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, which recently moved from Marina del Rey, Calif., to Los Angeles 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. Hydphobic coatings repel water. "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 Randall 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. Cathy Scotto of the Naval Research Laboratory and Carol Ashley participated in the sol-gel synthesis and applications parts of the study. 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.
Visuals available: Schematic diagram of the coupled passivant + sol-gel coating on a limestone surface.