This project addressed the poulticing of salts from carved stone and wall paintings by developing procedures for conservators on the use of desalination systems for building materials.

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The Desalination of Porous Building Materials project was the GCI's contribution to the European Commission–sponsored project, Assessment of Desalination Mortars and Poultices for Historic Masonry (Desalination), which included five institutional research partners and had as its primary objective providing those responsible for the care and maintenance of immovable cultural heritage with clear guidelines for choosing a desalination system for building materials.

The GCI's contributions to the EC-sponsored Desalination project included:

  • a case study on the historic residential building complex known as Madame John's Legacy in New Orleans
  • environmental scanning electron microscope (ESEM) and ion chromatography analyses on salt samples from the GCI, from case studies provided by project partners, and laboratory poulticing experiments
  • evaluation of the role of atmospheric humidity in the transport and accumulation of salts

The GCI also developed poulticing procedures to help conservators working on historic stone structures use less water to remove more salts from stone in a time-effective manner through better matching of the poultice properties and application method to the stone's substrate and environmental conditions. The procedures were published in the proceedings of the 11th International Congress on Deterioration and Conservation of Stone: 15–20 September 2008, Torun, Poland: and in the Journal of Cultural Heritage.

One of the more devastating effects of climate change is increased rainfall and flooding, as seen in Prague in 2001, in New Orleans in 2005 with the catastrophic levee failures following Hurricane Katrina and in Iowa in 2008, which experienced devastating five-hundred-year-level flooding.

Historic buildings located in coastal areas and along major rivers are especially vulnerable to flooding and its long-term effects. Flooding increases salt loads in structures by mobilizing or bringing salts into porous building materials resulting in cycles of crystallization, surface salt efflorescence, and damage to masonry. While flooding is a short-term event, the resulting wetting and drying cycle of thick stone walls often takes years to complete and may activate salts from deep within the stone that have accumulated from air pollution, deicing salts, or encroachment of brackish groundwater. Fifty years after sea flooding in parts of the Netherlands, salt weathering continues to be problematic in several churches in formerly flooded areas. In Prague, buildings never previously threatened by groundwater, rising damp, or salts are now seriously affected because of the higher groundwater table created by episodic flooding.

Treatment of salt weathering on historic structures and monuments often requires architectural and engineering interventions to control moisture—installation of a damp-proof course in the foundation, redesign of roof drainage systems to accommodate larger storms, or maintenance of architectural details that shed rainwater away from the foundation, doors, windows, and walls. A leaky roof, overflowing drainpipes, or condensation can contribute to the mobilization of salts.

Once these moisture issues have been addressed, a common treatment option involves the application of a desalination poultice to reduce the load of accumulated salts on these structures. The desalination poultices used have been based on a typical cleaning poultice and often contain a substantial amount of water. However, architectural conservators often experience unexpected outcomes when large-scale poulticing is undertaken—salts move in unexpected ways, come back quickly, are pushed deep into the stone, or are not removed by the treatment.

The Desalination of Porous Building Materials project developed poulticing guidelines to help conservators working on historic stone structures use less water to remove more salts from stone in a time-effective manner. This was accomplished by better matching the poultice properties and application method to the stones substrate and environmental conditions. This is important because using less water may actually remove more salts and result in more predictable extraction. The GCI-developed guidelines are based on a firm foundation of laboratory research and case studies.

Project laboratory research focused on experiments to characterize poultice working properties and behavior, discovering, for example, that poultice shrinkage is a key parameter since it controls the detachment of the poultice from the substrate. Experiments using NMR have documented the behavior of poultices in contact with substrates of contrasting pore size distribution, showing that the pore size of the poultice needs to be matched to the substrate for efficient salt removal. The GCI contributed to these experiments with IC and ESEM analysis.

The desalination case study undertaken by the GCI examined a warm, humid site, in contrast to the case studies undertaken by Desalination partners in France, Germany, and the Netherlands, which were centered on temperate to cold-climate sites. The site selected by the GCI was Madame John's Legacy, the oldest residence in the French Quarter of New Orleans.

At the Madame John's Legacy site, the main house was found to have a serious salt problem after a 1998 renovation had added cement render to the exterior, an air-conditioning system, and a plastic membrane under the brick floor, all of which changed the moisture flow exacerbating salt accumulation. In this study, four standard desalination poultices in two environments were used to test the application and efficiency of these materials. Field testing showed that poultice shrinkage and degree of detachment were important factors in the amount of salts removed because once a poultice has detached due to shrinkage, it stops extracting salts.

Results from laboratory analysis of drillings before and after poulticing at Madame John's Legacy found that leaving the poultice on the brick surfaces to dry for months instead of weeks, resulting in a significant reduction in salt content of well over 90% for two poultices. The high humidity at Madame John's reduced detachment problems in comparison with other Desalination Project case studies. Details of the results were presented at the 11th International Congress on Deterioration and Conservation of Stone held in Torun, Poland.

In May 2010 the workshop, Poultice Desalination of Porous Building Materials, was conducted by the GCI in partnership with the Louisiana State Museum at Madam Johns Legacy to further disseminate the work of the Desalination of Porous Building Materials to field-based conservation professionals. It included procedures for poultice desalination.

Page updated: January 2011