Project Objectives
Damage to porous materials from the interaction of moisture and salts is a widespread and complex problem in the conservation of mural paintings and stone. The action of salts also increases the corrosion rates of outdoor bronze and other metal objects. Consolidation treatments and other conservation interventions, such as rendering and artificial stone, often fail in the presence of high salt loads.

This project addresses questions of conservation applications concerning mitigation and desalination, and seeks to improve fundamental knowledge of decay factors and mechanisms.

The objectives of this project include:

• developing a better understanding of the fundamental salt damage mechanisms in porous materials at the microscopic level, with a comparison of salts with different solubility and hygroscopicity;
• testing and monitoring analytically in the lab and in the field various approaches to salt reduction/desalination, assessing the potential for secondary damage, and evaluating the efficiency of various treatments;
• developing and disseminating educational tools and information to enhance research and collaboration between conservators and scientists in the field of salt research.

Project Overview
"In time, and with water, everything changes."
- Leonardo da Vinci

The formation, transport, and crystallization of salts in pores is widely recognized as one of the primary causes of the deterioration of historical architecture, of structures in archaeological sites, and of archaeological objects. A better understanding of these mechanisms—and an exploration of options to deal with the problem of salts and moisture—holds great significance for the conservation of material cultural heritage.

The origin of the different salts that damage cultural heritage varies. Sea salt spray, deicing salts, and salinization of soil in arid environments are sources of alkaline chlorides. Air pollution often contributes to the formation of sulfate salts such as gypsum on calcareous materials, while indoor air pollutants such as acetic acid may corrode metal objects. Nitrate and oxalate salts are often related to biological activity. Alkaline sulfates may originate from the use of Portland cement in restoration.

Under certain conditions, the combination of different materials may lead to salt formation. For example, if excess moisture is present, gypsum and dolomitic lime tend to form damaging magnesium sulfates. Most building materials and the surrounding soil environment contain salts in low concentrations that can, over time, become concentrated at interfaces, and subsequently lead to damage.

The interaction of salt, environment, and stone is believed to depend on three primary variables: 1) the solubility and hygroscopicity of the salts and salt mixtures that are present; 2), the environmental conditions and the rate and amplitudes of environmental fluctuations; and, 3) the structure of the pore space in the surrounding matrix. A very important parameter affecting amount of damage appears to be the evaporation rate, which in turn influences the location of crystallization and the supersaturation ratio reached before crystallization.

Efforts to reduce the salt concentration in order to lower the rate of decay caused by the crystallization are a common—and critical—component of conservation measures. As part of the Salt Research Project, various approaches to salt reduction-such as poulticing, rinsing and sacrificial rendering-are being tested and monitored analytically. Tests have been carried out in the GCI laboratories and are validated in the field in order to evaluate the efficiency of various treatments and to estimate the risk of secondary damage to highly salt-laden surfaces.

Although conservation treatments take place on a macro scale, salts begin to crystallize in pores on a microscale. For that reason, the project is coupling the use of time-lapse video microscopy and environmental scanning microscopy for an in situ dynamic study of salt crystallization and dissolution. This aspect of the project can provide important knowledge of the underlying mechanisms of decay.

Finally, visualizing the processes involved on a micro-level can contribute to the education of conservation professionals by increasing their understanding of salt decay. The project is developing and disseminating educational tools and information—including visual materials depicting salt crystallization and dissolution—that can enhance the collaboration between conservators and scientists in the field of salt research.

Last updated: February, 2006