China’s first emperor broke the mold. He had himself buried with an army made of terra-cotta clay. Now insight into the careful crafting of those soldiers comes from the clays used to build them. Custom clay pastes were mixed at a clay-making center, new research suggests. The clays were then distributed to specialized workshops. Those workshops cranked out thousands of the life-size figures.

Roughly 700,000 craftsmen and laborers built Emperor Qin Shihuang’s palatial mausoleum (a type of huge tomb). It’s located in east-central China and was built between 247 and 210 B.C. A portion of those workers gathered clay from nearby deposits and prepared it in at least three forms, researchers now propose. On-site or nearby workshops used different signature recipes for their clay sculptures.

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Around 7,000 ceramic foot soldiers, generals and horses make up the army. Farmers digging a well accidentally discovered that army in 1974. Many researchers suspect the emperor would have regarded the ceramic statues as a magic army to protect him as he ruled in the afterlife.

Building and assembling the army was an enormous task. Workers poured clay mixtures into casts of torsos, limbs and other body parts. Then they assembled the bodies, taking care to create different facial features for each soldier. Finished statues are now mostly gray. But they were once covered in colored lacquers and likely fired in kilns. Most figures were placed inside one giant pit. Earthen walls formed 11 parallel corridors where statues stood in battle-ready rows.

Still, no debris nor workshops have ever been found that seemed firmly linked to the making of these statues. As a result, the number, size, location and organization of workshops involved in building the army all remain uncertain.

Analyzing clay

Patrick Quinn is an archaeologist at University College London in England. He and three Chinese colleagues studied the composition of clay from the site. The pieces were taken from 12 terra-cotta warriors and two acrobat statues found in a second pit. They also took samples from five clay bricks from the floor of the largest pit, clay fragments from inside three bronze waterfowl statues found in a third pit and part of an earthen wall in the acrobat pit.

Microscopic analysis of the samples revealed that the clay had come from deposits near the tomb’s location. But the recipes for different parts varied. Paving bricks contained only a mixture of dark and light clays. The clay used for warriors and acrobats had sand worked in. Sand and plant fragments were folded into a mix used to form the clay cores of the bronze waterfowl.

Sand may have made the clay more malleable. That could have helped in the shaping of ornate figures and increased the statues’ durability, the researchers suspect. Plant pieces also may have helped reduce the weight of core of the clay birds. A clay-processing site at or just outside the emperor’s mausoleum must have doled out the appropriate clay pastes to different workshops, the scientists propose. Potters in those workshops then made statues, bricks or other objects.

Many statue and waterfowl samples show also signs of having been slowly heated in kilns. The objects reached maximum temperatures of no more than 750˚ Celsius (about 1400˚ Fahrenheit). That’s lower by 150 ˚C (300 ˚F) or more than some previous estimates, the scientists say. Fires set in an attack on the tomb after the emperor’s death may have refired some of the clay. That could account for the temperature discrepancy. Quinn’s group published its findings in the August issue of Antiquity.

“I’m not at all surprised by the new findings,” says Robin D.S. Yates. He is an East Asian art historian at McGill University in Montreal, Canada. Archaeologists have previously found legal and administrative documents at two other Qin Empire sites. Those documents describe workshops that specialized in making various types of crafts, Yates says.

In some cases, artisans’ stamps and inscriptions on terra-cotta warriors match those on excavated roof tiles from Emperor Qin’s mausoleum. The markings suggest that some workshops made several types of ceramic objects, says Lothar Ledderose. He is an East Asian art historian at Heidelberg University in Germany. Markings on statues also indicate that artisans working at off-site factories collaborated with potters at local workshops to produce the terra-cotta army, he says.