Example: North American Copper Fragments
Two copper fragments from the American Midwest, circa 1650 CE, were sent to me for analyses. Because these fragments were from a post-contact archaeological site, they could have been (1) native copper or (2) European trade metal, either smelted copper or one of its alloys. Before European contact, Native Americans did not use metallurgical casting or smelting techniques. Instead, they used copper in its native metallic form. Such copper found as a metal in nature is known as native copper. It was worked by hammering and sometimes annealing nuggets into a desired form. Most native copper in the Midwest was obtained mainly from deposits around Lake Superior, the most extensive in the world. In addition, native copper may have been collected from glacial deposits, in which nuggets of “float copper” can be found in gravel carried southward by glaciers from copper deposits to the north.
Native copper is usually about 99.7% or more pure copper, whereas early post-contact European smelted copper was less than 99.0% pure. Studies have used the concentration of impurities within copper artifacts to establish if they were fabricated from native copper or European smelted copper. European metals, obtained by Native Americans via trade or other mechanisms, were sometimes refashioned into new tools or decorations. One extreme example is when in the 19th century the HMS Investigator, which had been outfitted with copper sheathing on its hull, was trapped in ice near Canada and was abandoned. Later, local Inuit removed and used this sheathing. In additional to smelted copper, copper alloys, particularly bronzes (principally copper and tin) and brasses (principally copper and zinc), entered Native American material cultures. Other elements can occur in bronzes and brasses, either added deliberately or included by accident, including phosphorus, iron, aluminum, silicon, manganese, nickel, and more.
The first fragment (pictured above) had no silver, and in my experience, small silver inclusions are quite abundant in Lake Superior native copper. I did find a few bismuth inclusions and trace amounts of gold and sulfur in the copper matrix. Bismuth and gold both occur in native forms. Native bismuth, though, is very rare and, to my knowledge, does not occur in the area. Most bismuth is obtained as a by-product in refining lead, copper, tin, silver, and gold ores. This is consistent with the fragment being smelted copper, not Lake Superior native copper.
The second fragment, recovered at the same site, was very different from the first one. It was certainly not native copper. The fragment was an alloy primarily of copper and zinc, a brass, with tin and lead. The grain boundaries were visible, having a few microns of Cu2O between the grains. The composition of the brass grains was about 79% copper, 16% zinc, 4% tin, 0.5% iron, and 0.3% nickel. There were also lead concentrations at the grain boundaries. This too is common in brasses. The exterior of this fragment was almost entirely Cu2O, depleted of tin and zinc. I concluded that the fragment is brass, almost certainly European trade metal, that had been reworked.
Both of these fragments, therefore, are not native copper and instead are reworked European metals.
5/30/07
Added:
Electron Microprobe Analysis in Archaeology
Electron microprobe analysis (EMPA), also known as electron probe microanalysis (EPMA), is an analytical technique that combines scanning electron microscopy (SEM) and compositional analysis using x-ray spectrometry. The ability to determine structure and chemistry of samples makes EMPA very versatile. This is a dominant analytical technique in geology, but it is not as commonly used in archaeology despite similar materials in studied both fields. Here I will post about topics in EMPA, artifacts I have analyzed, archaeological studies that use EMPA, etc. If there is a topic you'd like to see posted here, please let me know.
Ellery Frahm
Doctoral Candidate, Archaeology
Research Fellow, Geology & Geophysics
University of Minnesota - Twin Cities
