A statue of a different color.

A Statue of a Different Color

For almost a century, wind and rain sweeping across New York harbor have buffeted the Statue of Liberty. Faced with this constant assault, the statue's gleaming copper skin first dulled to a brownish color, then blackened before finally developing a sturdy green coat that has lasted for decades. But the statue's green patina, washed by acid rain, may be changing color again. A close look reveals a patchwork of darker areas that now poke through her weathered green coat.

Attention focused on these blackened areas about a year ago, after an aluminum scaffold was erected around the statue in preparation for an extensive renovation. This allowed corrosion specialist Robert Baboian, head of the electrochemical and corrosion laboratory at Texas Instruments, Inc., in Attleboro, Mass., to collect and analyze scrapings from small areas of the statue's surface.

Baboian's results suggest that acid rain may be converting a stable form of basic copper sulfate called brochantite, CuSO4 3Cu(OH)2, which formed naturally on the statue's copper surface, into a less stable form of copper sulfate called antlerite, CuSO4 2Cu(OH)2. With a slightly different composition and crystal structure, antlerite dissolves more readily in water and is more susceptible to wind erosion.

Thus, the antlerite-loaded green patina may be washing away, especially in areas exposed to the prevailing winds. This uncovers the underlying black layer, which consists mainly of copper sulfide and copper oxide. "We feel that acid rain is affecting at least the aesthetic properties of the statue,' says Baboian.

E. Blaine Cliver, chief of the National Park Service's preservation center responsible for the Statue of Liberty restoration, is a little more cautious about implicating acid rain. "A lot of these dark areas may always have been dark,' he says. "We're in the process now of having the patina mapped over time so we know what changes have occurred.' This involves careful computer analysis of black and white photographs of the statue as she appeared during her first century.

Last month, Cliver and Baboian also completed measurements of the thickness of the statue's skin to see if the copper's rate of corrosion was changing. Using an ultrasonic caliper, they took enough readings to conclude that no one side of the statue was weathering more than any other, and that the copper in green and dark areas had about the same thickness. Overall, the statue's skin, originally 96 mils (or 0.096 inch) thick, is only about 4 mils thinner than it was 100 years ago.

Baboian, with these preliminary results, guesses that acid deposition has not yet affected the copper's corrosion rate. Although the green patina may be washing away in some places, the copper is actually protected by its thin but tenacious and very compact black layer.

On the other hand, if the black layer starts to wear away faster than it forms at the underlying copper surface, Baboian says, "then we would see a drastic reduction in the thickness of the black, color changes and an increased corrosion rate.' The National Park Service plans to monitor the skin's thickness at selected points to track these corrosion rates.

Nothing can be done about the present color of the statue, says Cliver, because any skin treatment would have to be reapplied periodically. The statue will not even be washed using a detergent, as originally planned, because of fears of disturbing or damaging the patina.

Work on the Statue of Liberty is also generating some intriguing mysteries. On the arm holding up the torch, for example, is a single green copper plate surrounded by dark panels. Says Baboian, "The actual composition of the copper varies from one panel to another on the statue.' Analyzing and comparing these adjacent panels, he says, may suggest possible acid-rain-resistant formulations for copper roofing materials.