This is a photo of a small piece of native copper from Michigan. The bare copper here is exposed, giving it a “new” reddish-orange appearance. This is probably the result of an aggressive cleaning, as typically these pieces have a patina that is green, brown, red, or another color. If you look closely at the copper you can see dark red, glossy, string-like structures. These are likely cuprite, which is an oxide of copper (1). The clear crystals (at the edges of the copper at two and nine o’clock) may be calcite, the green crystals (on the right edge of the copper) epidote, and the matrix (the gray-green “host” stone the copper is on) is basalt (1).
Approximate Photo Location (Side 2)
Magnification: ~5.2X (crop)
Field of view: ~3/16” x 1/4“ (4.6mm x 6.9mm)
Images in focus stack: 50
Photo from a different section of the specimen.
Copper is one of a relatively small number of metals that can be found (in larger quantities) in nature in native form (2, 3). Others include gold, silver, and platinum (2, 3). It is a versatile material that has many applications due to its high electrical conductivity (second only to silver), thermal conductivity, ductility (i.e., ability to be stretched), malleability (i.e., ability to be shaped or compressed), and corrosion resistance (2, 4, 5, 6). It is also easily alloyed with different metals and can be used to create brasses, bronzes, cupronickels, gunmetals (a type of bronze), and other materials (2, 5)(i). Like aluminum and other metals, copper can in theory be continually recycled without losing any of its properties (2, 7). Copper is also an essential nutrient for humans and one of eight essential micro-nutrients for plants (8, 9). High levels of copper can, however, be toxic to both (8, 9). Copper is known to have anti-microbial properties (10, 19). Here are a few miscellaneous copper facts:
- The copper on the Statue of Liberty weighs 31 tons (62,000 lbs)(11, 12). The statue was a brownish color when installed in 1886 and turned green by 1920 (13, 14).
- An average small car contains 44 pounds of copper, and the average length of copper wiring in a car is nearly a mile (2). The average single-family home contains over 400 pounds of copper (15).
- The US nickel, dime, quarter, half dollar, and dollar all contain a higher percentage of copper than the cent (16). (Since 1982, the composition of a US cent has been 97.5% zinc and 2.5% copper)(17).
- The American Society for Testing and Materials lists over 500 different copper alloys (15).
- It is estimated that the global percentage of copper that is recycled from discarded products is over 50%, and the amount of recycled copper in use is between 10% and 25% (7).
- The average human adult has 50 to 120 mg of copper in their body, two-thirds of which is located in the skeleton and muscle (8). While uncommon, copper deficiency can cause anemia, connective tissue disorders, changes in lipid levels, osteoporosis, increased risk of infection, and other problems (8). Copper can be consumed through seeds, nuts, whole grain products, shellfish, chocolate, and other foods (8).
- Six copper alloy groups have been registered with the US Environmental Protection Agency as antimicrobial (18). This allows manufacturers to make public health claims, including: “Laboratory testing has shown that when cleaned regularly this surface kills greater than 99.9% of bacteria* within two hours” (19). These materials are marketed under the “Cu+” brand (20). Copper alloy door and sink handles, bed rails, toilet seats, and other items have been installed in medical facilities to decrease the spread of infection (10).
- Octopuses, squid, spiders, and some other animals have a copper-based protein in their blood called hemocyanin that transports oxygen and gives their blood a bluish color (21, 22). (In our case, an iron-based protein hemoglobin binds to (carries) oxygen and gives our blood a reddish color)(22).
i. In general, brasses are alloys of copper and zinc, bronzes are copper and tin, cupronickels are copper and nickel, and gunmetals are copper, tin, and zinc (4, 5). Variations of these are produced by adding or substituting materials (e.g., a phosphor bronze is composed of copper, tin, and phosphorous, and a silicon bronze is composed of copper, silicon, and manganese)(5). While copper alloying generally increases the strength of the material, it also decreases its thermal and electrical conductivity (5, 6). Brass alloys are easily shaped and machined (i.e., cut without deforming), retain some corrosion resistance, and are cheaper to produce than copper (5). Brasses can be strengthened by increasing the level of zinc, however this also increases the likelihood of structural weakening from stress-corrosion cracking and dealloying (4, 6). Gunmetals, on the other hand, have good corrosion resistance and tend not to have these structural susceptibilities (4). Lead can be added to gunmetals to ensure pressure tightness in items like valves and pumps (4). (The name “gunmetal” was coined from their past use in making gun barrels (4).) Cupronickels have advantages in marine environments, where they are resistant to salt water corrosion and biofouling (4, 5). Biofouling is the accumulation of barnacles, mollusks, or other organisms which can, for example, increase the drag on a ship’s hull or clog up piping (2, 4, 15).
1. Red material on copper? (2018). Retrieved from Mindat.
2. US Geological Survey. (2009). Copper—A metal for the ages. Retrieved from the USGS.
3. National Research Council. (2008). Minerals and materials uses in the United States. In Minerals, critical minerals, and the U.S. economy (Chapter 2). Washington, DC: The National Academies Press. Retrieved from the National Academies Press.
4. Powell, C., & Webster, P. (2012). Copper alloys for marine environments (CDA Publication No. 206). Retrieved from the Copper Development Association.
5. Wyatt, L. M., Coveney, V., Riddiford, C., & Sharpe, R. (1994). Materials, properties and selection. In E. Smith (Ed.), Mechanical engineer’s reference book. Oxford: Butterworth-Heinemann. Retrieved from Google Books.
6. Davis, J. R. (Ed.). (2001). Introduction and overview. In the ASM specialty handbook: Copper and copper alloys (#06605G). Retrieved from ASM International.
7. Graedel, T. E., Allwood, J., Birat, J., Buchert, M., Hageluken, C., Reck, B. K., Sibley, S. F., & Sonnemann, G. (2011). What do we know about metal recycling rates? Journal of Industrial Ecology, 15(3). Retrieved from the University of Nebraska.
8. National Institutes of Health (NIH), Office of Dietary Supplements (ODS). (2019). Copper fact sheet for health professionals. Retrieved from the NIH-ODS.
9. Kaiser, D. (2018). Copper for crop production. Retrieved from the University of Minnesota Extension.
10. Grass, G., Rensing, C., & Solioz, M. (2011). MINIREVIEWS: Metallic copper as an antimicrobial surface. Applied and Environmental Microbiology, 77(5). Retrieved from the National Center for Biotechnology Information.
11. The National Park Service (NPS). (2015). Statue statistics. Retrieved from NPS.
12. Thomas, M. (n.d.). Statue of Liberty. Retrieved from Purdue University’s School of Materials Engineering.
13. The New-York Historical Society. (n.d.). When did the Statue of Liberty turn green? Retrieved from the New-York Historical Society.
14. Felton, J. (2017). The true color of the Statue of Liberty–and it isn’t blue-green. Retrieved from IFLScience.
15. Copper Development Association. (n.d.). Copper facts: Copper the metal. Retrieved from the Copper Development Association. (FYI this link launches a print dialog window, which you can cancel.)
16. The United States Mint. (2019). Coin specifications. Retrieved from the US Mint.
17. Donahue, M. Z. (2016). How much does it really cost (the planet) to make a penny? Retrieved from Smithsonian.com.
19. Copper Development Association. (n.d.). Public health claims. Retrieved from the EPA Antimicrobial Stewardship. (Also see the EPA documents above for labeling).
20. International Copper Association. (2017). Executive summary: Antimicrobial copper initiative. Retrieved from the International Copper Association.
21. Nuwer, R. (2013). Ten curious facts about octopuses. Retrieved from Smithsonian.com.
22. Lutz, D. (2010). The many colors of blood. ChemMatters, February. Retrieved from the American Chemical Society.
Copyright © 2016–2019 by Aaron-Emile W. Osborn, all rights reserved.