Grant could help make tarnish history

University of Maryland and the Walters work to develop a technique that slows blackening on silver to a nearly undetectable rate

  • Materials scientists and conservators at the Walters Art Museum and the A. James Clark School of Engineering at the University of Maryland see Antoine-Louis Barye's "Walking Lion Striding Lion" as a prime candidate to receive a new, nanometers-thick coating.
Materials scientists and conservators at the Walters Art Museum… (Walters Art Museum )
November 01, 2010|By Mary Carole McCauley, The Baltimore Sun

Don't throw away the blackening silver tea service that you inherited just yet. A new microscopic coating being developed by the University of Maryland and the Walters Art Museum might get you sparkling sterling — without hours of elbow grease.

The university's A. James Clark School of Engineering and the Walters have received a three-year grant to develop a technique to slow the tarnishing of silver objects to a nearly undetectable rate.

The $500,000 project, which would involve refining a process called "atomic layer deposition," is being designed primarily to benefit museums such as the Walters, which have extensive collections of ancient silver statues, jewelry and drinking vessels.

But somewhere down the road — possibly far, far down the road — that same process could have everyday applications. Homemakers might start using their prized silverware on days other than Thanksgiving. Tiffany & Co. could apply the coating to its line of sterling jewelry to persuade consumers to splurge on a costly bangle.

"I can easily imagine that there could be an application to the jewelry industry," said Ray Phaneuf, a professor of materials science for the engineering school. "Silver is famous for tarnishing, polishing an intricate necklace can be a nightmare and polishing removes some of the silver metal. There's a lot of evidence that aluminum oxide may be a good choice for standing up to wear and tear. It's already used on bicycle rims, and they're constantly being abraded by the pedals."

The problem of tarnish is most severe for museums, which have large collections of fragile and irreplaceable silver items. Currently, officials at these institutions have two ways of protecting their silver, neither of them ideal: They can encase the object in Plexiglas, which slightly distorts its appearance and robs it of some of its three-dimensional beauty or they can glaze the object with lacquer, which is difficult, time-consuming, expensive and lasts perhaps 10 years. And every time the old lacquer is removed and a replacement coat is applied, it risks damaging the artifact.

"It's a huge problem," Phaneuf said. "We've inherited a giant store of precious silver relics from past cultures, and we really need to figure out how to preserve them for future generations."

The grant, from the National Science Foundation, allocates $330,000 to the engineering school and $80,000 to the Walters. In addition, the University of Maryland is kicking in $90,000 over three years to pay the salary of the graduate student who will conduct the chemical experiments.

The project builds on a technique discovered in the mid-1980s and later modified by a Finnish company for use in manufacturing semiconductors and solar chips. In essence, atomic layer deposition involves covering an object with a metal film no thicker than a single atom.

The process takes place in a "clean room" that is free of contaminants, pollutants and bacteria. The object to be treated is placed inside a vacuum chamber and exposed to a series of gases, which allows scientists to build up a protective oxide one layer at a time.

The technique "gives us an exquisite level of control, literally at the atomic level," Phaneuf says. "It's an effective strategy to reduce corrosion that preserves artifact appearance and composition."

In the case of silver, the chemical shield is composed of layers of aluminum oxide interspersed with occasional layers of titanium. Aluminum oxide is the same material that puts the grit in emory boards, and it has a strong resistance to sulfur. Alternating this chemical with titanium creates the chemical version of a corn maze that molecules of sulfur have difficulty penetrating.

Though the aluminum oxide compound, too, would eventually wear off, Phaneuf estimated that one application could last for nearly a century.

Best of all, according to Glenn Gates, a conservation scientist for the Walters, the compound can be applied in layers thin enough so they can't be seen and won't distort the appearance of the treasure they are protecting.

"The coating is invisible to the human eye because it is thinner than the wavelength of light," Gates said.

The Metropolitan Museum of New York conducted preliminary experiments with atomic layer depositing in 2008 at a thickness of seven nanometers, or roughly seven/billionths of a meter.

"At that thickness, aluminum oxide worked about as well as standard lacquer," Gates said. "But they didn't carry their experiments forward to determine the thickness that would strike the best balance between protecting the surface and optical clarity."

Chances are, a process that will save a priceless artwork will be even more effective for everyday objects, he said. Because the process conforms exactly to the shape of the object that it is coating, it is especially suitable for pieces of silver that are carved, engraved and otherwise heavily worked.

Gates has volunteered some pieces from his private collection — some Morgan silver dollars and 19th-century demitasse spoons — for the experiments.

If all goes well, the process eventually will be applied to artifacts in the Walters' collection, such as Antoine Louis Barye's 1865 piece, "Walking Lion." The sculpture has been lacquered twice since 1949, but in each case, the synthetic coating deteriorated. The sculpture currently is kept in an exhibition case.

"The famous walking lion has a complex shape," Phaneuf said. "It is difficult to coat with lacquer, and it is just the kind of work that might benefit from atomic layer deposition. If we can some day take it out of the box and put in on a pedestal, visitors to the museum will really be able to see and appreciate it."

mary.mccauley@baltsun.com

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