Featured What You Need to Know About: The Science of Toning

(Roosevelt dime, photo by Bob Campbell)

In response to a recent question by a member on another thread, I present here the science of toning. This article is adapted from an article I posted a couple years ago on the NGC chat boards. Save it, print it, reference it, use it in discussions of toning. While some simplifications have been made to make it easier to understand (BadThad – I know! ), the basic science is all here.

The science of toning is very interesting. Sometimes delving into the details of something takes away the mystery of it, but to a scientist and numismatist, it only serves to heighten the wonder of a coin. Toning on silver coins appears through something known as thin film interference of a layer of silver sulfide (Ag2S) on the surface of the coin. This silver sulfide is formed when the silver alloy reacts with hydrogen sulfide (H2S) in the atmosphere. Often, heat and humidity hasten this process. Hydrogen sulfide in large concentrations is very toxic and flammable, but small doses are almost always present in the environment. Hydrogen sulfide is responsible for the smell of rotten eggs, and is also present in flatulence, volcanic and natural gasses, some wells, and swamps. Other sources of sulfur which can affect coins are paper and cardboard holders, such as the mint sets issued in the 40’s and 50’s or early albums. The increased concentration in close proximity to the surface of the coin heightens the toning effect of these sulfur containing holders.

The chemical formula for the reaction process is:

4Ag +2H2S + O2 -> 2Ag2S + 2H2O

The reaction is accelerated by heat. As you can see, oxygen is essential to the toning process, and water is a byproduct. The combination of heat and humidity in close proximity to a sulfur source will produce toning the fastest – but no amount of meddling can replace the simple element of time.


(A beautifully toned 1958D Franklin, image by Bob Campbell)

As the layer of silver sulfide is formed on the surface of the coin, it gradually thickens. Different thicknesses produce the appearance of different colors. Note that silver sulfide in itself is actually black – this same compound is known as tarnish on silverware. The amazing colors are caused through a phenomenon known as thin film interference. You are probably familiar with the rainbow-like sheen of an oil slick on water, or the appearance of iridescent colors in a soap bubble. These manifestations of color in an otherwise colorless object are also due to thin film interference.


(Thin film interference in a soap bubble, image from Wikipedia)

The science and mathematics of thin film interference are beyond the scope of this discussion. Suffice it to say that a thorough analysis requires advanced calculus, geometry, physics, and optics. However, I will attempt a simple explanation. Imagine the surface of a coin, on which is a thin film of silver sulfide. Now imagine a beam of light striking the surface of this film at an angle. At this point, the beam is split in two – half of the beam is reflected directly off the surface of the film. The other half will penetrate the film, but it is refracted a certain amount. Whenever light enters another medium, it is bent according to the material’s index of refraction. This is why when looking at a fishbowl, the contents often appear distorted. The speed of light is actually slower in water than it is in air, so the image is bent. When the beam of light in the film strikes the surface of the coin, it is also reflected and exits the film. However, the two halves of the beam are now in different phases. Because the half of the beam that traveled through the film took a longer route, it lags behind the half of the beam which was reflected off the surface. The phase is shifted relative to the first beam, and this is the key to the colors.

Interference:

(diagram by the author)

Phase shift:

(diagram from Wikipedia)

Light is one of the mysteries of the universe. Sometimes it acts like a particle, and at other times it acts like a wave. In the case of thin film interference, light must be thought of as a wave. When two waves interact, sometimes funny things can occur. When two waves are perfectly in sync, they can combine to magnify their intensity, known as constructive interference. However, if the two waves are perfectly out of sync with each other, that is, when one wave is peaking, the other wave is at its bottom, the two waves cancel each other out. This is known as destructive interference. By taking a single beam of light, splitting it, and recombining it in just the right way, darkness can be created.

Constructive and destructive interference:


(diagram from Wikipedia)

It is this destructive interference which produces the colors we see. Destructive interference occurs when the thickness of the film is nλ / 2, which is to say, any multiple of one half the wavelength. Conventional notation among physicists is to denote wavelength with the Greek symbol for lambda, and it is usually measured in nanometers (abbreviated nm) when talking about light. Assuming you start with white light, the color you see will be the complementary color of the cancelled wavelength. For example – if the thickness of the film is such that it cancels yellow (with a wavelength approximately 580 nm) then you will see blue. This film would be approximately 290 nanometers thick. For reference, an atom of silver is about 0.16 nm thick, so these films are on the order of a couple thousand atoms thick. Toning layers cannot be more than about a wavelength thick, or else the light is absorbed by the film and will not produce colors. These thicker films of silver sulfide will appear brown or black.

The rainbow effect is caused when the film is not uniformly thick. Remember, different thicknesses produce different colors, so a film which gets gradually thicker towards the edges of a coin will appear to have a rainbow effect. It’s the same idea as when you get a gradual progression of colors when you refract light through a prism.


(1886 NGC MS-65* Morgan Dollar showing rainbow toning – in other words, a non-uniformly thick layer on the surface, photo by the author)

When a toned coin is dipped, it is the thin film that is removed. Using a precise enough scale, a coin can be weighed and then dipped. A perceptible amount of silver (or rather, silver sulfide – most solutions do not affect the underlying silver) is removed in the process. It is precisely because of this that proponents of white coins consider toning harmful to a coin. Also present on the surface of coins, and also on a microscopic scale, are tiny flow lines created when the metal flows during striking; it is these flow lines that cause the diffuse reflectivity we call luster. Because toning builds up substances on the surface, it can often overwhelm these flow lines. Toned coins often have somewhat muted luster depending on and due to the thickness of the toning. When these coins are dipped, the flow lines (and thus the remaining luster) are also eliminated in the process. The result is a dull, lifeless coin that appears unattractive and undesirable.

So there you have it, an introduction to the science of toning. One thing I love about this hobby is that I can sometimes foray into very different fields - optics and physics being one of them. I am a rocket scientist by degree, and now I am a nuclear physicist with the Navy, so I have a certain passion for physics. I hope you enjoy your toners, and I hope you enjoy them even more with a certain understanding of how they are formed.

~Jason Poe

References:
"Coin Chemistry"by Bob White
"Fundamentals of Optics" by Jenkin and Harvey
"Thin Film Interference" Boston University
Physical Optics

I’ll leave you with a final toner, an NGC MS-67 Mercury Dime, photo by the author.

 

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Wonderful post! Thanks!

 

In honor of such a fantastic post, I would like to post a phenomenal toned coin that represents the author's favorite series. Take a bow Jason, posts don't get better than that.




:thumb:

 

Very good article! Makes me think of Hawaii and all the lava that's around there. I don't know what I'd do with it, yet but I'm thinking. :thumb:

 

outstanding post, very nice and with such explanatory diagrams.

 

OutStanding post P~F Pi.
Until I read this post I could not figure out your choice of handle.
Carry ON!:bow: And thanks.

 

Newbie Question:

How do I nominate this for thread of the week?

 

As always....great job again!

 

I be wondering the same thing?
but I do not we are voting anymore on threads of the week??

 

you reply while I was typing my post.

 

... yes, I've been away for a while . . .

 

There are machines used in the semicinductor manufacturing industry that are used to deposit thin films on silicon wafer surfaces to act as dielectrics, or insulators. The color you see is dependent on the thickness if the film, (SiO2). Put it on a coin, and you can get any color you want.

 

Great post, and great coin Lehigh96!

Combine this with this thread, http://www.cointalk.com/t39837/ ,and you have a lot of good info on toned coins.
Thanks for the awesome thread!


LouieLot

 

Great write-up. It brings another question to mind. Is silver sulfide acting in the same way as copper oxide in that it's a natural progression of the metal's efforts to protect itself from the outside environment?
Guy~

 

It is a similar idea. Basically, many metals will form layers on their surface (known as passivity). A bare metal will corrode quickly, but as this passive layer forms, the corrosion slows. Silver sulfide is an example, copper oxide is an example, rust (ferrous oxide) is another example. Some metals react much more readily; for example, copper is far more reactive than silver. Because of this higher reactivity, the chemical processes in the thin films which form on the surface of copper coins are far more complicated - although the way light interacts with them is the same. Aluminum is even more reactive, and will form a thin layer of aluminum oxide within fractions of a second.

Thanks for all the kind words guys! Oh, and Doug, I didn't get too much into the science of Luster here, because I already treated the subject at length in another thread: Luster: A Guide for Beginners

 

Gold toning

Thanks for the excellent review of toning.

Is there a link for gold toning on this site?