About $230 million is being spent to renovate the Statue of Liberty in New York harbor, but when the famous symbol of freedom has its coming out party July 4, one sign of aging will still be there.
Miss Liberty, who has greeted millions of immigrants of all races and nationalities, is turning black.
"There is nothing that can be done about it," said Robert Baboian, a corrosion expert and consultant to the U.S. Park Service on the restoration of the statue.
He said the darkening is a result of sustained exposure to man-made acids.
The two-year-long restoration project consists mainly of rebuilding and strengthening the internal structure that supports the statue, and replacing the severely weathered torch.
The problem of corrosion on the left side of the statue could not be resolved during the restoration because experts are only now beginning to fully understand it, Baboian said. Furthermore, any corrective measures would have been temporary at best because it is not feasible to replace the complex web of scaffolding around the statue every year or two to rework the surface.
The blackening results from acid deposits from air pollution dumped on the statue by the prevailing winds and rains from the direction of Manhattan Island, which is on the left side of the copper statue. The statue, a gift from France, is celebrating its centennial this year.
The size of the 305-foot statue and pedestal make it impractical to restore the green patina that has corroded away, he said.
Even if that were possible, he said, "it would just turn black again."
Stain Getting Worse
Park officials have been aware of the problem for years, but the darkening has accelerated in recent years, according to Baboian. And it is spreading.
"At this time it's a cosmetic problem," Baboian said in a telephone interview. "But if it gets more serious, and the patina erodes even farther, then the patina would no longer serve the purpose of a protective layer," thus exposing the copper skin to damage.
Baboian, who heads the Texas Instruments Corrosion Laboratory in Attleboro, Mass., has collected a series of photographs that show a surprising acceleration in the darkening in recent years.
He said he has taken samples of the patina "from all over the statue" to compare the black areas with the green, and has measured the thickness of both the patina and the copper skin of the statue.
Measuring the Thickness
The skin itself has not changed, but in the darkened areas he found that the patina is from one-half to one-tenth the thickness of the green areas.
The green patina consists of a copper sulfate compound called brochantite, he said. In the black areas, brochantite has deteriorated into a less stable chemical which "is susceptible to washing away and erosion."
"In the dark areas, the green is being worn away," exposing the darker sulfate below, he added. "That's the mechanism."
"As far as the restoration is concerned now," he said, "there is nothing that can be done. The scaffolding is coming down and even if you could clean the statue and artificially patine it, you would have to go back every year or two years and recoat it. That just isn't practical."
He said similar acid deposits--from rain, snow, fog and wind--are degrading statues around the world. On smaller monuments, it is possible to protect the finish by cleaning it and coating it, he said.
Not a Practical Step
"But it just isn't practical to use the techniques that preservationists are using elsewhere" on a statue the size of Miss Liberty, he added.
Baboian is studying one panel on the statue's upraised arm that has resisted the degradation. That panel stands out as a green island surrounded by the blackening arm, apparently because of something in the copper panel itself that resists the acid.
He said it is not clear just what that agent might be.
The changing complexion of the statue is not expected to have any impact on a four-day extravaganza on Independence Day.
"It won't be a problem," said Dave Moffit, superintendent of the monument.