CAIRO — Its enigmatic face has been scarred by the hands of time and men, the sharp features of its former beauty seem to fade with each passing year. Yet this is not just any aging sculpture, but perhaps one of the world's most famous: The sphinx is gradually deteriorating. And like many of the world's ancient monuments, it sits in a nation so burdened by its own poverty that there isn't enough to go around for the populace, let alone for historical icons.
Though pollution, neglect, and looting have taken their toll on the sphinx and the nearby Giza pyramids, in many cases the legacies of the ancient world are dying a slow death by natural causes. In other parts of Egypt, the monuments emit salt and can "sweat" themselves into oblivion. In nearby Jordan, many of the magnificent reliefs from the Nabatean city of Petra are fading away from wind erosion, and some may be imperceptible in 100 years.
Now late-20th-century AD technology has offered a new technique that can help slow the disintegration of what the 20th-century BC left behind. By impregnating or injecting sandstone with silicon solutions, the natural stone is strengthened and chemically protected from further decomposition.
Despite such graphic descriptions, conjuring images of giant needles, application is commonly done by spraying monuments with the solution, or, for smaller statues, by dipping them.
In the process, pioneered by the Wacker-Chemie company in Munich, Germany, the solution forms an invisible protection against moisture and other elements. In slightly more technical terms, the silicon forms an inorganic, highly weather-resistant cement. Based on a formula of silicic acid ethyl ester, its substance reacts with atmospheric humidity to form silicic acid gel, or natural silica.
"This is able to restore cohesive strength to some deteriorated sandstones, when applied in sufficient quantities, and in appropriate conditions," says John Stewart, a conservationist with the British Museum in London who has overseen several preservation projects in Egypt.
A boon for Egyptologists
For specialists, the silicon process is helping preserve ancient Egyptian faades and engravings that were becoming illegible and imperceptible. Peter Dorman, an Egyptologist with the University of Chicago's Oriental Institute, says the silicon process is now being piloted at Luxor Temple, a structure first finished under the period of King Tutankhamen.
"The problem is that the temples are in contact with groundwater. There is visible salt damage," he says. "Salt that occurs in natural stone meets groundwater and destroys the exterior relief and breaks down the material. This [silicon process] tries to arrest the spread of the salts."
With that accomplished, his team can move on with their main task of documentation through photography and precise drawings. "We're convinced that now is the best time to capture these reliefs because they may not be there in a few years," Professor Dorman says. In addition to the water, he points to pollution, tourism, and overpopulation as the main culprits for the deterioration.
High cost of preservation
An earlier method aimed at further staving the decay of such structures included the application of a layer of tiny stones to the faade. That proved less effective than silicon injections. "Adding a layer of stones ... [is] just a stop-gap measure," Dorman says. "It just covers up the original, but you will still have water seepage and it [the structure] will deteriorate."
To be sure, the new process has its limitations. Application is slow and can only be completed during cool weather, ruling out the many hot months of the Middle East calendar. Moreover, it is costly.
For example, the silicon coating of just one temple in Luxor - under way this past winter - was an eight-month project that cost the University of Chicago and a number of private endowments about $140,000, Dorman says.
"It would be great to take a fire hose and coat the pyramids in the stuff, but this process is expensive," he says. That might rule out fortifying the Sphinx.
Petra, in Jordan, is another place where the process is being used. The famed, rose-colored rock city built by Arab Nabateans some 2,000 years ago served as a protected trading post during the Roman Empire. When it was "rediscovered" by a Swiss explorer in 1812, archaeologists began documenting Greek, Roman, Egyptian, and Assyrian influences on the city.
Many of Petra's relief sculptures are already so visibly worn that they look almost melted away. Here, too, a silicon-injection process is being piloted to keep the reliefs from wearing down any further. In Petra, where wind erosion is more of a problem than water, preservationists are using a different kind of stone strengthener developed by Wacker-Chemie.
"Our consolidants or strengtheners form a silicon-dioxide chain, a new binder for material for sandstone, brick, and other kinds of stone," says Helmut Weber, a scientist at Wacker-Chemie. "This silicon dioxide forms a new quality of stone so you get the old [original] strength of the materials."
But that is still no promise of immortality for the legacies of the ancient world, because the strengthening effect may be relatively short. Though it can be reapplied to achieve the same effect, the process is only known to have a durability of 20 or 30 years. "We cannot say it lasts more than that," says Professor Weber, "because we've only had the product for 25 years."