Zhang Tao draws unwanted attention at work every now and then. As he stands in front of historical buildings in Beijing, armed with a sophisticated drilling tool that may look like a weapon to untrained eyes, his seemingly public vandalism is interrupted and questioned by well-intentioned passersby.

"I am actually using a micro-drilling impedance analyzer, which helps me test the health of a building," the 39-year-old restorer explains to concerned onlookers.

"It is like drawing blood (from old structures) for tests. This tool is used abroad to determine the growth of trees," he further clarifies.

Most historical buildings in the city are made of wood, and Zhang and his team are entrusted with their restoration. With the misunderstandings quickly cleared up, Zhang gets down to examining the buildings, determined to preserve what is left, a job he has been dedicated to since 2005.

A few light taps on a pillar with his small hammer and Zhang can diagnose if something's wrong inside the woodwork. He zooms in with the impedance analyzer to detect decay and cavities in the timber. The technology, which is largely used by arborists or tree surgeons, can tell stages of rot and identify hollow areas and cracks.

As the micro-drill bores into the wood, the resistance of the timber changes the needle's rotation and speed. These variations are translated into a graph, which looks a lot like an electrocardiogram, or ECG, printout.

"Maintenance techniques followed by our predecessors are passed on and inherited, and modern technology can help us pinpoint problems in these historical structures," says Zhang. "Many of these buildings are more than 400 years old. They look sturdy on the outside, but may be decaying inside. If they collapse someday, it will be a great loss for all of us."

One of his most memorable experiences, he recalls, was detecting a 50-centimeter area of rot at the bottom of a pillar at the city's Yonghegong Lama Temple. "I was so glad I found it in the nick of time," he says.

Zhang has been working at an archaeological institute under the auspices of Beijing Cultural Heritage Bureau since graduation. The job has made him a regular at major historical sites across the capital, including the Temple of Heaven and Beihai Park. He is among the handful of protectors of several ancient structures in Beijing, which has more than 3,800 immovable sites boasting a history of over 3,000 years.

Zhang grew up in a neighborhood right next to the Temple of Heaven and is familiar with the grand architecture. "The imperial complex of buildings is indeed magnificent and I get to see it every day," he exclaims.

His knack for experiments and good science scores in senior middle school paved the way for a bachelor's degree in applied chemistry from Shandong University.

"After college, I received job offers from institutes for food quality testing, water analysis and ancient architecture restoration," he says.

Then, the archaeology institute was in charge of restoring the Hall of Prayer for Good Harvests at the Temple of Heaven. So, he took the restoration offer and started working on historical stone structures.

Although stones are much sturdier than wood or brick, they are susceptible to weather and erosion. "Sulfur and nitrogen oxides in the air turn into acid when it rains, and acid is bad for stone relics," he says.

Soluble underground salts, too, can cause problems when rainwater seeps into the soil and evaporation coats the stone surfaces with contaminants. "They can accelerate the accumulation of pollutants," he says.

When he first arrived at the site, he had to analyze a stone relic's constituents and the reasons behind its erosion before applying any protective material, including those that strengthen the foundation and shield the structure from the sun's ultraviolet rays.

He later obtained a master's degree in materials science and engineering from Beijing University of Chemical Technology. With time, Zhang became increasingly aware of the importance of testing and diagnosis. "It is like the first step in a hospital, also the most important one, before the course of medication can be determined and administered," he says. He found diagnosis was of great help in choosing protective materials. "If the weathering is serious, you choose materials with a strong potency," he says.

By 2008, Zhang had started focusing his attention on preventive tests for historical buildings, especially those made of wood. "I saw there was nothing new in the field of testing and protecting wooden structures," he recalls.

Most methods were ancient, and efforts were concentrated on structures that were already run-down. Zhang thought outside the box and decided that, maybe, prevention would be better than cure. He first introduced the micro-drilling impedance analyzer. "The detective needle offers a glimpse inside the woodwork. The needle is the size of natural cracks and does not cause damage," he says.

Since the tool was largely used in forestry at that time, Zhang worked hard to establish suitable parameters to determine the health of immovable historical structures. Today, he heads the scientific protection division of the Beijing archaeology institute. Along with his team, he has developed five national patents for preventive testing and restoration of historical buildings. They comply with industrial standards.

Zhang says he has seen significant development of preventive testing methods across the country over the past six years. Last year, the General Office of the State Council issued a plan for cultural relic protection and scientific innovation for the 2021-25 period. The plan categorically calls for more capacity for "preventive protection".

To make things easier, a greater number of science graduates are joining the field, says Zhang. One needs to acquire interdisciplinary knowledge for the job, including the architectural styles of various historical periods. It is about knowing more than just chemistry and physics, he adds.

Giving an old building a complete health check is no mean feat, Zhang warns. "Sometimes, we have to climb onto a roof, where dirt is layered so thick that even the best face masks cannot protect us," he says.

So far, Zhang has joined 10 restoration and design programs at major national cultural relic sites. He has also conducted over 40 safety tests of historical buildings in Beijing. Recently, he and his team finished the overall safety test of an architectural cluster at Xiaoxitian in Haidian district. The project was commissioned by the Beihai Park authorities.

"It is necessary to do preceding work for preventive protection and research-based repair," says Ge Huaizhong, deputy head of the Beijing institute. All the data from the tests will be studied to draft future restoration methods, he adds.

As of now, most ancient structures in China have been repaired on an emergency basis. The next stage is preventive protection, Ge says.

Zhang believes preventive protection will not just keep accidents from happening, but will save time and money. He says: "Our country has a great number of historical buildings. If we could save them all, how great would that be!"