In the 1850s, Eugene Viollet-le-Duc wrote that a historical monument should be rebuilt as it was meant to be, even if it was never finished, and could be carefully modified for modern utilization. Based on those principles, he added a sacristy to Notre-Dame in Paris in a style that matched the cathedral completed in 1250.
If his approach were followed today, restorers of Pre Rup temple in Angkor Archaeological Park would build the sixth lower tower that was planned but never erected. This would turn the monument into the architect’s dream for it, and not the way the Khmer people saw and used it a millennium ago.
“Today, no one would allow restoration as Viollet-le-Duc did,” said Azedine Beschaouch, scientific secretary of the International Coordinating Committee of Angkor. Nowadays, “restoration Viollet-le-Duc style” means excessive work; and yet, the Frenchman was viewed as a great architect and restorer in his time, Beschaouch said.
“This is part of the history of restoration techniques,” he said. “Things evolve.”
For example, in the late 1980s, the Archaeological Survey of India used a chemical to clean the stones at Angkor Wat. “This [product] was considered a highly advanced technique, and accepted by everyone in the field,” Beschaouch said.
“Then 20 years later, people found out that this product caused something unexpected—a chemical reaction that, after 10 or 15 or 20 years, blackens or yellows stones, changes their color,” he said.
“One of the biggest problems is that we don’t know the continuous behavior of material over 20, 30 or 100 years,” said Hans Leisen, project director for the German Apsara Conservation project. Restorers try to simulate conditions as they evolve over years, but numerous questions remain as to why a material may deteriorate and how long it will take it to do so, he said.
In the 1950s, people used acrylic resin and poly-resin; they ended up deteriorating rather fast, which has left the work to be redone, Leisen said. “This is the reason why, more and more, people try to avoid the use of any chemical product,” Beschaouch said.
So, oddly enough, restoration experts now conduct increasingly complex and advanced scientific studies in order to go back to ancient methods and material.
For instance, the mortar used at Pre Rup—a mountain temple built in 961—is based on an analysis of Khmer builders’ recipe. It consists of rock lime, sand, organic clay, brick powder and water, said Valter Santoro, the engineer in charge of the monument’s restoration project.
It also contains “cow glue,” Santoro’s polite way of describing cow dung. Cement, used by other generations of restorers, was not an option, he said. But it took three years of tests and studies to ascertain what would be right for the lower towers of the brick, sandstone and laterite monument.
Restoration teams face major challenges. They need to strengthen the structure of monuments weakened by vegetation and rain. They also must find ways to do so with the original material.
Nearly 100 years of restoration projects at Angkor has shown the danger of using today’s material to consolidate yesterday’s structures.
“We have discovered that stone itself has a history,” Beschaouch said. “If you put a stone from today—and some have a composition that retains atmospheric polluting agents—with a stone from 10 centuries ago, the one from 10 centuries ago contains no chemical agent, barred natural ones. You put them together, and there is a reaction as if you were putting together two elements that are antinomic—one rejects the other.”
Too often, people refer to stone as an inanimate object, he said. Not only does it have a history and age, he said, “but it has a life as one of nature’s physical and chemical element—a life that us, human beings, have no right to forget.”
Stones at Angkor had suffered over centuries. In sandstone, clay minerals swell in humidity and shrink in dry weather, eventually causing grains in the stone to disconnect, Leisen said. This, along with salt and bat droppings, led to flaking and scaling of apsara wall sculptures at Angkor Wat, he said.
The German team, which started working on the apsaras in 1995, has been testing remedies that include ethyl silicate mortar, which is chemically equal to quartz contained in the sculptures’ sandstone, and medical gauze for poultice designed to reduce salt content.
Consolidating a monument with original stones is no easy task. Of the 2,000 blocks dismantled so far at Prasat Suor Prat towers, 20 percent of them are broken, said Akazawa Yasushi, deputy director of the Japanese Government Team for Safeguarding Angkor, which is restoring the 12 towers built toward the end of the 12th century in Angkor Thom’s royal square.
Most of the towers dangerously lean backward, water from the nearby pond having seeped through the compacted sand underneath them, he said. Blocks of laterite, which is a light and porous material, have greatly deteriorated, some of them close to being crushed.
JSA has been testing various ways to repair laterite—steel bars and plates, polymer cement mortar, epoxy resin, lime mortar—and continues to monitor the stone’s response to treatment.
When there is no other way but to replace a stone, a new block is brought in from a quarry in Kompong Kdei in Siem Reap province where the composition of laterite is the same as stones used for the towers, Akazawa said. However, JSA experts don’t know whether the new blocks will react as the old ones did over long periods, and that concerns them.
The same approach—looking for solutions that will not traumatize the stones—is used to consolidate structures.
When restoration was launched at Pre Rup in 1995, the lower towers were cracked and one of them was about to topple. This was due to a number of factors—1,000 years of being exposed to the elements, water seeping under the foundations and lack of maintenance. The brick towers stand on a platform of laterite and sandstone that was decaying at different paces, Santoro said.
The restoration project, which is funded by Italy through the UN Educational, Scientific and Cultural Organization, began with a series of studies. Results showed that the towers moved 2 to 3 millimeters, and that temperature inside the structures varied by as much as 15 degrees Celsius, Santoro said.
The solution was to build boxes of reinforced concrete inside the towers, below the floor, to stabilize them, he said. Removing the sandstone blocks and putting them back after the concrete had set was quite difficult, said Heng Dara, a Cambodian engineer who has worked at Pre Rup since 1999. This took about six months per tower, he said.
The next step was to strengthen the towers by inserting steel bars into the thickness of the walls to make them invisible and to keep interference with the original bricks to a minimum, Santoro said.
A similar technique now is being used for the five upper towers, whose structural problems range from very small cracks to missing brick portions. Standing on top of Pre Rup’s 12-meter laterite pyramid, they have especially suffered from the rain and strong winds, said Kong Kanty, a Cambodian architect who joined the Pre Rup restoration team in 1995.
This even caused a lintel weighing close to 1.5 tons to fall off the northeast upper tower; it will be reinstalled with reinforced anchors, he said.
Looking at a monument such as Pre Rup, one should remember that this once was a structure in which all elements were in harmony, Beschaouch said. It needs to be cured as gently as possible close to its original state, and not just viewed as a thing to be fixed, he said.
In restoration techniques, two excesses should be avoided, Beschaouch said. First, extreme criticism. “One must not forget the time factor. Before Louis Pasteur, people did not know what rabies was, and people could die from it. We can’t say that doctors of the time were not good since the disease had not yet been discovered.”
The second excess is dogmatism, as in rejecting new techniques and keeping one’s own no matter what, Beschaouch said. “One must have the courage to change methods,” he said.