Stone in buildings is exposed to natural patterns of weathering, varying urban conditions, a legacy of repair and change, building quirks and outside events that interact in complex in complex and sometimes unpredictable ways to produce new patterns and new rates of stone decay. Stones can also have a ‘memory’ of the legacy of structural and chemical changes it has undergone since before it was quarried – legacies which can explain why stone continues to deteriorate long after air pollution levels have improved. Further, different stones react differently to different stresses, and require different solutions. A necessary part of building the knowledge to make informed decisions to conserve stone buildings is to understand why and where stone decay can and does occur.
The aim of this stone conservation research project was to gather information on the degree of degradation and erosion of stone monuments in the Republic of Ireland dating from the Neolithic through to the nineteenth century. The study examined 112 archaeological monuments in 14 counties, identifying and assessing the most significant forms of decay and impacts to stone monuments based on factual up-to-date information gathered through fieldwork and laboratory analysis. The archaeological monument types dated from the Neolithic to the nineteenth century and included megalithic tombs, cashels, Anglo-Norman castles, medieval tower houses, post-medieval buildings, and ecclesiastical heritage including churches, abbeys, round towers, high crosses and a range of smaller stone monuments including crosses, a cursing stone, leachta and grave slabs.
The most significant results and conclusions of the study are:
- 112 monuments from 14 counties comprising limestone, sandstone, granite and metamorphic rocks were assessed.
- The methodology proved effective in the assessment of any stone monument, of any period, of any stone type.
- All monuments studied showed some form of decay. 11% of monuments showed strong structural damage, 22% showed intensive stone decay and 27% showed mild structural damage.
- Intensity of decay does not increase with the age of the monument, but is primarily determined by the type and origin of the stone, and the stress it has undergone through the years.
- Stone dressings and carved sculptural detail tended to be more vulnerable than the masonry of the fabric, with 33.3% of monuments studied showing significant loss of carved detail. The topography and stress caused when the stone was carved or dressed may predispose the material to increased weathering rates.
- Biological colonisation was found on 97% of examined sites. Species such as trees and climbing plants induce the strongest damage including dislodgement of masonry.
- Only in aggressive polluted atmospheres is the environment more determinant on the type and rate of decay than the nature of the stone itself.
- Structural damage was found to be the most important threat to the integrity of 11% of the studied monuments, involving a danger of collapse. In these cases, the immediate response required by the intensity of structural damage far outweighs the damage caused by stone decay processes and mechanisms.
- Excluding stone sulphation in polluted areas, the most severe cases of stone decay were found on schist and sandstone.
- Calcite dissolution was the most common decay process of Carboniferous Limestone.
- All sandstone exhibited loss of carved detail.
- 32.5% of the monuments studied were repaired with modern Portland cement mortars. Of these mortars 33% showed lime leaching, 13% were generating salts and 10% were inducing flaking in the surrounding masonry.
The study examined the decay of some of the most significant stone types found in Irish monuments, and a broad cross section of monument types. The study was repeated a decade later for the Stone Monuments Decay Study 2010 to determine how/if stone decay had progressed, and whether new threats or conservation issues had arisen in the intervening decade.
Pavia, S. & Bolton, J. (2001) Stone Monuments Decay Study 2000: an assessment of the degree of erosion and degradation of a sample of stone monuments in the Republic of Ireland. The Heritage Council
This research was funded by The Heritage Council