A mysterious transformation

As I've mentioned a couple of times before (see, for example, https://www.cointalk.com/threads/suggestions-for-sellers-on-vcoins.357976/page-2#post-4325108), some months ago, when I didn't know enough to recognize the issue of artificial toning, I bought this Hadrian-Aegyptos denarius from Athena Numismatics.

Hadrian AR Denarius, 136 AD, Travel Series, Rome Mint. Obv: HADRIANVS AVG COS III P P, bare head right / Rev: AEGYPTOS, Egypt reclining left, holding sistrum in right hand and resting left arm on basket; to left, ibis standing right. RIC II-3 1482 (2019 ed.), old RIC II 297d (1926 ed.), RSC II 99, Sear RCV II 3456, BMCRE 801-3. 18 mm., 3.11 g., 7 h.

Seller's image:



When the coin arrived at the beginning of January, it looked even "bluer" and more artificial (if that's even possible!) than in the photo, especially on the reverse. All of a sudden, I really didn't like the way it looked. I started looking at some of Athena's other coins, and realized that the proprietor seemed to have a large number of coins for sale with a similarly artificial-looking blue patina. (You can see one current example at https://www.vcoins.com/en/stores/at...r_denarius_bold_portrait/1296224/Default.aspx, but I don't think there are any right now where it's quite as extensive as on mine.) After I joined this forum and asked about the coin, I was advised that the seller probably achieves the effect with liver of sulfur. When I asked here and on the ancient coins Facebook group I belong to if there might be any way of getting rid of the blue toning, people recommended a variety of possible approaches. The simplest approach was recommended by @Eukratides, who advised me to try soaking the coin for a short time on aluminum foil at the bottom of a glass or ceramic bowl, in a solution -- heated to a point just short of boiling -- made with 1.5 cups of distilled water, 1 teaspoon of baking soda, and 1/2 teaspoon of laundry detergent.

I finally decided earlier today to try that method, and went to retrieve the coin from its tray, where it had been sitting undisturbed for the last 4 1/2 months. I hadn't even looked at it closely in quite some time, and certainly hadn't done anything to it. I had never even tried rinsing it off to see if that changed anything. But somehow, most of the artificial blue tone had, as if by magic, disappeared by itself:





The only place where I could really see any remaining blue toning was the area above Egypt's sistrum. I probably should have left well enough alone, since the improvement was remarkable (although entirely mysterious to me), but went ahead and prepared the solution recommended by @Eukratides, although I let the coin soak in it for only a few minutes. This is what it looks like now; I don't see any remaining blue toning whatsoever:





Obviously, I'm much happier with how the coin looks now than I was with the way it looked when I bought it. But can anyone explain to me how 99% of the artificial toning disappeared by itself in the space of 4 1/2 months?

 

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Sounds like the artificial toning created a silver sulfide surface layer.

The aluminum and hydroxide solution perform a redox reaction:
The aluminum is oxidized while the silver sulfide is reduced to silver metal.
Here's someone writing a science lab for the same reaction:

OXIDATION: 2 Al(s) + 6 OH– (aq) –––> Al2O3(s) + 3 H2O (l) + 6 e–


REDUCTION: Ag2S(s) + 2 H2O (l) + 2 e– –––> 2 Ag(s) + H2S (aq) + 2 OH– (aq)


3 Ag2S(s) + 2 Al(s) + 3 H2O (l) –––> 6 Ag(s) + 3 H2S (aq) + Al2O3(s)


(I just cribbed the equations from their site. I was never great at electrochemistry.)

 

Thanks, but it's all Greek to me! I remember zero of my high school chemistry. So how did the blue toning mostly disappear on its own even before I put it in the solution?

 

How curious. Wish I had something more to offer!

 

I don't have an answer as to why the artificial patina suddenly disappeared, however, as someone who regularly restores ancient silver coins, I've grown a bit wary about using the aluminium / baking soda redox-method. Most ancient coins have a (desired) Ag2S (silver sulfide) patina; the greyish silver patina. The above mentioned redox reaction (middle line, describing the Ag2S >> Ag and H2S) is an extremely fast reaction that can strip a coin of its desirable patina within a few seconds.

In your case, where the artificial patina disappeared for an unknown reason in the relatively short time span of 4.5 months, I would have waited another 4.5 months for it to disappear entirely.

 

The color is not a real color but an optical effect caused by thin layers (e.g. an silver oxide or a sulfide layer). Like an oil-film on water or a soap bubble.

A blue color indicates a thickness of around 120nm. (nm =nanometer)

When the layer becomes thicker the color changes or disappears.


Look for interference (physical background) or here iridescence:

https://en.wikipedia.org/wiki/Iridescence

 

I have a denarius of Maximinus going the opposite way, took 3 years to turn from white to bleuish. Its in a wooden case together with other denarii, no further protection like a slab.

 

Very interesting. I believe I saw your coin on the facebook coin group yesterday. If it was indeed artificial toning your actions definitely approved the appearance of the coin. I only have one "toned" silver piece, an antoninianus of Phillip with a yellowish tone.

 

I've got a lot of coins with dark gray "cabinet" toning, such as this one:


But I only have one that has any sort of bluish tone to it:


No idea if the toning is natural or how long it has been there. The coin came with no provenance.

 

The other question is how long it will be there. If the irridescence is a produck of a micro-thin layer of toning, continued action will go gray and then black. Will this take a year or a century? The coin may be around to know. We won't be in some cases. I have a few coins that I have owned for over 50 years. They are darker grey now.

 

Thank you. Very interesting. I was assured by an experienced dealer that the baking soda/distilled water/aluminum foil process was harmless, so I figured I had nothing to lose. I left it in the solution (with the reverse side down, resting on the foil) for a total of only about 2-3 minutes before I took the coin out and rinsed it with plain distilled water. As far as I've been able to tell, it didn't do any harm, and had little or no effect on the coin's appearance beyond removing the last remaining trace of blue toning. (See the photos above.)

Still, in retrospect, I agree that I should probably have just let well enough alone and waited to see what happens over the next few weeks or months, given how much the coin had improved in appearance on its own. If I hadn't already mixed the solution before I went to retrieve the coin and discovered the change, I probably would have left things alone. But I had already begun the process, and so I continued. Even at my relatively advanced age, I sometimes don't think or act in the most logical way!

 

If you are lucky the blue color comes back with time.

Here is a maybe better link for the color formation:

https://en.wikipedia.org/wiki/Thin-film_interference

if you scroll down a bit you can see the steel colors which can be achived by heating, bu I wouldn't try this with a ancient coin.

 

Thanks. The blue toning on my coin -- which everyone agreed was artificial when I first posted a photo of it here, given the particular dealer's known penchant for applying artificial toning, and the number of coins he had for sale with a virtually identical blue tone -- was very blue in appearance regardless of the light conditions or the angle at which I viewed it. It sounds like the dealer didn't have to do very much to achieve the effect, whether he used liver of sulfur or something else, and that the layer of blue coloring was very thin however he achieved it. So it makes sense that it would be fairly easy to remove. What I still don't get is how it mostly disappeared all by itself! I checked the coin again this morning just to make sure I hadn't imagined the whole thing. And it still looks fine.

It seems that nobody can explain how the blue toning could have gone away by itself. (I posted the same question in the Facebook group last night, and the last time I checked, nobody had even tried to answer it.) Disappearing ink? (See http://www.chymist.com/Disappearing Ink.pdf.) But that apparently takes only minutes to fade. Not months.

Obviously, it was a miracle. The spirit of Hadrian decided to come back after 1,900 years and change the coin because he didn't like being blue?

 

Why would I want it to come back? I didn't like it, and am extremely grateful that it went away!

 

I can understand that it's possible for a coin to take on a bluish tone over time naturally, as when kept in a cabinet. The particular dealer seems to have decided to achieve the effect artificially. But I've never heard before of a coin going in the other direction without intervention, which is what happened here. Nobody has explained how that can happen. See my comment above explaining that I've decided that it must have been a miraculous event.

 

That's very attractive, however it happened. If my coin had arrived looking like that, I wouldn't have complained! Unfortunately, the artificiality of the tone was very obvious to me as soon as I had it in hand.

 

No, not at all.

Maybe the seller did some oxidation process or heated the coin to accelarate the process, but this kind of color appears also quite natural with time.

You start with blank silver, then it oxidation starts. A very thin layer is formed, which becomes thicker and thicker. In a certain thickness range you get a interference of layer with the visible light, resulting in nice colors, starting with yellow, gold and also blue, depending on the thickness of the layer.

But the growth of the layer thickness goes on, and with time you leave the range of interference with the visible light. Your coin becomes grey or even black.

This happens not for all coins. If the oxidation is imhomogenous you have a mixture of colors and the result is brown or grey.

 

Thanks. I hope you know I wasn't serious about the miraculous nature of the transformation! So this could happen -- changing from very blue to silver in color, comparing the first and second set of photos I posted -- in the space of a few months? Presumably not so very long after the seller did something to the coin to make it blue in the first place?

And there's no question at all that the particular dealer does something to silver coins to achieve that blue tone, whether through application of liver of sulfur or some other process. All you have to do is look at all the other identically blue-toned silver coins he has for sale or has sold in the past. And see how intensely bright and patchy and artificial-looking the blue tone was on my coin. Not to mention the same dealer's admitted practice of applying a fake desert patina to bronze coins, which has also been discussed in the past in this forum.

 

That you can do it artificially, you can see if you look at the colors of Titanium coins.

https://www.mdm-wholesale.com/collections/titanium-coin-collection/

For Titanium you can regulate the interference thickness perfectly by electrochemistry, and you get well defined colors. Maybe this works for silver too.

That you coin is still silver after it lost it's color could be expected. Imagine a silveroxide layer of e.g. 500nm thickness, to thick for interference colors, this is still only a 1/2000 milimeter thickness. Darker grey or black comes later.

 

I wonder if all of this dealer's silver coins with an artificial blue coloring lose it so quickly after he sells them. If anyone buys the coins because they actually like that tone, I imagine that they're disappointed.