Warrior nomads in Mongolia called the Xiongnu had a taste for the finer things in life, particularly gold jewelry, which they wore to their graves. Archeologists have wondered whether the gold objects found in second- and third-century B.C. Xiongnu burial sites originated from local deposits. Perhaps, researchers thought, the objects were gifts from the Chinese, who had a tumultuous relationship with their neighbors that alternated between warring and intermarrying.
Experiments using a synchrotron X-ray source now suggest that the Xiongnu gold originated from local panning in Mongolia instead of Chinese mines, says Martin Radtke, a physicist at Germany’s Federal Institute for Materials Research & Testing, in Berlin. Radtke’s team was invited to collaborate with researchers in Paris at the Center for Research & Restoration of Museums of France to pinpoint the likely geographical provenance of the Xiongnu gold (Anal. Chem., DOI: 10.1021/ac3025416).
Figuring out the gold’s provenance by a standard technique, such as studying the isotope composition of trace elements in the metal, was not possible, Radtke says. That’s because the method requires removing a small sample, which was not an option for the rare Xiongnu artifacts.
So the researchers turned to synchrotron radiation X-ray fluorescence (XRF), a noninvasive technique, and looked for the presence of platinum impurities in some 30 Xiongnu gold foils found in four tombs. They reasoned that if the Xiongnu had sourced the gold locally, they would have done so by panning, which leaves traces of platinum in the gold, Radtke says. Meanwhile if they found no platinum, the results would point toward a Chinese origin, since the mining techniques used by the Chinese during this point in history would have resulted in gold that was very low in platinum.
Unfortunately for the researchers, looking for trace platinum in gold using XRF is challenging because the signal for platinum is buried in the signal for gold. So the scientists employed a few experimental tricks to sidestep this problem.
First they adjusted the wavelength of the excitation energy to preferentially excite platinum signals. Next they measured or modeled the XRF spectra of all possible trace elements in the gold artifacts, such as tin, copper, and silver, and then subtracted these individual element spectra from the overall spectrum of the gold artifacts. The result was a spectrum that corresponded to that of platinum, suggesting that the origin of the artifacts was indeed the Xiongnu people themselves.
- Source: American Chemical Society