Project Objectives
This project addresses the knowledge gap concerning poulticing of salts from carved stone and wall paintings and works to establish guidelines for conservators on the use of desalination systems for building materials. Damage caused by salt crystals growing inside porous materials is one of the most common problems facing archaeological sites, monuments, sculptures, and wall paintings. To address this problem, more effective removal of salts from masonry is needed as is a strong scientific understanding of the problem.
The Desalination of Porous Building Materials project is the GCI's contribution to the larger collaborative project, Assessment of Desalination Mortars and Poultices for Historic Masonry (Desalination) organized under the auspices of the European Commission (EC), which includes five institutional research partners. Desalination is an EC-funded project with the GCI as full partner; however EC rules require non-European partners to provide their own funding.
The primary objective of the European Commission project is to provide those responsible for the care and maintenance of immovable cultural heritage with clear guidelines for an adequate choice of a desalination system for building materials.
In order to create guidelines, the following have to be reached:
- Formulation of a method to assess of performance of desalination systems
- Scientific knowledge of how desalination works and how salts are mobilized
- Recommendation for a new standard to test desalination systems
The GCI's contributions to the Desalination project include:
- 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, project partners' case studies and laboratory poulticing experiments
- evaluation of the role of atmospheric humidity in the transport and accumulation of salts
Project Overview
One of the more devastating effects of climate change is increased rainfall and flooding, as seen in Prague in 2001, in Iowa in 2008, which experienced devastating five-hundred-year-level flooding, and in New Orleans in 2005 with the catastrophic levee failures following Hurricane Katrina.
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 aims to provide poulticing guidelines to help conservators working on historic stone structures use less water to remove more salts from the stone in a time-effective manner. This will be 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. These guidelines will be based on a firm foundation of laboratory experiments (Component One) and case studies (Component Two).
Project laboratory research has 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 has contributed to these experiments with IC and ESEM analysis.
The desalination case study undertaken by the GCI examines a warm, humid site, in contrast to the case studies undertaken by partners in France, Germany, and the Netherlands, which are centered on temperate to cold-climate sites. The chosen site is 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 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 has shown that poultice shrinkage and degree of detachment are 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. Some details of the results were presented in a paper at the Stone 2008 meeting in Torun, Poland (see references).
Guidelines for desalination and a workshop in New Orleans to disseminate the guidelines are planned at the project's conclusion.
Last updated: April 2009
|
