SHAANXI, Northwest China — When Rong Bo and his team uncover a terra-cotta warrior, they have one thing on their minds: Protect the earth.

For a long time, archaeologists at the Emperor Qin Shi Huang Mausoleum focused on uncovering the roughly 8,000 clay soldiers buried in the giant necropolis, built for China’s first emperor in the ancient capital of Xi’an. But these days, they approach new excavations with extreme caution. Every grain of dirt may contain vital evidence.

“In the old days, the dirt surrounding an artifact might have been carelessly brushed away. Not anymore,” says Rong, deputy director of archaeology at the Emperor Qin Shi Huang Mausoleum Museum. “One misjudgment, one careless removal, and a relic’s history might disappear.”

The introduction of new soil analysis techniques has transformed researchers’ ability to restore the 2,200-year-old Terra-cotta Army over the past few years. Pigments and organic material that had leached from the clay figures over the centuries can now be accurately identified, and even in some cases reapplied, enabling restoration teams to recreate the warriors’ vivid paintwork and repair their original armor and weaponry. 

In the process, a new picture of Qin dynasty (221-206 B.C.) China is emerging: One of a society that was more colorful and more technologically advanced than many realized.

It has taken decades for China to acquire the scientific resources needed to preserve the relics uncovered from the sprawling 98-square-kilometer mausoleum, most of which has yet to be excavated. Even compared with other ancient sites, the necropolis is highly fragile, weakened by earthquakes, floods, as well as a fire — said to have been started by the rebel leader Xiang Yu around 200 B.C.

After the burial site was discovered by local farmers in 1974, work teams began digging at a frenzied pace, but they often unwittingly damaged the artifacts they uncovered. Most of the unearthed terra-cotta warriors were shattered and heavily worn, and early efforts to repair them were disastrous.

In 1979, a group of archaeologists wrote to China’s leaders in protest of the “ruinous” work being carried out in Shaanxi province, where excavations resembled a “potato harvest” and restoration efforts were turning the ancient statues into “cement men.” This led to the project being shut down for several years.

Chinese researchers eventually developed a method for reassembling the warrior fragments into whole statues — a painstaking process that required years of training — in the 1990s. But though this provided an impression of the Terra-cotta Army’s original appearance, it remained an incomplete and distorted picture.

Most notably, the Qin-era warriors were not monotone terra-cotta statues: They were sumptuously painted in a range of bright colors. Recent studies have confirmed that the most common color used to decorate the warriors was green, followed by purple, red, and blue. Contrary to popular belief, few of the soldiers under the austere Qin appear to have favored wearing black.

“Only the Qin rulers valued black, while the ordinary people seem to have preferred rich and bright colors,” says Jiang Liping, a historical researcher at Xi’an Polytechnic University. “The Qin army was mostly made up of ordinary people: When they put on their armor and went to the battlefield, they became soldiers.”

In many cases, the clay warriors retained their original coloring in the ground, but the paint would start to degrade as soon as it was exposed to air. Archaeologists have described watching helplessly as the statues they had uncovered visibly faded before their eyes.

“The multi-colored lacquer is buried in a moist environment for thousands of years, and then during excavation it is introduced to a much less humidified environment,” says Yang Lu, an expert on the use of pigments in ancient China from Northwest University in Xi’an. “If the painted relics are exposed to a much drier atmosphere for an extended period, the lacquer layers will shrink, deform, crack, and detach from the terra-cotta surface.”

For years, the researchers in Xi’an were powerless to prevent the lacquer from being damaged. Today, however, they are able to do so by spraying newly excavated objects with a preservative and then wrapping the object in a plastic membrane to preserve the humidity level. This method can keep the painted figures stable for more than 10 years.

In some cases, contemporary restorers are even able to reverse the effects of time by extracting pigment from the soil around a statue and reapplying it to the surface of the relic — a technique dubbed “paint layer reattachment.” 

Lan Desheng, a restorer at the Emperor Qin Shi Huang Mausoleum Museum, led the team that used paint layer reattachment to repair the famous general figurine. As one of only nine Terra-cotta Army generals uncovered so far, the statue is particularly precious, but it was in miserable condition when it was excavated in 2011: cracked and broken into 40 pieces, with parts of the lacquer layer peeled off.

It took Lan’s team three years to repair the general and restore his intricately patterned armor, but the 51-year-old tells Sixth Tone it was time well spent. “Now he is so beautiful,” says Lan.

The next frontier for the researchers is to find out more about the weapons and accessories that once adorned the warriors, which were often made using organic materials that have decomposed over time. To do this, they are increasingly moving entire chunks of the dig site to the museum’s 12 on-site laboratories, where they can perform detailed chemical analyses of the earth surrounding the terra-cotta fragments.

“An excavation site is like a crime scene, and we are like forensic experts,” says Luo Hongjie, a chief scientist in China’s “973” National Basic Research Program who has helped the museum introduce the new analysis techniques. “We have to deduce what happened in history from any trail of clues we find at the site.”

For a long time, archaeologists avoided removing artifacts whenever possible. Researchers had to use plaster to fix the objects in place during transportation, and breaking the mold sometimes damaged the relics. But the process has become much less risky in recent years, thanks to the discovery of more effective materials for temporary consolidants. 

In the mid-1990s, the Xi’an team began using cyclododecane (CDD), a waxy compound invented by scientists in Germany. Then, in 2010, researchers began switching to menthol, which is far cheaper and more readily available in China.

The chemicals proved to be a game-changer due to their ability to smoothly change states. Researchers can liquefy the consolidant on-site and apply it to a statue, where it gradually hardens, fixing the object in place. Once the relic has been safely transferred to the lab, the material can then be evaporated, leaving the statue untouched.

The technique has given the museum a new level of insight into Qin-era craftmanship, says Luo. He cites the recent unearthing of an intricate suit of stone armor as an example.

“The armor used to be a whole piece — 600 little bits of stone strung together,” says Luo. “The menthol technique fixed the pieces in place, allowing restorers to reassemble the armor in the exact order.

“Plus, the material used to string the pieces of stone together had rotted,” he continues. “Previously, all we could do was guess (what kind of material was used), but now we can actually find out, even if there’s just one molecule of it left in the soil.”

Sometimes, the chemical analysis provides intriguing hints about how large an undertaking building the Terra-cotta Army must have been. Yang Lu, of Northwest University, found that a range of different adhesives were used in the warriors’ colorful lacquer, confirming assertions by Sima Qian — the ancient Chinese historian — that the 700,000 workers involved in the project were sent to Xi’an from across the empire.

These adhesives were often applied in a slapdash manner, according to Yang, suggesting the artisans were under pressure to meet production targets. 

“You can imagine how the craftspeople worked: They just brushed a thin layer of glue onto the terra cotta and then applied the pigments,” says Yang. “This was another reason why the color often fell off.”

Other studies reveal a highly organized society with sophisticated production methods. An analysis of 40,000 bronze arrowheads suggests the warriors’ weapons were manufactured start-to-finish by autonomous workshops working in tandem — a process scholars have compared with the just-in-time production style honed in modern Japan. The grinding and polishing marks visible on the finished items, meanwhile, show that Qin housed the first society to employ industrial lathes.

Many more revelations surely await the archaeologists at Xi’an as they continue their work. Forty-five years on from its discovery, more than 90% of the ancient necropolis remains unexplored — including the chamber housing the emperor’s tomb.

According to Hou Ningbin, curator of the Emperor Qin Shi Huang Mausoleum Museum, it may take more than a century to finish uncovering the Terra-cotta Army. The researchers are not in a hurry.

“Unless we have the right technology and knowledge to protect these relics, we won’t excavate them,” says Hou.

Editor: Dominic Morgan.

(Header image: Unearthed terra-cotta figures after restoration in Xi’an, Shaanxi province. Courtesy of Rong Bo)