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Arsenic and Old Bronze

The work of Aslihan Yener, above left, proves that the APS is not only for the biological and physical sciences. This past spring, the Oriental Institute associate professor used a bending magnet beamline at the Synchrotron Radiation Instrumentation CAT—whose goals include attracting new research communities to synchrotron radiation—to scan an ancient tin-bronze figurine. Although Yener’s scan was one of the first times synchrotron radiation had been used on an archaeological object, it’s possible that the technique may be the biggest boon to archaeology since the introduction of radiocarbon dating.

Made in about 3,000 B.C. in southern Turkey’s Amuq Valley and excavated by Oriental Institute archaeologists in the 1930s, the 11-inch statuette is among the oldest known figures made from tin bronze, a copper-tin alloy. The figure’s chemical composition was characterized by spectroscopic methods in 1949, but the APS X-ray fluorescence study still revealed surprises.

For one, the scan showed that the figurine had been broken near the knee and repaired with a lower-melting bronze that contains arsenic. The repair was barely perceptible to the naked eye. Its quality, as well as the choice of soldering material, Yener says, indicated a higher level of technical skill in the artisans than the archaeologists had expected.

“What the APS promises to do is give us much more controlled and sensitive analyses,” says Yener. “It can also, theoretically, give us a CAT scan–like image—that is, we will be able to look into the inner macro- and microstructure of the figurine without damaging it. You don’t have to damage it, you don’t have to chop off a part of it, in order to understand how it’s made.”

In November, Yener and colleagues from the Synchrotron Radiation Instrumentation CAT and the Field Museum began a series of tomographic analyses, looking at the interior structure of the figurine, a samurai sword in the Field’s collection, and other artifacts. As Yener says, the process is similar to a medical CAT scan, in which horizontal X-rays are taken of a person’s body, with the resulting series of scans used to reconstruct a three-dimensional picture of the interior.

“These types of techniques are very well developed for soft tissue and fossils,” Yener notes, “but materials such as metals are a different story altogether. It will be quite difficult, I’m told, but it is certainly worth pushing the envelope.”

The challenge comes from the extremely high energies needed to penetrate the metal and see inside. A typical X-ray can’t penetrate more than 20 microns into a metal object. But beams from the APS can penetrate much further, and should not do any damage to the artifact.

Just in case, the researchers will start by experimenting with dummy samples. Not only are the actual artifacts valuable enough to require a guard at the beamline, they are also delicate. With the dummies, researchers can relax while they fine-tune the measurement method, performing dry runs to prepare for the real thing.

“What the tomographic image will allow us to do,” explains Yener, “is to understand the manufacturing history of the figurine. Was it cast? Was it hammered? What temperature was it worked at? Was it coated with some other metals?

“We’d like to be able to define technological styles—that is, how things were made in particular areas and particular times. Embedded in the technology is a set of cultural choices that were made in order to make a particular tool or artifact.” Knowing about such choices will, she says, “add another dimension to our understanding of these ancient cultures.”

Yener is also using the X-ray fluorescence technology at APS to study the chemical composition of layers of sediment that accumulated over thousands of years. Drilled from the bed of a relic lake in the Amuq Valley that was drained in the 1950s and ’60s, the deepest and oldest layers of the sediment core have been dated to the end of the last ice age. Changes in the composition over time, Yener explains, tell the stories of erosion, pollution, changes in irrigation and hydrology, and the advent of mining and smelting in the region.

“We’re able to reconstruct the environment of that valley over time,” she says. “It covers the period of the development of human civilization in the region.”

A side benefit to the collaboration these projects require, Yener notes, is that “the scientists we work with come at this problem from an entirely different point of view. They contribute a great deal of insight and ask questions that we in the archaeological community may not have thought of.” She and scientists from the U of C, Argonne, the Field Museum, and the University of Illinois hope to develop a National Science Foundation Science and Technology Center at Argonne. Devoted to the study of ancient technology and environment, the center would have access to an APS sector. The group’s pre-proposal passed the initial NSF review in May; the group is now submitting a full proposal, with a funding decision to be made by next summer.—D.S.

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