The existing literature and knowledge across many disciplines, including stone conservation and heritage management, is founded on an ability to react and adapt to changing situations, based on the knowledge and experience gained through past and current events. However, both reactive and anticipatory responses to threats to buildings are largely based on the premise that the past is the key to the future. predicted rapid changes in climatic conditions anticipated throughout the 21st century, results in a situation where past events can no longer be relied upon for driving future decision-making. Many of the established truisms and work practices of stone decay, cleaning and conservation may have to be amended, altered or abandoned when confronted with new challenges.
The existing stone conservation literature is too-often centered on understanding stone deterioration and providing appropriate solutions on a very simplisitic basis. Stone conservators are focused on solutions for ‘limestone’ problems and ‘sandstone’ problems, though these are very complex materials, containing sub-types of widely varying stone properties, durabilities, and responses to soiling and new environmental conditions. As the 21st century progresses, heritage professionals will see more complex interactions between stone decay and soiling processes and changing environmental conditions. There is a need for growing interaction between heritage professionals to respond to increasingly complex future architectural conservation problems. There will be a number of key shifts, which cannot currently be accurately quantified, but can be flagged as areas of concern. These include:
Stone Decay Processes: Stone decay is an extremely complex field, and many recent broad overviews of risk to cultural heritage have been forced to over-simplify stone decay processes in order to summarise one or more threats, at the risk of misconstruing or misinterpreting key concepts in how stones weather and soil.“Stones” are not a homogenous group, with over 1400 bedrock groups in the Republic of Ireland, each one of which may contain between one and five stone types, and each stone type may contain different ‘beds’. Granites, limestones, sandstones, siltstones, mudstones and a diverse range of igneous and metamorphic rock types were used in all periods of Irish architectural heritage. These stones display wide variability in physical properties, vary in their susceptibilities to different decay processes, show different decay forms when they deteriorate, and will not all respond in the same manner to changing environmental conditions.
Stone decay forms (such as flaking, scaling, and granular disintegration) cannot be universally attributed to a particular decay process. Many decay forms can present themselves in response to more than one decay process, and some may originate from two or more decay processes acting synergistically. Further, many stone decay processes are not based on gradual wearing down or retreat of a stone surface, but are episodic in nature. The stresses accumulated over a number of centuries within an apparently stable stone, combined with recent alterations in atmospheric pollution and/or environmental conditions and/or inappropriate conservation intervention can trigger rapid breakdown of a stone. The new alterations in environmental conditions brought about by changing climatic conditions project a scenario where relatively minor temperature and moisture fluctuations are likely to cross durability ‘thresholds’ for a number of stone types, especially porous sandstones, likely to lead to sudden, dramatic recession and failure of stone surfaces.
Salt Decay Processes: Salt decay is potentially the most damaging process to affect Irish building stone through the 21st century. Warmer, drier summers form conditions suitable for the accumulation of salts in building stones, without the “washing” effect of the frequent precipitation events which have been naturally removing salts from building surfaces. Any salt deposition is likely to act in combination with fluctuating temperature and moisture levels resulting in damaging salt crystallisation, especially in the east and south-east of the country. However, salts of marine origin known to be extremely damaging to architectural heritage, and were detected at low levels to coastal and inland unpolluted environments in Ireland. If these salts begin to accumulate to harmful levels under new climatic conditions through a decrease in natural rainfall “washing”, there is significant potential for salt-based decay to occur in all parts of the country.
Soiling Processes: Current projections for a decrease in summer rainfall will both decrease the effect of this natural “washing”, and increase the time available for dry deposition of airborne particulate matter on building surfaces. Building stones in urban areas act as passive repositories for gaseous and particulate airborne pollutants entrapped by dry and wet deposition, and the composition of soiling changes to accommodate new classes of air pollutants as air quality changes. New soiling in urban areas is most likely to be based on vehicle emissions. Traffic pollution tends to be more severe at street level than roof level, and rapid soiling is likely in areas of heavy traffic. The new soiling is also likely to be distributed in markedly different patterns to historical soiling, controlled by traffic emissions and removal or redistribution of soiling by wind-driven rain.
Diesel-emitting traffic in areas with granite, limestone and sandstone buildings may see a return to sulphate deposition resulting in black gypsum-rich crusts. However, there will be variability in the composition of black-coloured crusts due to the multi-pollutant nature of modern airborne particles. Some of the emissions will promote the growth of black biofilm soiling in urban contexts, which can be best removed with a mild biocide. Some of the crusts are likely to contain significantly more iron-bearing components than previous soiling crusts, leading to the possibility of significant and potentially irreversible staining if the soiling is allowed to remain on the building surface, or if the soiling is attempted to be ‘cleaned’ without first analysing the composition of the soiling layer, and the interface between stone substrate and soiling crust. And much of the soiling is likely to at ground level rather than to the eaves and cornices where conservation and cleaning efforts are currently focused.
It will become increasingly important to correctly identify the components of black-coloured urban soiling. New soiling will be most visible to buildings which have been cleaned in the last two decades. In this situation, where records of the past cleaning system and methods are likely to have survived, it will become crucial to correctly identify the nature of the new soiling, as repeating a cleaning method previously known to be successful could cause irreparable damage to the building exterior, especially in an urban context where there is a risk of mobilising soluble salts within the masonry.
Storm Action: The increasing frequency and severity of storm events over the winter months is likely to lead to an increase in wind and water abrasion at high levels, with increases in dissolution to areas of run-off. Increased winter precipitation will also lead to problems associated with saturated masonry, with possible mobilization of salt-loading accumulated over the warmer, drier summer months. However, the projected overall increase in winter temperatures should lessen the risk of freeze/thaw damage, especially to urban areas. The effect of wind action is well known, especially to roofs, However, stone roof detail in conditions of more frequent storm events are likely to show increased rates and severity of stone decay, and stress and damage to any metal fixing elements securing the stone.
Coastal Erosion Processes: In the Republic of Ireland, the sea has progressively encroached on the land through a continuing process of erosion and gradual inundation since the end of the last Ice Age. Current climate change projections indicate an increase in relative global sea-level, and an increase in precipitation events and storm events likely to cause temporary increases in local sea levels, leading to coastal flooding and coastal erosion. Any rise in sea-level on the shoreline of Ireland is likely to produce a range of impacts, unlikely to be uniform, and influenced by factors such as shoreline type, topography and local variables. However, certain coastal types such as tidal deltas, low-lying coastal plains, beaches, islands (including barrier islands), coastal wetlands, and estuaries may face greater risk due to their physical characteristics, with consequent increased risk to any architectural heritage thereon. However, inundation alone does not necessarily lead to significant deterioration of monuments on the coast. Many structures such as harbours and coastal fortifications were intended to be partially immersed. Vulnerability mapping based simply on projecting a rise in sea-level is unlikely to be informative of the risk to architectural heritage unless considered in conjunction with the response of the shoreline to change through coastal erosion, and the position of a building or structure in relation to the damaging inter-tidal zone. It is not simple sea-level rise, but the factors associated with sea-level rise which have the potential to cause the most damage to architectural heritage – increased wave and storm activity, increased coastal erosion and changes in climate and weathering parameters, and the landward movement of the damaging inter-tidal zone.
The vulnerability of a building to coastal erosion and storm action is most likely to be based on the nature and durability of the local shoreline, and the position of the building in relation to the high water mark. Buildings most likely to be dramatically affected by changes in sea level include those constructed at the high water mark, such as coastal cottages, harbour and fisheries structures, piers, jetties, bridges, light houses, coast guard stations, Martello towers and other coastal fortifications. In addition to the rate at which mean sea-level may rise, any resultant changes in tidal amplitude, especially ascending levels of the highest tides will be of the greatest relevance to coastal architecture. Storm surges caused by winds and atmospheric pressure changes can result in a temporary rise in sea-level of up to several metres on a lee shore, and the production of significantly high-energy waves can result in dramatic loss to land and to buildings.
Biological Colonisation: Historic buildings, and especially ruined structures, provide a specialised environment which can support a wide range of flora and fauna. However, biological colonisation and biodeterioration processes are important contributors to stone weathering. Previous studies in Ireland have shown biological colonisation to have a significant impact at over 90% of historic stone buildings examined. Biotic communities and species will respond to climate change, though the nature of that change is unknown at a building scale level. Shifts in established equilibriums are to be expected. These may most noticeably be seen in shifts from bioprotection to bioweathering as natural species adapt to the new climatic conditions, and the habitats and species found on our built heritage changes, and a re-assessment of how to balance biodiversity with the protection of our architectural heritage. The current common practice of periodically removing all biological growth from building surfaces may need to be re-assessed in the light of changing biological growth, with greater consideration of the potential for bioprotection of stone surfaces, especially in the warmer, drier summer projected for the east and south-east of the country.
This research project was funded by The Heritage Council through the Architectural Research Grant Scheme.