Copper artifacts unearth new cultural connections in southern Africa

Chemical and isotopic analysis of copper artifacts from southern Africa reveals new cultural connections among people living in the region between the 5th and 20th centuries according to a University of Missouri researcher and colleagues.

People in the area between northern South Africa and the Copperbelt region in central Africa were more connected to one another than scholars previously thought, said Jay Stephens, a post-doctoral fellow in the MU Research Reactor (MURR) Archaeometry Lab.

"Over the past 20 to 30 years, most archaeologists have framed the archaeological record of southern Africa in a global way with a major focus on its connection to imports coming from the Indian Ocean," he said. "But it's also important to recognize the interconnected relationships that existed among the many groups of people living in southern Africa. The data shows the interaction between these groups not only involved the movement of goods, but also flows of information and the sharing of technological practices that come with that exchange."

Mining copper ore

For years, scholars debated whether these artifacts, called rectangular, fishtail and croisette copper ingots, were made exclusively from copper ore mined in the Copperbelt region or from Zimbabwe's Magondi Belt. As it turns out, both theories are correct, Stephens said.

"We now have tangible linkages to reconstruct connectivity at various points in time in the archeological record," he said. "There is a massive history of interconnectivity found throughout the region in areas now known as the countries of Zambia, Zimbabwe and the Democratic Republic of the Congo. This also includes people from the contemporary Ingombe Ilede, Harare, and Musengezi traditions of northern Zimbabwe between at least the 14th and 18th centuries A.D."

To determine their findings, researchers took small samples from 33 copper ingots and analyzed them at the University of Arizona. All samples were carefully selected by researchers from archeological samples found in the collections of the Museum of Human Sciences in Harare, Zimbabwe, and the Livingstone Museum in Livingstone, Zambia.

"We didn't want to impact the display of an object, so we tried to be aware of how museums and institutions would want to interact with the data we collected and share it with the general public," Stephens said. "We also want our knowledge to be accessible for the individuals in these communities who continue to interact with these objects. Hopefully, some of the skills linked with these analyses can be used by whomever wants to ask similar questions in the future."

Stephens said copper ingots are excellent objects for this type of analysis because they often have emblematic shapes that allow archaeologists to identify specific markings and follow changes over different time periods.

"By looking at their changes in shape and morphology over time, we can pair those changes with how technology changed over time," he said. "This often comes from observing the decorative features produced from the cast object or mold, or other surface attributes found on these objects."

Gathering scientific evidence

Once the samples arrived at the University of Arizona lab, researchers took a small amount of each sample -- less than one gram -- and dissolved it with specific acids to leave behind a liquid mixture of chemical ions. Then the samples were analyzed for lead isotopes and other chemical elements. One challenge the team encountered was a lack of existing data to match their samples with.

"One part of the project included analyzing hundreds of ore samples from different geological deposits in southern Africa -- especially ones mined before the arrival of European colonial forces -- to create a robust data set," Stephens said. "The data can provide a scientific foundation to help back up the inferences and conclusions we make in the study."

Historical connections

Stephens said the data they collect is one of the only remaining tangible links that exist today to those precolonial mines in Africa.

"Unfortunately, large open pit mines have destroyed a lot of the archaeological sites and broader cultural landscapes around these geological deposits," he said. "This makes it a challenge to reconstruct the history related to these mines. It's a concerning development, especially with the global push toward more electric vehicles which use minerals like copper and cobalt found in the Copperbelt."

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How a record-breaking copper catalyst converts CO2 into liquid fuels

Since the 1970s, scientists have known that copper has a special ability to transform carbon dioxide into valuable chemicals and fuels. But for many years, scientists have struggled to understand how this common metal works as an electrocatalyst, a mechanism that uses energy from electrons to chemically transform molecules into different products.

Now, a research team led by Lawrence Berkeley National Laboratory (Berkeley Lab) has gained new insight by capturing real-time movies of copper nanoparticles (copper particles engineered at the scale of a billionth of a meter) as they convert CO2 and water into renewable fuels and chemicals: ethylene, ethanol, and propanol, among others. The work was reported in the journal Nature last week.

"This is very exciting. After decades of work, we're finally able to show -- with undeniable proof -- how copper electrocatalysts excel in CO2 reduction," said Peidong Yang, a senior faculty scientist in Berkeley Lab's Materials Sciences and Chemical Sciences Divisions who led the study. Yang is also a professor of chemistry and materials science and engineering at UC Berkeley. "Knowing how copper is such an excellent electrocatalyst brings us steps closer to turning CO2 into new, renewable solar fuels through artificial photosynthesis."

The work was made possible by combining a new imaging technique called operando 4D electrochemical liquid-cell STEM (scanning transmission electron microscopy) with a soft X-ray probe to investigate the same sample environment: copper nanoparticles in liquid. First author Yao Yang, a UC Berkeley Miller postdoctoral fellow, conceived the groundbreaking approach under the guidance of Peidong Yang while working toward his Ph.D. in chemistry at Cornell University.

Scientists who study artificial photosynthesis materials and reactions have wanted to combine the power of an electron probe with X-rays, but the two techniques typically can't be performed by the same instrument.

Electron microscopes (such as STEM or TEM) use beams of electrons and excel at characterizing the atomic structure in parts of a material. In recent years, 4D STEM (or "2D raster of 2D diffraction patterns using scanning transmission electron microscopy") instruments, such as those at Berkeley Lab's Molecular Foundry, have pushed the boundaries of electron microscopy even further, enabling scientists to map out atomic or molecular regions in a variety of materials, from hard metallic glass to soft, flexible films.

On the other hand, soft (or lower-energy) X-rays are useful for identifying and tracking chemical reactions in real time in an operando, or real-world, environment.

But now, scientists can have the best of both worlds. At the heart of the new technique is an electrochemical "liquid cell" sample holder with remarkable versatility. A thousand times thinner than a human hair, the device is compatible with both STEM and X-ray instruments.

The electrochemical liquid cell's ultrathin design allows reliable imaging of delicate samples while protecting them from electron beam damage. A special electrode custom-designed by co-author Cheng Wang, a staff scientist at Berkeley Lab's Advanced Light Source, enabled the team to conduct X-ray experiments with the electrochemical liquid cell. Combining the two allows researchers to comprehensively characterize electrochemical reactions in real time and at the nanoscale.

Getting granular

During 4D-STEM experiments, Yao Yang and team used the new electrochemical liquid cell to observe copper nanoparticles (ranging in size from 7 nanometers to 18 nanometers) evolve into active nanograins during CO2 electrolysis -- a process that uses electricity to drive a reaction on the surface of an electrocatalyst.

The experiments revealed a surprise: copper nanoparticles combined into larger metallic copper "nanograins" within seconds of the electrochemical reaction.

To learn more, the team turned to Wang, who pioneered a technique known as "resonant soft X-ray scattering (RSoXS) for soft materials," at the Advanced Light Source more than 10 years ago.

With help from Wang, the research team used the same electrochemical liquid cell, but this time during RSoXS experiments, to determine whether copper nanograins facilitate CO2 reduction. Soft X-rays are ideal for studying how copper electrocatalysts evolve during CO2 reduction, Wang explained. By using RSoXS, researchers can monitor multiple reactions between thousands of nanoparticles in real time, and accurately identify chemical reactants and products.

The RSoXS experiments at the Advanced Light Source -- along with additional evidence gathered at Cornell High Energy Synchrotron Source (CHESS) -- proved that metallic copper nanograins serve as active sites for CO2 reduction. (Metallic copper, also known as copper(0), is a form of the element copper.)

During CO2 electrolysis, the copper nanoparticles change their structure during a process called "electrochemical scrambling." The copper nanoparticles' surface layer of oxide degrades, creating open sites on the copper surface for CO2 molecules to attach, explained Peidong Yang. And as CO2 "docks" or binds to the copper nanograin surface, electrons are then transferred to CO2, causing a reaction that simultaneously produces ethylene, ethanol, and propanol along with other multicarbon products.

"The copper nanograins essentially turn into little chemical manufacturing factories," Yao Yang said.

Further experiments at the Molecular Foundry, the Advanced Light Source, and CHESS revealed that size matters. All of the 7-nanometer copper nanoparticles participated in CO2 reduction, whereas the larger nanoparticles did not. In addition, the team learned that only metallic copper can efficiently reduce CO2 into multicarbon products. The findings have implications for "rationally designing efficient CO2 electrocatalysts," Peidong Yang said.

The new study also validated Peidong Yang's findings from 2017: That the 7-nanometer-sized copper nanoparticles require low inputs of energy to start CO2 reduction. As an electrocatalyst, the 7-nanometer copper nanoparticles required a record-low driving force that is about 300 millivolts less than typical bulk copper electrocatalysts. The best-performing catalysts that produce multicarbon products from CO2 typically operate at high driving force of 1 volt.

The copper nanograins could potentially boost the energy efficiency and productivity of some catalysts designed for artificial photosynthesis, a field of research that aims to produce solar fuels from sunlight, water, and CO2. Currently, researchers within the Department of Energy-funded Liquid Sunlight Alliance (LiSA) plan to use the copper nanograin catalysts in the design of future solar fuel devices.

"The technique's ability to record real-time movies of a chemical process opens up exciting opportunities to study many other electrochemical energy conversion processes. It's a huge breakthrough, and it would not have been possible without Yao and his pioneering work," Peidong Yang said.

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6,500-year-old copper workshop uncovered in the Negev Desert's Beer Sheva

 A new study by Tel Aviv University and the Israel Antiquities Authority indicates that a workshop for smelting copper ore once operated in the Neveh Noy neighborhood of Beer Sheva, the capital of the Negev Desert. The study, conducted over several years, began in 2017 in Beer Sheva when the workshop was first uncovered during an Israel Antiquities Authority emergency archeological excavation to safeguard threatened antiquities.

The new study also shows that the site may have made the first use in the world of a revolutionary apparatus: the furnace.

The study was conducted by Prof. Erez Ben-Yosef, Dana Ackerfeld, and Omri Yagel of the Jacob M. Alkow Department of Archeology and Ancient Near Eastern Civilizations at Tel Aviv University, in conjunction with Dr. Yael Abadi-Reiss, Talia Abulafia, and Dmitry Yegorov of the Israel Antiquities Authority and Dr. Yehudit Harlavan of the Geological Survey of Israel. The results of the study were published online on September 25, 2020, in the Journal of Archaeological Science: Reports.

According to Ms. Abulafia, Director of the excavation on behalf of the Israel Antiquities Authority, "The excavation revealed evidence for domestic production from the Chalcolithic period, about 6,500 years ago. The surprising finds include a small workshop for smelting copper with shards of a furnace -- a small installation made of tin in which copper ore was smelted -- as well as a lot of copper slag."

Although metalworking was already in evidence in the the Chalcolithic period, the tools used were still made of stone. (The word "chalcolithic" itself is a combination of the Greek words for "copper" and "stone.") An analysis of the isotopes of ore remnants in the furnace shards show that the raw ore was brought to Neveh Noy neighborhood from Wadi Faynan, located in present-day Jordan, a distance of more than 100 kilometers from Beer Sheva.

During the Chalcolithic period, when copper was first refined, the process was made far from the mines, unlike the prevalent historical model by which furnaces were built near the mines for both practical and economic reasons. The scientists hypothesize that the reason was the preservation of the technological secret.

"It's important to understand that the refining of copper was the high-tech of that period. There was no technology more sophisticated than that in the whole of the ancient world," Prof. Ben-Yosef says. "Tossing lumps of ore into a fire will get you nowhere. You need certain knowledge for building special furnaces that can reach very high temperatures while maintaining low levels of oxygen."

Prof. Ben-Yosef notes that the archeology of the land of Israel shows evidence of the Ghassulian culture. The culture was named for Tulaylât al-Ghassûl, the archeological site in Jordan where the culture was first identified. This culture, which spanned the region from the Beer Sheva Valley to present-day southern Lebanon, was unusual for its artistic achievements and ritual objects, as evidenced by the copper objects discovered at Nahal Mishmar and now on display at the Israel Museum in Jerusalem.

According to Prof. Ben-Yosef, the people who lived in the area of the copper mines traded with members of the Ghassulian culture from Beer Sheva and sold them the ore, but they were themselves incapable of reproducing the technology. Even among the Ghassulian settlements along Wadi Beer Sheva, copper was refined by experts in special workshops. A chemical analysis of remnants indicates that every workshop had its own special "recipe" which it did not share with its competitors. It would seem that, in that period, Wadi Beer Sheva was filled with water year-round, making the location convenient for smelting copper where the furnaces and other apparatus were made of clay.

Prof. Ben-Yosef further notes that, even within Chalcolithic settlements that possessed both stone and copper implements, the secret of the gleaming metal was held by the very few members of an elite. "At the beginning of the metallurgical revolution, the secret of metalworking was kept by guilds of experts. All over the world, we see metalworkers' quarters within Chalcolithic settlements, like the neighborhood we found in Beer Sheva."

The study discusses the question of the extent to which this society was hierarchical or socially stratified, as society was not yet urbanized. The scientists feel that the findings from Neveh Noy strengthen the hypothesis of social stratification. Society seems to have consisted of a clearly defined elite possessing expertise and professional secrets, which preserved its power by being the exclusive source for the shiny copper. The copper objects were not made to be used, instead serving some ritual purpose and thus possessing symbolic value. The copper axe, for example, wasn't used as an axe. It was an artistic and/or cultic object modeled along the lines of a stone axe. The copper objects were probably used in rituals while the everyday objects in use continued to be of stone.

"At the first stage of humankind's copper production, crucibles rather than furnaces were used," says Prof. Ben-Yosef. "This small pottery vessel, which looks like a flower pot, is made of clay. It was a type of charcoal-based mobile furnace. Here, at the Neveh Noy workshop that the Israel Antiquities Authority uncovered, we show that the technology was based on real furnaces. This provides very early evidence for the use of furnaces in metallurgy and it raises the possibility that the furnace was invented in this region.

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