning and its corresponding color.


Soap bubble refraction of light as an example

                                                                          Color of the refracted light as a function of the film thickness [Isenberg, 1992]



The difference between natural and accelerated toning is not clearly defined and highly subjective but don’t tell that to the dealers and certifying houses. In general, anything that quickly causes toning is considered accelerated. A clear example of accelerated toning is to use concentrations of chemicals in a laboratory to tone a coin. In this example, it's possible to get a toned coin in seconds but the chemicals to do this, such as Hydrogen Sulfide, is very toxic.

Nonetheless, these novice attempts at toning coins are generally different from coins that toned more gradually. They tend to exhibit certain signs that are said to be well known in the collecting community and relatively easy to detect. Suffice it to say that there is a delineation of colors that often is not present on a coin that was subject to accelerated toning. (see color delineation on coin below) Certain patterns and color combinations it is said can also only be achieved through natural toning and not through accelerated toning. There is a possibility some extremely skilled individual can produce a coin that can fool the experts, but this would be uncommon, but not impossible, as they also know of this accelerated toning downside.