Toned $2 1/2 Gold Indian! Put your shades on....

Beautiful gold coin!

 

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To be clear - it is most likely *not* the gold toning, but rather the copper and silver alloy which is reacting to form the colors.

I've never seen platinum coins do so, but that doesn't mean they won't. It's probably more a function of the fact that platinum has, until very recently, not been widely used as a coining metal. With only a couple of exceptions, platinum is almost always a bullion metal. All of the old platinum coins I have seen have been untoned. Most of them have a subtle patina, but that's it. For example, the US struck a couple of pattern Bust halves in platinum. The only country to strike platinum coinage for circulation (as far as I know) was Russia in the early 19th century, but they abandoned that idea after a short time:

http://coins.ha.com/itm/patterns/18...8-7729.s?ic4=ListView-ShortDescription-071515

http://coins.ha.com/itm/russia/worl...-25372.s?ic4=ListView-ShortDescription-071515

http://coins.ha.com/itm/russia/worl...-22566.s?ic4=ListView-ShortDescription-071515

 

FWIW, pressure also melts solids.

Totally immaterial. If mixed properly, every 4th atom will be copper. (4th rather than 10th because a gold atom is 2.7 times as heavy as a copper atom.) I guarantee that would be enough to tone the entire coin.

 

If gold can't tarnish, how does it have oxides?

https://en.m.wikipedia.org/wiki/Gold(III)_oxide

 

Anhydrous Au2O3 can be prepared by heating amorphous hydrated gold(III) oxide with perchloric acid and an alkali metal perchlorate in a sealed quartz tube at a temperature of around 250 °C and a pressure of around 30 MPa.[3]
Because we don't normally store our coins at 250°C at 30 MPa. Gold has existed for millions of years inside mountains through every weather, laying at the bottom of oceans in salt water, survived volcanoes, etc. and it still looks like (most of it anyway) gold when found in nature.

 

Well, let me break it down and tell you why you're wrong: a 90% gold coin is mixed by weight, not number of atoms. The atomic weight of gold is 197 (by the way, the gold atom is noticeably larger than copper - but gold is denser, meaning that on the macro scale the same mass will take up less space). That means for a weight of 197 grams of gold, there will be Avogadro's number of molecules (6.022 x 10^23). The atomic weight of copper is 63 (for a weight of 63 grams, there will be 6.022 x 10^23 molecules).

So then, we can ratio that down to 90 grams of gold (which would be 2.75x10^23 molecules) for every 10 grams of copper (which would be 0.955 x 10^23 molecules). The ratio thus becomes about 2.88 gold molecules for every copper molecule. (and thus, just as @rlm's cents said, every 4th atom will be copper)

As for the number of molecules in the surface layer itself (here, I will make a number of assumptions, but you'll get an appreciation for the order of magnitude, which is the important part): The average radius of a gold atom is 144 pm (picometers), the copper atom is 128 pm. We'll just use an approximation of 140 pm as an average radius, for ease. If the radius of an atom is 140 pm, that means its effective area (its 2-dimensional projection) will be .0615 nm2 (square nanometers). The surface area of a $2.5 gold is 254 mm2 (square millimeters). Thus, I would expect roughly 4.13x10^15 molecules on the surface plane of the coin. Assuming a perfectly homogenous alloy (which might not be the best assumption, but it makes it easier), if 1/4 of those are copper molecules, then I would say that there is plenty of chance for those copper molecules to react and tone.

(As I have discussed elsewhere, the layer of the film which causes toning must be 1/2 the wavelength of the light of the complementary color that we see - blue being created by a film about 290 nm thick. It isn't just the very surface layer. If you look at the width of the atom, we're talking about 1000 atoms deep for the toning - which means there are even more copper atoms available for toning).

Science.

 

Sorry, Doug, but in this case you are 100% wrong. If you do not believe me, read physics-fan3.1.

 

Doug, please read and think about what I wrote. The copper is uniformly distributed over the entire surface, 1000 atoms deep (and indeed, throughout the entire volume of the coin). The impurities aren't "drawn" to the surface (as you describe in a scientifically-dubious post) - they are already there, as a function of the alloying process. You can't divide it and say this 25% is copper and this isn't - there is a (more-or-less homogeneous) mix of copper compounds and gold uniformly spread throughout the surface of the gold. The copper molecules are reacting, the gold molecules aren't.

If it is the gold itself forming compounds, please provide a scientific explanation of the compounds being formed. Gold is incredibly non-reactive, and won't easily form compounds under normal circumstances.

I can't recall seeing any .999 gold coins which have toned. Please provide the promised pictures of them. (Not being familiar with these, I would surmise that the toning is very slight - and would still argue that it is the copper impurities which are reacting, not the gold itself).

 

Cool looking coin, but in my opinion nothing beats toning on Morgan Dollars

 

I have no idea why if 25% of the surface is toned, the whole coin would not look toned. Have you ever made chocolate milk. It is not 25% chocolate, but the whole glass is brown. When you mix yellow water (die not the other) with blue water, it is 100% green.

If you could possible see each atom, it might be limited to 25% toned, but your eyes are far from that fine. The toned copper atom blends with the gold atoms and makes the entire surface look like it is tone just like the chocolate milk.

 

The copper molecules and the gold molecules are acting independently of each other. The copper isn't toning the gold, and the gold isn't impeding the copper from reacting. What one molecule is doing has absolutely no impact on what the other molecule is doing.

Very attractive coins. What was the actual purity? I'll bet it wasn't 100% (because that isn't possible). Based on my understanding of the technology of the time, the best they could get was 0.99 - which still leaves an awful lot of copper atoms left over.

That is an incredibly bold claim to make. Science is formed of hypothesis, research, experimentation, and conclusion. Are you saying you know more than the scientists who have researched this? I know gold coins tone, I'm not arguing that with you - but the gold itself is not what is reacting.

You still have not provided a logical and reasonable scientific explanation for your hypothesis. What compounds are being formed by the gold? How is it doing that? I need more proof than "Because I say so." Until then, I'm going to side with science.

 

I am not in the mood to get into the weeds with you guys since its been 25 years since my last physics class, but love the posts. The one thing I would reiterate is the affect of smoke. I bought a byzantine coin once, (pretty high gold content), and was surprised it was a "toned" color. Well a soak in acetone later and it was bright gold color again. I have literally bought entire collections from a smokers house, with a large percentage of the coins looking lightly toned. So another aspect to always consider in this discussion is surface depositions of light films giving "toned" appearances. Like Doug believing his eyes, I believe mine. That toned gold coin became "untoned" with a quick cleaning.

 

So smoke makes blues and purples?

 

You and I have had this discussion before. Heat generated by surface friction is enough to melt molecules on the surface. It's called blast friction. If there is any temperature rise at all, its origin is the surface atoms.

 

I don't believe the gold itself tones, but it does have an electrical charge that can attract other ions to its surface.

 

I simply do not know on gold. The byzantine was just dark. On silver it will produce many colors. It does not have to be cigarette smoke, any kind of aerial deposition can leave a thin film on the surface of a coin changing its color appearance. My aunt's good silver would look very pretty a few weeks after she cleaned them due to smoke residue. Coins in my hand have included silver sets with the smoke smell so thick on the cardboard I had to take the coins out of the folder outdoors, and wash my hands after. Some of those coins would lose their "color" after a soak, while others would still be toned, (but might change colors).

Sorry, I was not trying to say i had direct experience with highly toned US gold that was only due to smoke. Just pointing out its another consideration for some coins.

 

Don't argue with Doug...it's like wrestling with a pig, you ain't gonna' win and the pig just enjoys it.