Bake-It-Yourself Basalt

Making rock doesn't always require immense tectonic forces or eons to pass by. The ancient Mesopotamians, in fact, cranked out custom-made rocks in a couple of days, according to a report in today's Science. The discovery suggests their technological skills were much more developed than previously thought.

The faux stone turned up during an excavation of the ancient city of Mashkan-shapir, about 80 kilometers south of Baghdad, Iraq. Archaeologist Elizabeth Stone of the State University of New York, Stony Brook, and her colleagues discovered several slabs of what looked like basalt, roughly 80 centimeters by 40 cm by 8 cm thick. Basalt was the grinding stone of choice throughout the Near East for thousands of years because the hard surface was pocked with vesicles--small, sharp-edged holes where gas was trapped during cooling--making it ideal for crushing grain. But the nearest sources of basalt were about 1000 kilometers distant--a long way to haul such heavy objects.

Stone's team sought the origin of the slabs by comparing their composition with basalts from around the world--and even the moon--but nothing matched except some small basaltlike chunks from a potter's kiln that had overheated in another ancient city. Since kilns throughout the region were built with silt, the researchers tried baking silt in their laboratory. By heating silt to just below the point of complete melting, about 1200 degrees Celsius, they could reproduce the synthetic basalt complete with vesicles.

Stone says that building a large-enough oven that could be controlled at high temperatures was a technological leap. If the temperature rose just a few degrees too high, the silt would turn to glass, any gases would escape, and it would be useless as a grinding stone. "One of the really fascinating things about this is the narrowness of that band that it's heating it to," she says.

The southern Mesopotamians lacked many raw materials such as stone, metal ores, and large trees, but they were fabulously rich in silt, so other researchers think the theory makes sense. "It makes a very satisfying and very persuasive story," says David Killick, a technology historian at the University of Arizona in Tucson. Stone thinks the synthetic basalts may be widespread. "We suspect that there are lots of other sites in Mesopotamia that have these things as grinding stones and nobody ever paid any attention to them," she says.