EVER since Egyptian stone workers propped up the damaged right arm of the monumental statue of Ramses II with crude stone blocks in the 2nd millennium before Christ, civilizations have sought ways to protect their valued monuments. But only in the past 20 years has a ``science'' of stone conservation emerged as physicists, chemists, biologists, and geologists have joined up with art historians and archaeologists to save stone monuments worn with age and damaged by modern pollutants.
In the early 1980s, four of Rome's most important imperial monuments were targeted for conservation care: the Arches of Constantine and Septimius Severus and the Columns of Trajan and Marcus Aurelius.
All four, now shrouded in protective green netting, were built as ``triumphal'' monuments by the Roman emperors whose names they bear, from Trajan (born AD 53 and emperor from 98 to 117), under whom the empire reached its largest extent, to Constantine (280-337), under whom Christianity became the religion of the Empire.
The hundreds of carved marble scenes depicting the emperors' conquest of Thracians and other foreign tribes - their heads shown staked on posts and their chests pierced with the imperial cavalry's lances - were meant to convey both Rome's unrivaled political power and its engineering and sculpting skills.
But after more than 1,600 years, the monuments themselves have been ``conquered'' - beaten down by exposure to diesel fuel, home heating emissions, pelting rain, and earthquakes.
``You can't imagine what it was like when we first climbed Marcus Aurelius' Column six years ago,'' said Cinzia Conti, a conservator. ``We didn't have any idea what to do.'' (See related story, next page.)
Since 1981, conservation teams numbering up to 50 members have analyzed the thick black incrustations, lesions, pockmarks, and powdery, disintegrating surfaces, and worked to restore the stone with laboratory-learned conservation techniques, centimeter by centimeter.
The language of conservators reveals their increasingly technical approach.
First, the stone is ``preconsolidated.'' This means that particularly weak areas that cannot even stand up to cleaning are coated (by brush or cotton wool) with a chemical solution that penetrates the stone and ``consolidates'' it.
Second, the stone is ``nebulized'' - delicately showered (under a plastic covering) with air-compressed water propelled from hissing nozzles. This cleans the stone with a sheet of steady mist, almost like a sauna.
The stone is then picked over and cleaned with micropneumatic drills, tiny rotating metal drills, scrapers like those used by dentists to clean teeth, and simple toothbrushes. A neutral stuccolike mixture of lime, marble, and sand powder is applied to large lesions.
Finally, the stone is ``consolidated'' with another deep-penetrating chemical solution, and a final protective water-resistant covering may be applied.
But despite the high-tech advances in stone conservation, conservators of the monuments, who will finish their work in the next two years, do not see their efforts as eternal solutions. ``We are just trying to keep things on a kind of holding pattern,'' said one.
Since the organic chemical compounds used to consolidate the stone will start to wear off within 10 years, they see regular maintenance, with checkups every three to five year, as critical.