Can ancient stone avoid salt attacks?

From Boston, at a meeting of the Materials Research Society

Ancient buildings, monuments, and sculptures can’t escape harsh environmental conditions, including air pollution and salty ground and ocean water. Once salt water seeps into stone, salt crystals remain in the stone’s small, internal pores after the water evaporates. In time, as the crystals grow, they can exert stone-crumbling pressure.

Researchers now have a new means for defending stone treasures against crystal growth, reports George W. Scherer of Princeton University, who’s made a new protective coating.

In lab tests, Scherer and his coworkers dissolved a carboxylic-acid-containing polymer in the organic solvent tetrahydrofuran. Then, they immersed small limestone cubes in the solution so that the polymer could penetrate the cubes’ internal pore networks. Polymer-treated cubes held up well to subsequent submersions in salt water, but untreated ones lost their strength and disintegrated, says Scherer.

Salt crystals exert pressure as they grow in a stone’s pores because there are repulsive forces between the crystals and the pores’ walls, says Scherer. With the new polymer coating, he suspects, the surfaces become more attractive to the salt crystals, which then grow to the surfaces rather than repel them.

Other researchers are planning tests of the polymer coating on ancient walls around the medieval Aegean city of Rhodes in Greece, says Scherer. He notes however, that before the technique can be implemented widely, he and his colleagues need to find a related polymer with a similar stone-saving action but that will dissolve in a more environmentally friendly solvent than tetrahydrofuran.

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