A team of researchers from Princeton has developed a low-cost and nontoxic treatment to help preserve iconic stone structures exposed to decay.

Developed by the researchers in collaboration with an international team, the treatment involves the application of a thin film of a calcium compound called hydroxyapatite, which is formed by the reaction of a water-based phosphate salt solution and calcite, the mineral that makes up marble. The solution seeps into and binds cracks in the marble's surface, creating a stronger building or monument that’s also more resistant to environmental pollution and rain.

The study not only discovered that hydroxyapatite has excellent sticking properties but also that their method has the advantage of being based on a nontoxic solvent (water), being able to penetrate deep inside marble cracks, and reacting in just 24 hours.

Using a calcium compound commonly found in bones and teeth, Princeton researchers led by George Scherer, the William L. Knapp '47 Professor of Civil Engineering, are developing a method to protect marble structures from environmental damage. These images show decay of marble structures in the Certosa di Bologna cemetery in Italy. (Photos by Enrico Sassoni, visiting postdoctoral research associate, Department of Civil and Environmental Engineering)

The team conducted preliminary tests using hydroxyapatite at the Certosa di Bologna; the researchers believe the new treatment developed at Princeton could help preserve artworks in the cemetery from the effects of time and weather. For instance, the tombstone of Antonio Basoli, an 18th-century artist and engraver, is crumbling in a characteristic pattern termed ‘sugaring’, so called because the marble crystals are similar in appearance to sugar grains. In such instances, the hydroxyapatite treatment works a bit like glue. Scherer says they are using thin layers of the coating so that it is not noticeable.

Further tests will now look at the possibility of further improving the hydroxyapatite treatment by electrodeposition. The Princeton researchers are also planning to start a pilot application of the hydroxyapatite treatment on some sculptures in the park of the Palace of Versailles in Paris, in collaboration with restorers and researchers in charge of the sculptures’ preservation.

A single application of the treatment has the potential to protect a marble structure for decades.