Field Laboratory at Howden Minster
The Chapter House at Howden Minster. This building dates to 1388 and has important stone carvings that should be preserved.
Beginning in 2004, the Getty Conservation Institute collaborated with English Heritage to study the rapid stone decay found at structures built from magnesian limestone. To this end, the team 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 power plant, 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 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 Conservation Institute 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 Institute delineated magnesian limestone’s 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 historical acidic air pollution—in the Institute 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 then 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 Institute. The rate of decay was documented via a time-lapse field camera. 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.
That same year the materials scientist research group, led by 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 Minster’s 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 are the most promising conservation approaches. Project results were presented at several international meetings. These include recent laboratory research focused on clays in magnesian limestone, treatment trials and decay documentation, simulation and modeling.
In 2009 the project characterized 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 showed that the clay content of magnesian limestone selected for use in buildings was generally low (0.1 to 1 weight percent) and that barium sulfate was a common component. These low clay levels do not appear to have a significant effect on magnesian limestone behavior. The wide range of composition of magnesium limestone documented from quarries is matched on buildings, with highly variable rates of loss.
More detailed work on salt behavior included thermomechanical analysis, which showed that expansion of stone contaminated with magnesium sulfate salts occurs during drying, followed by relaxation of the stress during dehydration of the precipitated salts. ESEM/STEM experiments show that hydration of single crystals of the lower hydrates of magnesium sulfate is a through-solution crystallization process that is only visible at a small scale (~µm). It is followed by growth of the crystal prior to deliquescence. This demonstrates that crystallization pressure is the main cause of the stress induced by salt hydration and rewetting of lower hydrate salts. In addition, drying-induced crystallization was found to be kinetically hindered at high concentration, which was attributed to the low nucleation rate in highly viscous magnesium sulfate solutions.