The damage that salt crystallization can cause in ancient stone monuments has long been recognized by conservators and conservation scientists. Recent scientific research at the GCI has increased understanding of the ways in which salts can harm cultural heritage. Because of this new work, a wider range of potential mitigation strategies can now be considered.
As part of an ongoing research project, Carlos Rodriguez-Navarro, GCI research fellow, and Eric Doehne, GCI associate scientist, used tools such as time-lapse video and the environmental scanning electron microscope to investigate in detail the behavior of salt-laden stone. The findings from their research help explain why certain salts are more damaging than others and what parameters are likely to be important in determining the extent of the damage. How concentrated the salt solution becomes before the salts crystallize appears to be a key damage factor. Rapid cooling or drying—as the result of wind, for example—was shown to greatly increase damage. Changes in the internal surface roughness of the material may also delay the onset of crystallization. The work also found that the extent of the damage also greatly depends on the properties of the solution, such as its surface tension.
This research opens the way to considering the use of certain materials, such as some surfactants, that might modify the salt solutions and thus reduce the damage they cause. In the meantime, buffering the site of a monument as much as possible from rapid cooling or rapid drying—the result of wind, sun, or low relative humidity—can help reduce the amount of damage from salts. Researchers in Australia, for example, have proposed reducing salt and thermal damage by planting a row of trees in front of a sun-exposed, cliff rock-art site. Another technique is the use of temporary, porous, sacrificial material placed in the area of current damage; this may help move the site of crystallization to above the layer to be preserved.
Much more research is needed to provide a solid foundation of knowledge regarding this complex phenomenon, but future work by the GCI research team and other groups can be expected to shed additional light on these problems. Inquiries regarding the GCI's work can be made directly to Eric Doehne (firstname.lastname@example.org). Scientific articles on the research will be added to "Research Webstracts" on the GCI's Web site (www.getty.edu/conservation) as soon as they become available.