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KEEPING GRANDMA’S silver tea service nice and bright has always been a pain—until now. Researchers in the University of Maryland’s Department of Materials Science and Engineering have applied nanoscience to the problem with a process called atomic layer deposition. ALD is a common technique in the semiconductor industry, but this time even The Walters Art Museum in Baltimore is interested.
The 29 March 2013 issue of Science, the magazine of the American Association for the Advancement of Science, suggested this item (see http://goo.gl/CLkCB). Here, I offer a summary with some amplification of my own.
The world of nanotechnology is an amazing one. One nanometer (abbreviation, 1 nm) is a billionth of a meter, 1 x 10-9 m. An analogy: 1 nm is to a meter (39.37 inches) as a marble is to the size of Earth. Atoms have diameters in the range of 0.1 to 0.5 nm. Hence, in fact, you can think of nanotechnology as pushing individual atoms around for one reason or another.
ALD does just this. University of Maryland researchers coated silver pieces with multiple layers of protective aluminum oxide, each layer only one atom thick. Because of each layer’s thinness, nooks and crannies of these samples could be reached without compromising inherent color or texture.
By contrast, conventional polishing can remove silver along with the sulfide tarnish. Do it often enough with Grandma’s plated pieces, and you’ll rub through the plating.
Avoiding this degradation, museums typically apply thin coats of lacquer or similar nitrocellulose compounds to their artifacts, with a discoloration of the surface being one shortcoming. Another is the life of the coating, perhaps no more than 20 years.
A report to the American Physical Society (http://goo.gl/Jmjyw) gives details of the University of Maryland investigators’ work. To compare ALD coatings of 5-100 nm with the current nitrocellulose strategy, they devised accelerated tarnishing in an “aging chamber” subjecting the test pieces to hydrogen sulfide. Advanced techniques—reflectance spectroscopy and an integrated sphere spectrophotometer—were used to measure thickness of tarnish layers and to compare lifetimes of the different preservation strategies.
Researchers found contrasting failure modes of ALD versus lacquer. Insufficient layers of ALD could have pinholes in their films; the lacquer would fail because of non-uniformity of thickness. However, a thin ALD was still found to protect the silver five times longer than even a 1000-nm nitrocellulose film.
The next step is testing ALD on actual museum artifacts, with The Walters Art Museum in Baltimore contributing an intricately figured 15th Century Spanish cross. Researchers note there’s no danger to the artifact; ALD is completely reversible.
Nanoparticles can also be removed—if necessary, atom by atom. ds
© Dennis Simanaitis, SimanaitisSays.com, 2013