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Ancient seal technology shows its age

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2:26pm, February 18, 2003
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Somehow, ancient Mesopotamians carved elaborate scenes on the sides of crystal cylinders just centimeters tall. They even did this in reverse, so a cylinder would imprint its owner's mark when rolled across soft clay.

Archeologists have long guessed at engravers' techniques by studying their designs. Now, researchers have used modern technologies to illuminate the ancient ones, and they've found that Mesopotamians adopted one of their most efficient engraving methods some 1,500 years later than thought.

Cylinder seals originated about 3500 B.C., when early carvers began engraving soft stones, says Margaret Sax of the British Museum in London. Ancient people used the cylinders to roll their stamps of approval or ownership onto clay tablets, granary doors, and even the necks of sealed jars.

Over the millennia, new technologies arose for carving harder materials. Wheel cutting, the most efficient method for cutting tough varieties of quartz, used a tiny engraving disk at the end of an axle, which the engraver powered with the sawing motion of a bow. During the past 30 years, archeologists had placed the onset of this technology in the second half of the fourth millennium B.C.

Sax and her colleagues used scanning electron microscopy to identify the signatures of microflaking, filing, drilling, and wheel cutting in moldings of engravings they made themselves.

Then they made silicone molds of many of the museum's 400 quartz seals and compared the microscopic features. Their results, published in the June Antiquity, show that the engraving wheel probably didn't make its debut until the middle of the second millennium B.C.

Some features archeologists previously believed were due to wheel work were actually microflaked or filed with different tools, says Sax.

The electron microscope allowed her to see details others hadn't. "I looked at the texture of the features, the shape of the features, the depth of the features, and the orientation on the seal," she says.

By testing different tools, Sax and her colleagues also found that engravers started using copper tools charged with an abrasive quartz powder in the third millennium B.C. By the second millennium B.C., the researchers' analyses suggest, engravers used copper or bronze tools charged with emery powder.

"You obviously can't see this with the naked eye," comments Richard Zettler of the University of Pennsylvania Museum of Archaeology and Anthropology in Philadelphia. "So you really do need that electron microscope to look in great detail. I think it's a real step forward."

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