Elemental Phosphorus
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Color of copper alloys
Hello guys,
I recently was pondering the color of copper alloys, because I was looking at some different coinage, and what occurred to me is that the color of
copper alloys varies widely, for example, the color of cupronickel, the common alloy in American coinage (the nickel, dime, quarter, half dollar, and
those old very big dollar coins) is purely silver, with no coppery lustre. However, copper-zinc alloy, copper-aluminum alloy, and copper-tin alloys,
among others, all retain color. The same is true with gold, where some gold alloys remain colored, while others, like Au-Pd or Au-Ni are silver. In
fact, just 10% of nickel is sufficient for Au to no longer be gold-colored.
I had assumed that the color of copper was due to the small energy difference between the [Ar] 4s1 3d10 ground state and the [Ar] 4s2 3d9 excited
state, which would absorb some blue light, lending an orange color. But I realize this is assuming I'm dealing with a gas or solution, not a solid,
since the bulk metal will have a 'sea' of electrons rather than these discrete electron shells, right?
I must admit that condensed matter physics is not my forte, but Wikipedia said that copper's color is attributable to a low-energy plasma oscillation,
which I can accept. If I understand correctly, it seems like essentially the condensed-matter analog of an electronic energy state transition, where
the color is due to a low energy valence band to unoccupied band transition.
However, this doesn't explain to me why the addition of a fraction of another metal can remove this color. I assume it must modify the energy of the
valence/first unoccupied bands? If this is the case, why do different metals have different effects? Someone suggested the reason why zinc doesn't
change the color much and nickel does is because zinc's [Ar] 4s2 3d10 structure means it doesn't alter the electronics much. But what about the
nickel? (or tin, Al, etc.)
Wikipedia gives an equation for the plasmon frequency:
https://en.wikipedia.org/wiki/Plasmon#Role
But I am trying to see how the Ni atoms affect things. I assume they must change the conduction electron density? But weirdly, the density of states
of nickel seems to be closer to that of copper than zinc is to copper. (Cu and Ni both have a lot of states close to Fermi level, Zn less so). So if
anything, it would seem to me that zinc ought to affect the color more.
If anyone has any insight, I am very curious.
Also, if I could somehow make cold copper gas, or dissolve copper metal in solution, would it still be orange? Or should the plasmon energy be much
different from the atomic orbital transition energy?
I am still pretty new to this stuff. But any answers would be appreciated!
Edit: The order of the photos is: 1. Copper DOS 2: Nickel DOS 3: Zinc DOS.
I don't know what's wrong with the first photo, but if you click it it's normal.
  
[Edited on 26-2-2025 by Elemental Phosphorus]
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chempyre235
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Unfortunately, I don't have much insight, just some more copper-containing alloys that are/were included in US coinage:
- So-called "white pennies" (1860s and 1870s) were cupro-nickel (88% Cu, 12% Ni).
- Sacagawea dollars (2000-2008), innovation dollars (2018-present), presidential dollars (2007-2016, 2020) were made of a manganese brass (88.5%
Cu, 6% Zn, 3.5% Mn, 2% Ni), and still show some color from the copper.
- Most pre-1965 US coinage (dimes, quarters, half-dollars, silver dollars) were 10% Cu, 90% Ag.
- The "war nickels" from 1943-1945 were some kind of German silver, with 56% Cu, 35% Ag, and 9% Mn.
[Edited on 2/26/2025 by chempyre235]
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Elemental Phosphorus
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Interesting! A while back I found a war nickel in some change I got, and I had heard that they contained silver, but had no idea it was a manganese
alloy.
Never heard of a white penny though! And that's a different mix from today's 75-25 cupronickel.
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chempyre235
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Yeah, the white pennies were only made during the last years of the flying eagle cent, and the first years of the indian head penny.
Note that the different copper-colored alloys are still somewhat different in appearance:
- Bronze is darkest;
- 1944-1946 pennies were made with recycled bullet casings, and slightly lighter than bronze;
- The zinc alloy of the 1970s was somewhat lighter and redder than the bronze;
- The pure copper plating on modern (1983-present) pennies is still lighter and redder than the alloys.
I too find this study interesting, and am inclined to know the reason for these color variations, though I hadn't given it much thought before.
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Sulaiman
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I used to clean electronic assemblies or circuit boards after repairing them.
depending upon which cleaning solution(s) I used,
plain pure copper surfaces appeared in different shades. (reddish to brownish hues)
I assumed it was something to do with redox but I never investigated it.
CAUTION : Hobby Chemist, not Professional or even Amateur
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