What You Need To Know About the Science of Toning

[physics-fan3.14]

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.

 

A great example of a superb mint set toned coin (my coin, photo by Bob Campbell):

 

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.

 

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.

 

(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.

 

Thin film interference (image courtesy of TomB):

 

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, as in the coin at the top of this post. 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.

 

An example of the rainbow effect on toned coins (my coin, photo by Bob Campbell):

 

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.

 

~Jason Poe

 

References:

"Coin Chemistry"by Bob White

"Fundamentals of Optics" by Jenkin and Harvey

"Thin Film Interference" Boston University

Physical Optics

[robec1347]

Thanks for the great post Jason. That was a very interesting and informative read even if some was beyond my intellect.

[Bugmann1974]

Awsome! Thanks for the tutorial Jason (thumbs u

[realone]

Something all should read and this my friends ends the long argument that toning is something that removes metal from the coin and is harmful to a coin. Class over you can go home .

Thanks!

[bsshog40]

Pretty cool there jason. And I thought all this was caused by a frying pan and oil. Seriously, very informative. Science was not one of my best subjects. Come to think of it, not many were.

[Coindude]

Sounds like I need a big bowl of beans and a nice closed room for me and my coins to hang out in!!

 

Nice work there Jason.

[Jim Beam]

Coindude: Now that I am done laughing , I want to wish you the best of luck with your toning experiment. Let us know how it went and what the results were. You may be on to a new method of NT.

[WoodenJefferson]

Substantial reading. Much to absorb…I’ll get back to you.

 

[Smilie Puss]

physics-fan3.14

 

It seems to me I read something in the October issue of the Numismatist in Letters to the Editor and on ANA's web site of this same explanation. However, it didn't go into phase shifts diagrams or as detailed as you did but did mention something about rainbows and why we see colors in an oily residue on water. I think the person was trying to give information that wasn't explained in a previous article "The Magic of Toned Coins" or something like that. I think I liked his explanation because it didn't get as complicated in theory and formulas as you did.

 

As a coin collector I am not sure I am that interested in that much detail. The Numiamatist article was basic enough to give me an idea of this color thing on coins without the involved science lesson. Not that there is any thing wrong with explaining the way you did but you lost me some where around constructive and destructive interference.

 

You must be quite intellegent and you certainly demonstrated, I think, that you know something about the subject not that I could verify any of it with my limited educational background.

 

I hope you don't take offense to what I said but it is only my opinion. What matters , I think, is the toining natural or artificial? Where coin doctors at work? Or how long was the coin in the dip-it-t-do-da solution to strip off the top layer of silver to make it dull and lifeless. Perhaps you could enlighten us on that subject in your next science lesson.

 

Check out the article and see if you agree. Perhaps then the rabbit will be definitly out of the hat!

[RareSov]

Do NGC have a detector and what would the bodybag say. Farted on

[RareSov]

Great post, I love em WHITE ! of course green gold and blue toning can look great too

[Conder101]

Very well done, presented in a simple straight forward method that should be easy to understand. It isn't difficult folks. What was presented here is nothing that wasn't in a ninth grade general science class. OK maybe first quarter high school chemistry as well for the equation.

[physics-fan3.14]

Very well done, presented in a simple straight forward method that should be easy to understand. It isn't difficult folks. What was presented here is nothing that wasn't in a ninth grade general science class. OK maybe first quarter high school chemistry as well for the equation.

Thanks guys. I included the pictures to help make it easier to understand. One thing a scientist always wants to know is "Why?" and I think the why of toning not only helps us appreciate it more, but if we know how it works, it helps us distinguish AT/NT much better. It also helps us understand what we are looking at - for example, why the luster on the top coin has been muted somewhat, and why that coin would appear lifeless if dipped. Or it helps us understand the natural progression of a rainbow toned coin - and how that multicolored coin you thought was natural is actually AT because the color progression doesn't follow the rules of physics. Maulemall wanted a detailed explanation of why Stephanie's Peace dollar was AT in a recent thread, and the basic understanding of the "why" of toning is a foundation to understanding that.

[MAULEMALL]

Very well done, presented in a simple straight forward method that should be easy to understand. It isn't difficult folks. What was presented here is nothing that wasn't in a ninth grade general science class. OK maybe first quarter high school chemistry as well for the equation.

Some of us didn't get to the 9th grade... Some of us left home and went to work before they could. Granted,, Later in life some of us got a GED and then went to college but the whole High School thing just didn't happen for some of us. So pleeeease taaaalk sloooower.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[Conder101]

No offense, I'm a high school dropout myself. But I assume most of the people here did go to high school and many of them have some college. (I also went the GED route but I did because high school was a waste of time for me. My only regret there is that I didn't do it earlier. I then went on to college and currently hold degrees in Chemistry and electronics. I reget I never finished my degree in computer science. Someday I will go back and get it because I can't keep up with my nephew anymore.)

[RGT]

That is a very good and understandable explanation. In fact, I thought that is was a bit over-simplified in a few places. But then, I have an MS degree in physics and worked in optics several years ago, so I am not the average reader here.

 

TomB has a very good scientific explanation of toning on his web site that does not contradict yours but takes a slightly different approach. I think you may have taken one of your drawings from there.

 

TomB's Science of Toning

 

A couple of comments did occur to me as I read your post. With regard to a thicker film becoming brown or black you say that it is due to the absorption of the transmitted portion of the wave. I don't know what the absorptive properties of silver sulfide are and you may be correct. Another factor that would also cause the same effect is the coherence length of the light source. For those who don't know, the coherence length is the distance the light travels before the phase changes due to the random nature of the light source. If the coherence length is less than twice the film thickness the phase of the incoming light will have no correlation to the phase of the reflected light emerging from the film and the interference will be random instead of constructive or destructive. The coherence length of sunlight in the visible wavelenghts is around 800 nm so a film much thicker than 400 nm will not produce any colors. Other light sources can have longer coherence lengths and would display colors with thicker films. This explains why the light source can be important in seeing the color and why sometimes the coin looks more washed out in natural sunlight than it does under artificial light.

 

Another thought on this relates to the viewing angle. As can be seen in your image the distance that the light travels through the film is dependent on the angle that the light hits the surface. That means that the effective thickness of the film will depend on the angle. This explains why some coins tend to change color as you rotate and tilt them in the light.

 

One last thought regards your chemical equation. I'm no chemist but that equation does not look balanced to me.

 

Overall, a very nice job. (thumbs u

[MAULEMALL]

No offense, I'm a high school dropout myself. But I assume most of the people here did go to high school and many of them have some college. (I also went the GED route but I did because high school was a waste of time for me. My only regret there is that I didn't do it earlier. I then went on to college and currently hold degrees in Chemistry and electronics. I reget I never finished my degree in computer science. Someday I will go back and get it because I can't keep up with my nephew anymore.)

No offense was taken... I went into the Marines and sucked up all of the GI bill I could use.. since I was in Aviation I continued that.. Got through it and found out working for Sikorsky that I hated corporate America.

 

 

You know you just don't appreciate it if you don't have to work for it.

 

[RI AL]

Jason, definitely a "print 'n save" post. I don't understand it all (barely made it through 9th grade biology let alone even REACHING Chemistry)...they stuck me in the Music track... being a pianist an all......

RI AL

[physics-fan3.14]

RGT, thanks for the comments. Yeah, I did lift the drawing from TomB, it was about the best picture showing the ray paths that I could find.

 

Interesting about the coherence of light. I hadn't come across any of that in my preparation for this post. I did see a bit about polarity of light and changing the polarity by passing through filters on the same orders of thickness.

 

I was going to include the bit about different viewing angles, but by then the post had already gotten rather long and I didn't want to completely overwhelm everyone.

 

As for the formula, I got that from Wheimar White's "Coin Chemistry." Mostly an anti-toning tome, but it does have some interesting science. The formula is balanced as far as I can tell.

[RGT]

Coherence is something that isn't usually discussed unless you are talking about lasers, but it does play a role in interference too.

 

As far as the formula, I see 4Ag on one side and only 2 on the other. Someone may complain about a loss of metal in this process.

 

Of course I haven't had to balance a chemical equation in 20 years so I'm probably missing something.

[physics-fan3.14]

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

 

You should see 2AG2S - I can't really do subscripts on here. It would look much better with proper Latex coding

[Smilie Puss]

 

One last thought regards your chemical equation. I'm no chemist but that equation does not look balanced to me.

 

 

Yes I think you are correct. However, coinage metals contain two metals and is know as an alloy or so I was told in kindergarden

 

Now for the formula: Lets just stick with moist hydrogen sulfide and silver/copper alloy for the moment:

 

2Ag + H(2)S ---------> Ag(2)S(s) + H(2)^

2Cu + H(2)S ---------> Cu(2)S(s) + H(2)^

 

Yes? Balanced? Think so or is the chemistry more complex? What is this green stuff I see on copper roofs? Cheese? Or does copper react with CO(2) to give copper carbonate. Or perhaps its copper sulfate from acid rain. You know all that junk that the coal fired power plants put into the air that results in sulfuric acid.

 

Recall air is composed of oxygen, nitrogen, carbon dioxide and other gaseous stuff and yes water vapor or the condensed form, rain.

 

So before we start giving a science lesson in coin toning lets get all are ducks in order.

 

And really the purpose is to enlighten and inform coin collectors. Yes?

 

[supertooth]

Hello Folks---- This subject is the kind that always needs to be broached at times. Any knowledge that we can acquire from these threads is the kind of thing that I envisioned as being the purpose of the WYNTK threads.

 

I congratulate Jason on a fine effort and for wanting to share and explain this knowledge.

 

Wish to especially thank Randy [ RGT] for his response. An extremely knowledgable reply---with wonderful additional information.

 

One needs to use both logic and science to help to figure out the AT vs. NT issue. Looking at a lot of coins helps too. Great job guys. Bob [supertooth]

[Smilie Puss]

Do NGC have a detector and what would the bodybag say. Farted on

 

RarSov

 

Congratulations!

 

Your not that far off the mark. However, I am not so sure a 'fart' contains much hydrogen sulfide. If it did you might not be alive to day especially if you ate a lot of beans and lived in a closet. Hydrogen sulfide is a posionous gas in high concentrations. There were reports , however, in WW I of many Germans found dead in the trenches with empty sauerkarut (lots of sulfur in cabage) cans in evidence. But this report could not be confirmed.

 

But I suppose we could get some of that stimulus money in grant form and do a study on the GCMS (Gas Chromatrograph Mass Spectrometry) analysis of 'farts'. Who knows how many people we could put to work and off the unemployment rolls. Our economy would recover in no time.

 

Truck on RarSov

[RI AL]

You guys lost me about 15 posts ago...

[RGT]

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

 

You should see 2AG2S - I can't really do subscripts on here. It would look much better with proper Latex coding

 

I'm sure that's my problem. If I actually knew the chemical formula for silver sulfide I wouldn't have a problem. So, the second 2 in 2Ag2S is a subscript. That makes sense. I just need to quit pretending I remember any of my chemistry.

 

Smilie Puss, your formulas look like something taking place in solution. I'm not sure that's relevant here but maybe it could take place in a humid environment. Now, I am really going to shut up and quit showing my ignorance of chemistry. I'm sure a real chemist will come along and straighten us out.

[Smilie Puss]

RGT

 

In solution perhaps but the point was only half of the toning story is been told. Copper is part of this process. In addition the reaction takes place in air not just pure oxygen.

 

The end result is the same silver and copper form a sulfide or am I missing something here?

 

If the toned coin is cleaned using a solution of thiourea a faint smell of hydrogen sulfide is detected. What was chemically bonded to the silver and copper as a sulfide are now as a gas H(2)S^ and the surface silver is almost as it was depending on the contact time the coin was allowed into the thiourea solution. Yes?

[physics-fan3.14]

Yes, smilie, the chemical situation is far more complicated than I have simplified it to be. However, the conversion of silver to silver sulfide through reactions with hydrogen sulfide is by far the predominant mode of toning on silver coins. It is the most common and most important, thus the one I discussed. Copper is also a much smaller part of the alloy (roughly 10%). Yes, there are numerous reactions which occur, and numerous different compounds which can be created, but silver sulfide is the one most important. The toning modalities of copper, nickel, and gold coins are completely different - but my focus here was on silver coins. If anyone knows the primary chemical reactions which occur on these coins, I would love to see them.

 

Oxygen is the important element of air because it is so highly reactive. Oxygen comprises 20.9% of the atmosphere, Nitrogen is 78.1%, and the remaining 1% is a wide variety of other things. Nitrogen does not react readily, being an mostly inert diatomic gas. The bonds of elemental nitrogen are some of the strongest in nature, which means that it does not readily react with other elements. Thus, air for the purposes of this discussion can be simplified to just Oxygen.

 

However, the basics of thin film interference, the process through which the actual colors appear, are the same for all the different compounds.

[Smilie Puss]

 

And really the purpose is to enlighten and inform coin collectors. Yes?

 

The title of this subject was " What You [Need} To Know About The Science of Toneing"

 

So then, where 'all' coin collectors enlightened and informed? Perhaps, but I think a few, including myself, that were a bit overwhelmed and confounded in scientific theory. I may want to know why one sees color from a toned coin but a 'need' to know is akin to 'I need air to breath'.

 

As Reptiva? said in "Fiddler on the Roof". It doesn't change a thing but its always nice to know. "Do you love me"?

 

Peace be with you.

[TomB]

You should reference your sources. Obviously, I recognize the illustration that I made and that I have posted on my website, which is present in your post. It also appears in close proximity to text that accompanies it and that I essentially wrote, too. This usage without reference causes one to question how much of the text is your contribution to the post and how much belongs to other authors. As an example, I wonder where you obtained the phase shift diagrams because they are so well done. They may be yours, but they may be from another source. Signing the end of the post with your name also infers that this is original work or research, which is something that would be incorrect in this case.