A formation of terracotta infantrymen unearthed during the previous excavations. [Cheng Qianjun]

"To dig or not to dig, that is the question," says Jiao Nanfeng, president of the Shaanxi Archaeology Institute in Xi'an.

Jiao's Shakespearean-like dilemma reflects his concern at the worldwide interest in the third excavation at the mausoleum.

The latest dig has aroused global fascination since it began 10 days ago but has also provoked concern about the risks to the integrity of the ancient site, regarded as one of the Eight Wonders of the World.

Jiao wants to play down expectations about the speed and extent of future discoveries. "We should all be aware that any preservation technology can't guarantee protection of a relic forever," he says.

"We can't afford any risk of damage to the world-class heritage site."

Located near Xi'an, capital of northwestern Shaanxi province, the terracotta army was built as part of a massive burial complex for Qin Shihuang (259-210BC), the first emperor of a united China.

Discovered in 1974 by a peasant digging a well, it has since become one of China's best-known images. An exhibition of 20 figures, and dozens of other artifacts from the tomb, broke all records on its 2007 tour of London and the United States.

The statues are symbols of an amazing ancient world that fires the imagination; its enormous scale is simply mind-boggling.

The scale of the task ahead is equally daunting. So far, only 2,000 statues have been unearthed but another 6,000 remained buried in the three pits under examination. The current dig is expected to take five years and double the quantity of unearthed statues in pit 1, the biggest pit, from 1,000 to 2000. At least 4,000 more will remain buried, possibly forever.

Moreover, the three known pits, which archaeologists have focused on for the past 35 years, represent only a small percentage of what really lies beneath. What the other sites - estimated to be around 600 - contain is still a mystery and some people at the museum have no great appetite for new excavation.

They would rather focus on the big picture.

Museum curator Wu Yongqi says the most pressing task for archaeologists is to establish definitively what they are dealing with, not just the three pits housing the terracotta warriors and horses, but the whole burial complex.

The museum is therefore surveying the entire burial complex to get a better idea of the pits' locations, through drilling and partial excavation.

Experts are excited by the new dig but believe the 6,000 statues unearthed so far were severely fire-damaged by a rival army shortly after Emperor Qin Shihuang's death.

The first formal excavation, from 1978 to 1984, produced 1,087 clay figures.

The second excavation, in 1985, lasted a year and was cut short for technical reasons, including the preservation of the remaining color on the warriors' bodies and armor.

Archaeologists began the third large-scale excavation of the terracotta warriors on June 13, after a halt of over 20 years. [Yuan Jingzhi and Niu Yixin]

Since then, there have been major technological advances, especially in color preservation, but the ongoing excavation has once again raised public concern over relic preservation.

"Most of the warriors and horses were colorfully painted when they were created," says Yuan Zhongyi, the museum's former curator and its first lead archaeologist. "The excavation of some richly colored clay figures in past excavations is strong proof."

However, Yuan says most of figures had lost their colors even before being excavated, suffering damage from fire and constant floods from the nearby Lishan Mountain as they slept underground for almost 2,200 years.

"The chance of excavating a well-preserved colorful warrior is very slim," says the 77-year-old expert. "For a few whose colors remain well preserved, they also risk losing their luster and turning an oxidized gray once exposed to the air. The lacquer would peel off and dissolve, the whole process takes only five to six minutes."

Since 1990, the museum has been working with experts from Germany on the protection of colored drawings on the warriors and horses.

So far, scientists have come up with a polyurethane liquid to preserve the lacquer and have succeeded in protecting several hundred warriors from further decay, according to Zhang Zhijun, deputy director of the museum's Protection Department.

Despite the success so far, no one can guarantee that the technology will be successful in the long run, says Yuan. "Only time can tell," he says.

Moreover, another challenge in the color preservation procedure - recovering the mineral paints dissolved in the soil and returning them to the surface of the clay figures - remains unsolved.

Gradual digging, not hasty excavation, is necessary

Jiao understands the public appetite for the new excavation to swell the museum's current collection, which has remained almost unchanged over the past 24 years. "But we have to think twice over the scale of excavation," he says.

"Personally I don't think there will be any unexpected discoveries. We know too much about the pit to be surprised."

However, Cao Wei, the museum's vice director, expects the excavation to complete the historical puzzle and provide a full understanding of our past.

Moreover, "in comparison with 20 years ago, we've made much progress in terms of preservation technology, it would be unreasonable to suspend excavation for ever".

Cao says the museum hopes to better understand how the tradition of burying clay figures evolved from the Qin Dynasty through the Han Dynasty.

Some clay figures have also been found in other tombs dating back to the Han Dynasty but are miniature, not life-sized as are those in Xi'an.

"How did that happen? We hope to find more clues," says Cao.

Yuan's 35-year-experience of excavating and preserving terracotta warriors and horses has warned him to always expect new problems with any archaeological discovery.

He recalls many challenges the terracotta army has faced since its discovery: erosion by rain and snow before the massive plane hanger-like shed was completed; mold because of humidity, and decay from exposure to coal dust produced by local industry.

Yuan was the first archaeologist sent to the site in 1974 and thought "it would only take a couple of weeks to excavate."

"But here we are 35 years later, still at work," he says with a chuckle.

(China Daily June 23, 2009)