Discover Copper Online

Spring 1994

Protecting Bronze Against Pollutants

Preserving the hundreds of outdoor bronze statues in The Big Apple is a never-ending process, according to Jonathan Kuhn, curator of monuments for New York City's Department of Parks.

"Attacked by car exhausts, bird droppings and acid rain, our statues require the year- round ministrations of two trained restoration crews," said Kuhn. First, they use high-pressure water - around 2,000 pounds per square inch (psi). That's followed by water mixed with detergent or abrasives to remove unsightly encrustations.

Nowadays, abrasives such as sand and fine glass beads have been largely replaced by pulverized walnut shells. One reason is because both sand and glass are harder than bronze, and they would remove some of the statues' surface along with the encrustations. The softer shells are blown against the encrustations with a sand-blasting machine at the comparatively low pressures of 40 to 80 psi.

Sand is still used occasionally to remove particularly heavy corrosion, says Alan Cox, who heads a four-person restoration crew for the Parks Department. As much as possible, the crew tries to retain the naturally weathered patinas on statues undergoing restoration.

Bronze statue of George Washington in Union Square, New York, before and after conservation (left). The statue, erected in 1856, was sculpted by Henry Kirke Brown.

Image courtesy of New York City Department of Parks & Recreation.

After the statues are cleaned, they are coated with synthetic microcrystalline wax (beeswax is too acidic) and /or with Incralac. This clear acrylic lacquer from the Stanchem Company, East Berlin, Connecticut, (and others) is made under license to the International Copper Association, New York City. One of its primary ingredients is benzotriazole, which protects copper against corrosion.

Depending on the conditions the statue is subject to, it is coated with from five to as many as eight coats of Incralac, said Mark Rabinowitz of the Central Park Conservancy. This private group devoted to preserving the park provides a three-person restoration crew to restore statues. For example, statues in fountains receive eight coats.

After the Incralac dries, the statues are coated with synthetic wax to bring out highlights, Rabinowitz added. The wax is applied either cold or after the statue has been warmed locally. Rabinowitz prefers warm waxing "because wax warmed by the hot surface penetrates into the pores of the bronze."

The lacquer coatings can last from two to ten years, depending on where the statue is located. If it's at a busy intersection, like the statue of Commodore Vanderbilt near Grand Central Terminal, the remaining lacquer may have to be removed and replaced after just a couple of years.

Because the solvents in Incralac are noxious, restoration crews wear respirators when they spray statues with the preservative, according to Rabinowitz. The crews also wear the respirators when they clean statues with powdered shells.

"Exposed bronze is not just at risk in crowded cities," Kuhn adds. "Because of acid rain, the bronze monuments at the rural site of the Battle of Gettysburg are also threatened." Bronze, however, withstands pollutants much better than pure zinc. So far, two century-old zinc statues in New York's parks have been recast in bronze because they deteriorated to the extent that arms were falling off.

The Parks Department buys Incralac in five-gallon drums, about $140.00 per drum. Quarts and 10-ounce aerosol cans are also available. Other Incralac manufacturers include: Conservation Materials, Sparks, Nevada; Custom Aerosol Packaging, Piqua, Ohio; and Mass & Waldstein Co., Carlstadt, New Jersey. The detergent used to remove encrustations is Orvus Paste, about $70.00 for a four-gallon case, and is purchased from a distributor called Talas in Manhattan.

Stanchem: 203/828-0571.

Conservation Materials: 702/331-0582.

Custom Aerosol Packaging: 800-237-6765.

Maas & Waldstein Co.: 201-933-1000.

Talas: 212/736-7744.

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