Their findings, reported in the journal _American Antiquity_, illuminate a vital facet of Cahokian society and culture.
Cahokia was one of the largest and most complex urban centers in prehistoric North America. It flourished from around 700 to 1400 CE in what is now southwestern Illinois.
Over 100 copper plates have been recovered from ceremonial and elite contexts at Cahokia, some of them embossed with intricate engravings. Archaeologists have long wondered where the copper came from and how prehistoric Native Americans working in stone, bone, wood and other organic materials were able to shape the soft yet durable metal into works of art.
New trace element analyses of metal fragments indicate that the copper was mined from deposits located in present-day Michigan's Upper Peninsula. To determine the prehistoric metalworkers' techniques, the researchers turned to ethnographic accounts of historic copper working and conducted experiments using various tools, including stone and copper hammers, anvils and wooden mallets, to replicate Cahokian copperwork.
The researchers observed that cold working--or working below the metal's recrystallization temperature--was ideal for thinning the copper sheets and creating decorative engravings, while hot working--heating the metal past its recrystallization temperature--was useful for casting.
Dr. Kelly Graff, assistant professor of anthropology in the UIC College of Liberal Arts and Sciences and lead author of the study, said that these ancient artists were incredibly skilled and knowledgeable.
"They understood metallurgy extremely well and adapted their copperworking techniques based on the desired shape of the final object," Graff explained. "They weren't simply shaping copper because they could. This process was intimately connected to broader ideological, social and cosmological systems of meaning within Cahokian culture."
The researchers also compared the chemical fingerprints of the Cahokia copper samples with samples of historic Native American copper artifacts curated at seven Midwest museums and the Smithsonian Institution. The trace elements in the prehistoric, historic and contemporary copper items yielded a "fingerprint" that is unique to Lake Superior copper, supporting the claim that the copper used at Cahokia was mined from the Upper Peninsula.
"The copper trade networks that stretched from the Great Lakes to Cahokia and beyond were central to both the region's economy and its cultural identity," said Graff. "Obtaining and working with copper was no simple feat. It took planning, time and skill, but the end results were incredibly valuable, not just as raw materials but also as symbols of power, prestige and religious authority that connected past, present and future Cahokian communities."
