The amulet doesn't look like much: A lopsided, six-spoke wheel barely an inch across, swollen and green from corrosion.
But the 6,000-year-old object, uncovered from the ruins of a Neolithic farming village in Pakistan, holds clues about the ancient world it came from. And the effort to decipher those clues required some of the most sophisticated technology of today.
In the journal Nature Communications on Tuesday, scientists describe how they used a powerful synchrotron beam to analyze the tiny amulet on a microscopic level, revealing secrets about its origins that were once thought lost.
Peering through the corrosion, “we discovered a hidden structure that is a signature of the original object, how it was made,” said lead author Mathieu Thoury, a physicist at Ipanema, the European center for the study of ancient materials. “You have a signature of what was happening 6,000 years ago.”
The study relied on an imaging technique called full-field photoluminescence. The researchers shined a powerful light at the amulet, exciting electrons in the atoms that compose it so that they emitted their own light in response. By analyzing the spectrum of this emission, the researchers could figure out the shape and composition of parts of the amulet they couldn't see.
The technique revealed something surprising: countless tiny, bristle-like rods of copper oxide scattered throughout the interior of the amulet. Their structure was very different from the copper-oxygen compounds that pervade the rest of the object as a result of heavy corrosion over the course of thousands of years.
Thoury believes that ancient metallurgists were trying to craft the amulet out of pure copper, but inadvertently allowed some oxygen in during the production process. Those early copper oxides hardened into the microscopic bristles in the amulet's interior.
Their existence, paired with the fact that the amulet is not symmetrical, also suggests that the amulet was made via a process called lost-wax casting — one of the most important innovations in the history of metallurgy. The age-old process, which is still used to make delicate metal instruments today, involves crafting a model out of wax, covering it in clay, and baking the whole thing until the wax melts out and the clay forms a hard mold. Then molten metal is then poured into this cavity and cooled until it hardens. When the mold is broken open, a perfect metal model of the original wax structure remains.
At 6,000 years, the amulet is the oldest known example of this technique. Eventually, lost-wax casting would be used to produce countless functional objects — knives, water vessels, utensils, tools — as well as jewelry, religious figurines, impressive metal statues of gods, kings and heroes. The technique helped societies transition from the Stone Age to the ages of copper and bronze and gave rise to new and powerful types of culture. We have it to thank for the incredible bronze Buddha at Tōdai-ji temple in Japan and Faberge eggs. Investment casting, which is based on the process, is now used to produce equipment for NASA that has flown to the International Space Station and Mars.
In terms of beauty or sophistication, the amulet cannot rival its more famous successors. But Thoury finds it impressive in other ways. Not only did the amulet's creators use a new casting technique, they also opted to craft the amulet entirely from copper — a rare and unusual choice, since pure copper is hard to acquire and corrodes more easily than an alloy.
“It is not the most beautiful object, but still it holds so much history,” he said. “It shows how the metalworkers at the time were so innovative and wanted to optimize and improve the technique.”
Mehrgarh, the ancient settlement where the amulet was uncovered 35 years ago, is already known as a “crucible” of innovation, Thoury added. The first evidence of proto-dentistry was uncovered at the site, which is more than 600 miles southwest of Islamabad. It also contains some of the most ancient evidence of agriculture and the oldest ceramic figurines in South Asia. It's thought that this small farming community was a precursor to the entire Indus Valley civilization, one of the most important cultures in the ancient world.
“I’m really impressed that these people at the time were so keen on experimenting,” Thoury said. As a scientist, that's an impulse he knows well.