December 2, 1999

Berkeley Lab Science Beat

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When relying on potsherds instead of people as their sources, archaeologists can learn a lot about ancient cultures. Layers in which fragments are buried suggest the age of the culture; changes in pottery design or decoration give hints about a culture's evolution and contact with others; and a fragment's composition may indicate where a population lived and how widely it traveled or traded.

In culturally complex areas, however, where various settlements were manufacturing and trading wares of similar composition, the standard methods of archaeology -- excavating and comparing decorative styles and materials -- are not always sufficient to unambiguously determine whether an artifact was actually made where it was found.


This is where new methods of materials analysis known as instrumental neutron activation analysis (INAA) and x-ray fluorescence (XRF) come in. The work of Berkeley Lab's Frank Asaro, Robert D. Giauque and their colleagues has shown that these methods can help solve archaeological problems, as demonstrated in a recent collaboration with archaeologist David Adan-Bayewitz of Bar-Ilan University in Israel, in which XRF was applied to pottery from more than 20 sites in northern Israel. Asaro formally retired from the Lab in 1991, although he says he continues to work full-time "and often longer, just for the fun of it."

Determining the origin of ancient pottery -- or provenance -- can help archaeologists better understand the relationships and influences between peoples. Correlating pottery fragments found in excavations with the ancient workshops in which they were made helps illuminate the trade and cultural contacts among ancient settlements.

Determining Provenance

By measuring the abundance of trace and major elements in a pottery fragment, and comparing these with those in the soil and artifacts at known pottery workshops, researchers can often determine provenance. "INAA has been the method of choice for measuring elemental abundances because of its great precision and reliability, but it has some disadvantages," says Asaro. "One is that INAA requires a nuclear reactor, and XRF does not. Another has to do with more complicated sample preparation and analytical procedures. INAA results can often take several months to complete, even for a small number of samples."

Asaro and Giauque, who work in the Environmental Energy Technologies Division, collaborated with Adan-Bayewitz to demonstrate that a new methodology of x-ray fluorescence (XRF) can be used to determine pottery provenance with comparable measurement precision to INAA. XRF does not require a nuclear facility, relying instead on a conventional silicon x-ray spectrometer with an x-ray tube -- widely available lab equipment. A single investigator can complete XRF results in as little as a few hours.

The research team analyzed pottery samples from ancient sites in the Galilee and neighboring areas in northern Israel using both the standard INAA method and XRF. The ages of the artifacts ranged from the first century B.C. to the fourth century A.D. The researchers demonstrated that both methods were effective in measuring provenance. Moreover, Asaro says, "XRF was also able to distinguish better than INAA between two sets of pottery from nearby sites that were very similar in composition.

"We showed that XRF holds promise as a cheaper, faster method than NAA for determining archaeological provenance while being equally effective," concluded Asaro. A paper reporting their results was published in Archaeometry earlier this year.

"To the best of our knowledge," says Adan-Bayewitz, "the kind of extensive intra-regional provenance study we are engaged in has not yet been conducted elsewhere ... In most provenance studies, the questions asked might be whether the pottery is local or imported, or whether the pottery was made in one country or another -- for example, in Iron Age Cyprus (1200-586 B.C.) or ancient Israel. In our work, our interest is in site-specific provenance: whether the pottery was made at site x or at site y, which might be as little as two miles away. This definition of the provenance problem aims at a better understanding of mobility and contacts within a region. But in order to achieve this, the analytical techniques employed must be capable of distinguishing between pottery of similar composition made at nearby sites."

New Testament connection

One reason for the interest in the Galilee area, and why this work is important, is that the region was home to several major cultures. The most important texts of Rabbinic Judaism were compiled there during the Roman period. The disciples of Jesus are said to have lived there, as did several major pagan communities. "The relationships between the peoples of this region in the Roman period continue to be of intense scholarly interest," says Adan-Bayewitz. "An example of how our work has contributed to these questions is the problem of rural-urban conflict in Galilee. It had been widely accepted among New Testament scholars that the `Jesus movement' was comprised of village Jews who were alienated from the cities, with their populations of rabbis and hellenized Jews. Support for this hypothesis was found in the New Testament in the absence of any reference to the two Galilean cities, Sepphoris or Tiberias, despite their proximity to villages discussed extensively in the Gospels. For example, Nazareth is only about 6 kilometers from Sepphoris.

"Our work has demonstrated that about three-fourths of the common kitchenware used at Sepphoris in the Roman period came from the Galilean village of Kefar Hananya, about 27 kilometers away. These data show that there was continual contact between village and city. It is common pottery -- by far the most prevalent artifact to survive from antiquity -- analyzed with high precision chemical techniques, that provides quantifiable and statistically meaningful data, opening a window on local contact and relationships in antiquity."

Tools of physics for archaeology

The road to techniques of analysis used in this and many other archaeological studies began in the 1950s at Berkeley Lab. Early in his career, as one of Isadore Perlman's first Ph.D. students (Perlman was head of the Chemistry Division and associate director of Berkeley Lab), Frank Asaro studied the excited states of the nuclei of heavy elements by measuring the states of their alpha particle emissions, and later, gamma ray emissions. During the 1950s and early 1960s, Asaro, Perlman and their colleagues did groundbreaking work in this field, which contributed crucial evidence for a now accepted model of the nucleus called the unified model.

In the mid-1960s, Perlman became interested in archaeology. He and Asaro realized that they could use the gamma ray emissions of trace radioactive elements in a pottery sample to measure very accurately the abundances of elements in the sample, pinning the sample's origin to a unique location where the materials were quarried or dug. After bombardment by neutrons, the elements in the sample would emit gamma rays of various discreet energies, revealing unique signatures.

By comparing the gamma rays of the unknown sample with known reference samples -- measurements that they had been making for more than a decade -- they determined the concentrations of elements in the sample, and with it, the origin of the pottery. The paper they published in 1969 on their work became a landmark, and the most heavily cited in its field.

"How good was Perlman at choosing new fields?" asks Asaro. "I thought I would take three months off to do this. I made that decision in 1967, and I'm still doing this work 32 years later."

In the years that followed, Asaro, Perlman, and other colleagues conducted a number of studies which solved some archaeologically significant problems. With Michal Artzy, Perlman and Asaro demonstrated in 1967 that an innovative Middle Bronze Age style of pottery known as Palestinian bichrome, long considered to have been manufactured in Palestine, was actually manufactured in Cyprus and exported to Palestine.

Bichrome was first discovered by the great English archaeologist Sir Flinders Petrie and later found throughout the Middle East.

Turning their attention to Egypt in 1973, Asaro and his colleagues began studying the Colossi of Memnon, two 50-foot hewn-quartzite statues located near Luxor. These guardians of Pharoah Amenhotep III were built before 1,200 B.C. In 27 B.C., an earthquake felled half of the north statue, and the damage was not repaired until 200 A.D. by order of Roman emperor Septimius Severus. Archaeologists thought that the quartzite had come from a quarry 100 miles away near Aswan. Using neutron activation analysis, Asaro and his co-workers showed that the original rock for the statues came from quarries in Cairo, 420 miles away, and that the Romans subsequently used stone from the nearer Aswan quarry to repair the statue.

And in 1977, Asaro and Helen Michel demonstrated that the Plate of Brass, a much-celebrated artifact said to have been left on the California coast by Sir Francis Drake in 1579, was not authentic. Neutron activation analysis showed that zinc content was too high and the impurity levels too low for sixteenth century English manufacturing techniques. The brass in the plate, which was found under a rock in 1936, was most likely manufactured no earlier than the first half of the nineteenth century.

Dinosaur extinction collaboration

The work that is perhaps best known to the general public was Asaro and Michel's collaboration with Luis and Walter Alvarez in measuring the abundance of iridium in a layer of rock found throughout the world that was formed at the end of the Cretaceous era, 65 million years ago. Begun by the father-and-son Alvarez team of Berkeley Lab and UC Berkeley, the work led the group to propose that the extinction of the dinosaurs was caused by an asteroid collision with the Earth.

About half of the measurements of iridium abundances from samples of the Cretaceous-Tertiary boundary layer throughout the world were done by Asaro and Michel using an instrument now called the Alvarez Iridium Coincidence Spectrometer.

Asaro, Giauque and Adan-Bayewitz continue their collaboration and are continuing to refine the precision of the techniques.

Says Adan-Bayewitz, "Our work on the development and application of high-precision XRF will make this high-precision analytical method available to other researchers, enabling them to investigate such problems of mobility and trade in other regions and periods."