The Salt Research project improved scientific understanding of the way stone deteriorates due to the action of water and salts and developed effective mitigation and conservation procedures. By addressing questions of conservation applications concerning mitigation and desalination, and by improving fundamental knowledge of decay factors and mechanisms, the project:

  • through fundamental research on salt damage, developed a better understanding of salt damage mechanisms in porous materials at the microscopic level, with a comparison of salts with different solubility and hygroscopicity;
  • tested and monitored analytically in the lab and in the field various approaches to salt reduction/desalination, assessed the potential for secondary damage, and evaluated the efficiency of various treatments; and
  • engaged in education and dissemination by developing and disseminating educational tools and information to enhance research and collaboration between conservators and scientists in the field of salt research.

The Salt Research project helped to train and inspire researchers whose work in this area is sponsored by the European Commission. As a result of work undertaken by the GCI's Salt Research Project and ongoing research by other institutions and organizations, a specialized international conference on the topic of salts and porous building materials, Salt Weathering in Buildings and Stone Sculptures (SWBSS) is now being held at regular intervals—2007, 2008 and 2011. In addition, an ongoing experts workshop, Crystallization in Porous Media (CrysPOM), is also being held.

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

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.

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 mechanismsand an exploration of options to deal with the problem of salts and moistureholds 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 the amount of damage appears to be the evaporation rate, which in turn influences the location of crystallization and the supersaturation ratio reached before crystallization.

Overview
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—were tested and monitored analytically. Tests were carried out in the GCI laboratories and were 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 coupled 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 provided important knowledge of the underlying mechanisms of decay.

Finally, visualizing the processes involved on a micro-level has contributed to the education of conservation professionals by increasing their understanding of decay processes. The project developed and disseminated educational tools and information—including visual materials depicting salt crystallization and dissolution—that have enhanced the collaboration between conservators and researchers in the field of salt damage.

Available Project Videos

Sodium Chloride (NaCl) crystallization and dissolution with change of relative humidity (RH) both in rate and magnitude (ESEM) (2:12 minutes)
Sodium Nitrate (NaNo3) crystallization and dissolution versus RH change (ESEM) (0:55 minutes)
Sodium Chloride (NaCl) crystallization/dissolution (macro photography) (1:11 minutes)
Sodium Chloride (NaCl) crystallization/dissolution under the influence of simulated wind from a fan (macro photography) (1:27 minutes)

Last updated: January 2011