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Archaeological Riddles

01.10.2003

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When a stone tablet containing a 15-line inscription apparently written by a Biblical king of Israel in the 9th century B.C.E turned up in the hands of an Israeli collector, it seemed a priceless treasure. An initial team of experts studying the "Jehoash tablet" declared it to be authentic. They based their conclusions on, among other things, radiocarbon dating conducted in a laboratory in Florida.

 
Radiocarbon dating measures the concentration of carbon, which can be found in two forms (i.e., isotopes): a common, stable form called C12 and a less common form called C14. The latter is radioactive and decays over time. Because the initial ratio of C14 to C12 is a given, and the radioactive C14 atom decays at a known rate, the age of an object can be deduced from comparing the ratio of C14 to C12.

Dr. Elisabetta Boaretto. Radiocarbon dating

Dr. Elisabetta Boaretto. Age and authenticity

 
While age of the stone itself cannot be gauged by radiocarbon dating, the Florida lab team tested samples of the patina -  layers of natural build-up on the surface of undisturbed rock. Finding carbon-containing material, they performed radiocarbon testing and produced a time estimate of 2,250 (plus or minus 40) years ago. Calibrating the results put the date sometime between 390 and 200 B.C. The experts deduced that the carbon had come from wood that grew and was burned around that period, which would mean the inscription on the tablet had been produced earlier.
 
Enter Israel's Antiquities Authority, which formed two separate teams of experts. Working on a voluntary basis, they tested the tablet with nearly every method known to archaeology to determine its authenticity. Dr. Elisabetta Boaretto, who heads the Radiocarbon Dating lab at the Weizmann Institute, was asked to evaluate the results of the Florida radiocarbon dating.
 
To help answer the question: "Is it authentic?" she had to ask: "Could it have been faked?"
 
While the results of the lab tests were not in doubt, several other aspects of the testing led her to question the relation between the radiocarbon date of the charcoal and the authenticity of the tablet inscription. For one thing, the patina contained several materials, including clay and charcoal. Boaretto suspected that the Florida lab may have dated them together. And because the C14 in clay comes from organic matter that could have mixed with the clay at any time, its presence could have skewed the results.
 
More worrying, however, was the fact that no one could say exactly where the tablet had been found or by whom. In a proper archaeological dig, radiocarbon dating goes hand in hand with analyzing the context in which the artifact is found -  the chronology of the layers above and below and the age of other artifacts in the same layer. Dating would be considered conclusive only when all the evidence matched. In the case of the Jehoash tablet, there was only a bit of charcoal to go by.
 
And yet, in spite of the irregularities, the charcoal is clearly very old. In explanation, Boaretto points to the radiocarbon lab, where labeled cardboard boxes containing all kinds of artifacts, including charcoal, sit on open shelves. "A person who knows some archaeology could easily come to the lab, express interest, and even request a little of this or that."
 
Her conclusion was that the radiocarbon dating did not prove the age of the tablet or its authenticity. The rest of the team also came up with results that were at odds with what should have been found had the tablet been truly ancient. The second team formed by the Antiquities Authority to examine the writing on the tablet also reached the same conclusion. Thus the "Jehoash tablet" was declared a fake.
 

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