Need help regarding copper alloys... 1

[MrEureka]

I'm a numismatist, venturing here because Google has failed me.

I'm in possession of what I believe to be an 1857 cent, which I believe to be in an experimental alloy. (For those that may be interested, there were many such things struck in the 1850's. These experiments led to two actual changes in the composition of new coinage over the course of the decade.)

My piece has been tested via SEM-EDX and appears to be:

Copper 88.2%
Nickel 10.5%
Zinc 0.8%
Iron 0.4%

The standard alloy at the time was:

Copper 88%
Nickel 12%

I'm trying to figure out if the divergence from the standard alloy was accidental or intentional. What I need from your group is an understanding of how the introduction of Zinc and Iron into the standard alloy would have changed the properties of the material, especially with respect to color.

Any help would be tremendously appreciated!

 

[MikeHalloran]

http://www.copper.org

Mike Halloran
Pembroke Pines, FL, USA

 

[MrEureka]

Will try them. Thanks.

 

[NickE]

I dont think that SEM EDX (EDS) has a fine enough resolution to tell the difference btw 10.5% Ni and 12% Ni.

Also It seems to me that without electrolytic refining the Zn could be tramp from the Cu ore body. And the Fe could be from the Ni.

If I'm way off here I'm sure someone will be able to tell me I dont know what I'm talking about.

(thats your invitation to correct me.)

Nick
I love materials science!

 

[MrEureka]

Nick - Thanks for the info. Do you know of any better non-destructive processes? - Andy

 

[NickE]

X-Ray Florescence (XRF) and/or X-Ray Diffraction (XRD) were the analytical methods I was always taught had the highest accuracy. You might need some correction for the surface roughness of the coin, the interaction volume (the amount of material excited by the incident X-Rays) should be large enough to give a good composition though.

SEM-EDS can get you close, if you have samples of coins in each alloy that will really help. Also if the EDS system is properly calibrated with good standards the accuracy is better.

I definately wouldn't use high magnification, and as large an acceleration the machine will support will get more representative info.

 

[MrEureka]

Thanks again!

 

[EdStainless]

The other advantage of SRF is the high voltage will give deeper penetration of the Xrays, this should help minimize the surface effects.
Do you have other coins to check as a reference? You will want to do as many as possible in the same setup to get a better feel for the actual differences.
My guess is that the Zn and Fe are common enough impurities that they came along in the process rather than being deliberatly added.
In modern CuNi alloys the limits for Zn and Fe are usually 1%. At these levels the Zn doesn't hurt, and hte little bit of Fe increases mechanical properties.

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[kenvlach]

MrEureka,
Is this the coin in question?

"Diameter: ±19 millimeters
Metal content: Copper ±88%, Nickel ±12%
Weight: ±72 grains (±4.7 grams)
Edge: Plain

1857 saw the end of an era as the old, bulky Large Cents were replaced with a new, smaller One Cent .... The metal composition was changed from nearly pure Copper to a Copper-Nickel mixture that gave added strength and a lighter color, earning the coins the nickname "White Cents." To ensure widespread distribution, the Mint produced over 17,000,000 of the new Small Cents, nearly six times the production of Large Cents in the previous year."
--- Collectors Universe, Inc.

http://www.coinfacts.com/small_cents/flying_eagle_cents/1857_flying_eagle_cent.htm

Similarly, from the US Mint website, for the cent coin:
"The composition was pure copper from 1793 to 1837.
From 1837 to 1857, the cent was made of bronze (95 percent copper, and five percent tin and zinc).
From 1857, the cent was 88 percent copper and 12 percent nickel, giving the coin a whitish appearance.
The cent was again bronze (95 percent copper, and five percent tin and zinc) from 1864 to 1962."

http://www.usmint.gov/about_the_mint/fun_facts/index.cfm?action=fun_facts2

Regarding EDS, WDS & XRF analytical methods.
Electron microprobe analysis (EMPA), uses wavelength-dispersive spectroscopy (WDS) so has diffracting crystals to separate the x-rays excited from the sample. With high purity standards and a flat, polished sample surface, accurate to 0.01 wt%.

SEM with EDS (energy-dispersive spectrometry, EDAX is an analyzer brand name), is accurate to about 0.1 wt% for standardless analysis, maybe 0.025% with standards. Has pretty much displaced WDS since cheap to attach to an SEM and simpler construction, with a single multichannel analyzer allowing line and area scans (rastering) w/o affecting relative sample-to-detector geometry.

XRF has interference problems when analyzing metals of similar atomic weights. Although an X-ray beam may pass deeper into a specimen than an electron beam as used in EDS or WDS, the fluorescing x-rays detected are from a much shallower volume, 0.1 micron vs. characteristic x-rays from a 1 micron deep volume in EDS or WDS. Makes it useful for plating thickness & composition measurements. Takes good standards and very good software to be accurate.

Regarding the 1857 alloy. I agree that the iron content likely is an impurity, mostly in the Ni. But, the Zn seems too high to be accidental. I tried to find refined copper & nickel analyses ca. 1857 but so far unsuccessful. Analyzing other coins of similar vintage as suggested above is a good idea.

At that time, most of the copper matte (semi-purified sulfide) produced in the US & elsewhere was shipped to Swansea, Wales, for treatment -- roasting to blister copper and then fire refining (oxidizing off less noble metals and sulfur) + poling (inserting wood poles to deoxidize) to produce tough pitch copper. Note: the electrolytic refining of copper on a commercial scale began in 1869 at Pembrey in Wales. -- A Hundred Years of Metallurgy, pp. 128-132 & 140-141 (1963).

In Handbook of the Non-Ferrous Metallurgy, vol. 2, p. 248, (1945) Lidell gives analyses of blister copper from 5 smelters -- 98.4-99.5% copper, av. 0.12% Fe, av. 0.10% Ni, av. 0.035% Zn; also As, S, Sb, Pb, Ag & Au in lesser amounts. Subsequent fire refining + poling improves the purity. The current specification ASTM B216, 'Standard Specification for Tough-Pitch Fire-Refined Copper,' gives compositions Cu 99.88% (min), Ni 0.05% (max), Se + Te 0.025% (max), As 0.012% (max), and everything else < 0.004%. No iron and definitely no zinc.

I didn't find a composition for refined nickel ca. 1857, but note that 1857 was a few years before electrolytic refining and many years before the Mond (carbonyl purification) process. Iron & cobalt are usually the greatest impurities in smelter nickel prior to electrolytic refining. Circa 1936, commercial purity nickel was about 99.2% Ni (+Co), 0.4% Fe, 0.2% Cu, 0.1% Mn, 0.1% Si, 0.1% C, 0.005% S. Composition from Metals Handbook, 1936 Edition, p. 1257.

Of possible interest:
"Nickel silver is a metal alloy of copper with nickel and often but not always zinc. It is named for its silvery appearance, and contains no elemental silver."

http://en.wikipedia.org/wiki/Nickel_silver

Various 19th century uses & trade names of nickel silver:

http://pages.zoom.co.uk/leveridge/nickel1.html