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Project Objectives
The rapid deterioration of magnesian limestone structures in the north of England has been a serious problem for more than one hundred years. While air quality in England has improved during this period, the rate of stone loss in these structures has not slowed. Thus far, conventional stone conservation treatments have not been successful in mitigating this decay, and large-scale stone replacement has been proposed to deal with the problem for buildings such as York Minster and the world heritage site of Fountains Abbey.
The Magnesian Limestone Project, a collaboration with English Heritage and Princeton University, seeks to further our understanding of this rapid deterioration on three fronts: stone characterization, decay mechanism or damage process research, and treatment testing. The overall goal of the project is to provide a scientific foundation for the development of conservation interventions for magnesian limestone. This work is being undertaken through collaboration and exchange between conservators (English Heritage), conservation scientists (GCI), and engineers (Princeton University).
Project Overview
Since 2004, the GCI has collaborated with English Heritage to study the rapid stone decay found at structures built from magnesian limestone. To this end the team has used Howden Minster—a ruined monastery and chapter house dating to 1388 and managed by English Heritage—as a field laboratory. Located a few miles from Drax, Europe's second largest coal burning powerplant, Howden Minster suffers from rapid flaking of stone surfaces thought to be related to past air pollution and accumulation of salts in stone pores. The project team has found that while current air pollution levels are much improved, the accumulated salts are continuing to cause extensive damage, especially in areas sheltered from rain.
In 2005, the GCI characterized the physical and chemical properties in a range of magnesian limestones and determined which of these is the most durable stone for stone replacement projects. The following year the GCI delineated magnesian limestones decay process based on analyses of samples from Howden Minster and successfully simulated the primary decay mechanism—cyclic crystallization of accumulated magnesium sulfate salts from historic acidic air pollution—in the GCI laboratory.
Team members from English Heritage analyzed the structure of the walls at Howden and completed a condition survey, the preliminary results of which were presented to the UK Cathedral Architects Association.
In 2007, treatment trials for salt removal were undertaken by English Heritage at Howden and evaluated by the GCI. The rate of decay was documented via a time-lapse field camera installed by the GCI. A weather monitoring station was installed at Howden Minster by English Heritage to correlate climate data with decay parameters and patterns. English Heritage also investigated the structure of the building and performed test treatments. The project team held a workshop in York to share current research results on the problem with architects, scientists, and conservators.
Also in 2007, the research group of materials scientist George Scherer of Princeton University joined the project to help investigate important aspects of the different damage mechanisms and to test potential interventions.
In 2008, the time-lapse photography and environmental monitoring systems data revealed the prominent role of condensation events in the salt decay process in Howden Minsters chapter house. The installation of a roof over the chapter house walls in 1984 led to the drying out of interior walls and the accumulation of surface salt efflorescences. Removal of salts and management of moisture appear to be the most promising conservation approaches at this stage in the project. Project results were presented at several international meetings (see Related Publications), these include recent laboratory research focused on clays in magnesian limestone, treatment trials and decay documentation, simulation and modeling.
In 2009 the project is characterizing a wider range of magnesian limestone samples, determining the role of clay in magnesian limestone decay and documenting the behavior of the complex salt mixtures found at sites such as Howden Minster. Results show that the clay content of magnesian limestone selected for use in buildings is generally low (0.1 to 1 weight percent) and that barium sulfate is a common component. Even these low clay levels may have a significant effect on magnesian limestone behavior. This result may lead to better ways to select more durable replacement stone.
Related GCI Work
Results from the GCI's Desalination Project, an independent undertaking to improve the body of knowledge concerning poulticing of salts from carved stone and to establish guidelines for the use of desalinization systems for building materials are expected to aid in providing useful solutions for magnesian limestone treatment.
Last updated: April 2009
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