Managed and Operated by Sandia Corporation
ALBUQUERQUE, NM LIVERMORE, CA
News Release MEDIA RELATIONS DEPARTMENT, MS 0167
ALBUQUERQUE, NEW MEXICO 87185-0167
PHONE: (505) 844-8066 FAX: (505) 844-6367
FOR IMMEDIATE RELEASE
January 21, 1997
NY Metropolitan Museum of Art, Sandia work together
to preserve statues and infrastructure
ALBUQUERQUE, N.M. Marble statues, the nation's bridges and tunnels, and
the building called the Pentagon share a common problem: limestone ingredients
make them vulnerable to pitting from acid rain and spalling from freeze-thaw
cycles that may follow.
The same problems can afflict concrete blocks that entomb radioactive
components from decommissioned commercial nuclear reactors and discards of the U.S. military.
For these reasons, New York's Metropolitan Museum of Art, which collects
statuary, has teamed with scientists from Sandia National Laboratories,
concerned with preserving the nation's security, to produce an inorganic coating
that increases by a factor of ten the longevity of powdered calcite -- the basic
component of limestone -- when calcite is submerged in a solution similar to
mildly acid rain.
The Met provided Sandia a variety of limestones as well as technical
information about statue preservation.
The Getty Conservation Institute in Marina Del Rey, Calif. helped establish
criteria for the coating to be acceptable to the art community.
Sandia developed the coating, which is chemically similar to glass, with
funds from its Laboratory-Directed Research and Development program, which
finances speculative defense-related research. Because the coating is inorganic,
it does not react as readily with acid rain as organic-based materials.
Curiously, the coating is not intended to be a sealant that forms a water-
tight skin around the statue or cement. Instead, the coating forms within the
pores of the masonry and strengthens it.
"These are not hydrophobic coatings," says George Segan Wheeler, a research director at the Metropolitan Museum of Art. "They would be seen, and look terrible.
"This is more like re-creating cement that had been eaten away. It's not
like putting Thompson's Water Seal on your deck. The coating is not a water
repellent but a consolidant."
According to Sandia researcher Kathryn Nagy, "Water can always seep into a
statue or infrastructure component. Ordinarily, it migrates to the surface and
evaporates. A waterproof coating wouldn't let that happen." Trapping water
within masonry can lead to severe damage due to freeze-thaw effects, chemical
reactions, and bacterial and fungal growth.
However, for simpler cement objects created only to store radioactive
materials, a more hydrophobic form of the coating could "create another barrier,
like the Russian Matrushka dolls that fit one inside the other, to slow leaching
of radioactive wastes," said Sandia scientist Randy Cygan.
A brief history of lime
Difficulties in protecting limestone-based materials (marble is compressed
limestone, and cements are made with lime) have been recognized since the late
1800s. Organic coatings such as resins and epoxies bond well to limestone but
break down in sunlight and water. Inorganic coatings resist sunlight and water
but bond well only to quartz, feldspar and silicate rocks. Unfortunately for
artifact preservation, the majority of statuary is made of limestone and marble.
The problem has been to create an inorganic coating that effectively bonds
to these limestone-based (more technically, carbonate mineral-based) materials.
While several commercial coatings are available on the market, the museum
and defense researchers thought a better product could be produced through use
of modern technology rather than by the trial-and-error testing done formerly of
necessity.
The coating in development at Sandia is made of a two-sided molecule called
a "passivant" that better couples with its organic side to the carbonate
structure, and its inorganic side to an overlying inorganic protective layer
involving a silicon-based polymer.
When she announced the Sandia results at a meeting of the Materials Research
Society in Boston in early December, Nagy showed why scientific observation is
sometimes superior to so-called common sense. She said that contrary to what
one might expect, the attaching molecule, or passivant, "that binds least
strongly gave the best protection. We think that those molecules binding
strongly are actually enhancing the dissolution rate."
How modern technology attacks the problem
Computer modeling saves time by eliminating inefficient molecular
arrangements from laboratory testing. The modeling, under the direction of
Cygan, also provides an idea of the degree of protection provided by other
molecular arrangements.
According to Cygan, "The total energy of the chemical system was monitored
as a function of the atomic positions based on the energy contributions of bond
stretching, bending and torsion of the binding molecule."
Monte Carlo simulations of the sequential packing of individual molecules as
they bound to the calcite surface helped evaluate the expected surface coverage
of the molecules.
The coating is applied in an alcohol solution called a sol-gel, developed at
Sandia under the leadership of chemist Jeffrey Brinker. When the solution
dries, only the coating is left.
Geochemists measure the dissolution rates of calcite powders when attacked
by one substance -- simulated acid rain -- and protected by another -- the
inorganic coating. Through use of magnetic resonance imaging, Sandia scientists
Roger Assink and Todd Alam watch the progress of moisture through limestone when
different coatings are applied. University of New Mexico graduate student
Sudeep Rao measures the mechanical strength of coated limestone cores.
The preliminary product is a bendable glassy coating a few molecules thick
that coats like glue. It is silicon-based -- a kind of glass -- rather than
carbon-based, like plastics.
Sandia is a multiprogram DOE laboratory, operated by a subsidiary of
Lockheed Martin Corp. With main facilities in Albuquerque, N.M., and Livermore,
Calif., Sandia has major research and development responsibilities in national
security, energy, and environmental technologies and economic competitiveness.
#
Media contact: Neal Singer, 505-845-7078 [email protected]
Tech contact: Jeff Brinker, 505-272-7627 [email protected]
Kathryn Nagy, 505-844-5337 [email protected]
Visuals available: Schematic diagram of the coupled passivant + sol-gel coating
on a limestone surface.
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