Researchers at the University of Maryland are attempting to solve the age-old problem of tarnished silver by developing a nanoscale coating method. |
Anyone
who’s ever polished silver knows that keeping the tarnish at bay is
never ending work. But, you may not know that polishing also rubs away
some of the precious metal, whether it’s your grandmother’s silver bowl
or a 19th century museum treasure.
“We’re
always looking for some kind of barrier that will protect the surface
so we don’t have to keep polishing it,” says Terry Drayman-Weisser,
director of conservation and technical research at the Walters Art
Museum in Baltimore.
Twenty
miles from the museum, materials scientist Ray Phaneuf and his team at
the University of Maryland are working on a small solution to this big
problem. With support from the National Science Foundation (NSF),
they’re producing and testing a protective coating so thin, you can’t
see it with the naked eye.
“The
method that we use to apply it is called atomic layer deposition. So,
literally, we’re able to control the thickness of the film at a
sub-nanometer level,” explains Phaneuf.
Using
a special reactor inside a clean room, they apply nanometer thick films
of aluminum oxide to a sample silver wafer about the size of a silver
dollar. Phaneuf says the films conform to the recesses and protrusions
of the silver, creating a protective barrier.
Art
conservators say atomic layer deposition, or ALD, will have to pass
rigorous testing before they use it to protect irreplaceable treasures.
At
the lab, the coating is put through a series of tests. Using a
spectrometer the research team measures how light reflects off the
surface of a test wafer, and how the ALD coating affects the wafer’s
color.
Another
test measures how quickly sulfur penetrates the coated wafer. Sulfur is
what tarnishes silver. The test will help determine how many layers of
coating will be needed to keep the silver shiny. In another controlled
chamber, the team heats a coated wafer to speed up the tarnishing.
Phaneuf says this helps scientists figure out how long a barrier will
last.
“Part
of the challenge is to determine what the optimal thickness is that
keeps sulfur off the silver surface. Eventually, thermodynamics tells us
that the sulfur will diffuse through any layer we put down. The denser
the layer, the slower the diffusion,” explains Phaneuf. “So we’ll start
with films that may be a few nanometers thick and investigate the
efficacy of these films all the way out to maybe a few hundred
nanometers. If we can increase the lifetime of these films to a century,
you may not need to do this very often.”
Art
conservators won’t give ALD a thumbs-up until they can show that it
works better than the lacquers they are using now, which have to be
reapplied every decade or two. The conservators also will have to be
able to remove the coating without damaging the piece.
“When
it comes to art objects, the less treatment the better,” says Glenn
Gates, a scientist at the Walters Art Museum. “The standard treatments
that use lacquers or nitrous cellulose coatings can give off a plastic
look. The ALD coating is very, very thin, and orders of magnitude
thinner than the wavelength of light; the idea being that it’s going to
impact the aesthetic presentation of the object much less than a thick
organic lacquer coating that we generally apply these days.”
If
ALD proves a shining success, silver works of art will remain at their
best for future generations to enjoy. And for many of us, it may mean
never polishing silver again.
