Stephen McCabe

Understanding the decay of stone-built cultural heritage

The problem of the decay and conservation of stone-built heritage is a complex one, requiring input across many disciplines to identify appropriate remedial steps and management strategies. Over the past few decades, earth scientists have brought a unique perspective to this challenging area, drawing on traditions and knowledge obtained from research into landscape development and the natural environment. This paper reviews the crucial themes that have arisen particularly, although not exclusively, from the work of physical geographers — themes that have sought to correct common misconceptions held by the public, as well as those directly engaged in construction and conservation, regarding the nature, causes and controls of building stone decay. It also looks to the future, suggesting how the behaviour of building stones (and hence the work of stone decay scientists) might alter in response to the looming challenge of climate change.

Impacts of fire on stone-built heritage

Abstract Fire is a major threat to stone-built cultural heritage and this paper is a review of the existing research into fire damage on building stone. From early research based on anecdotal evidence of macroscopic observations, scientists have moved on to develop various techniques for approaching the investigation of fire damage to stone (high-temperature heating in ovens, lasers, real flame tests), different aspects of the damage that fire does have been learned from each, developing understanding of how microscopic changes affect the whole. This paper seeks to highlight the need for a greater awareness of the threat that fire poses (and the need to take precautionary measures in the form of fire-suppression systems), of the immediate effects, and of the long-term management issues of natural stone structures which have experienced fire.

The use and meanings of ‘time of wetness’ in understanding building stone decay

Moisture is a well-documented, and crucial, control on the nature of stone decay. The term ‘time of wetness’ has frequently been adopted to describe how long a stone block is wet, with a view to understanding the impact of this on decay processes. Although this term has proved conceptually useful, it has been used in different ways, by different groups to mean very different things. For example, the time of wetness for a stone block surface (the traditional understanding) may be very different from that of a block interior, controlled by the different dynamics of wetting and drying in those zones. Thus, surface wetting will occur regularly (sometimes swiftly followed by drying, depending on the time of year), with block interior wetting requiring the accumulation of surface moisture to penetrate to depth (more likely in autumn and winter months), and drying out much more slowly. This relatively new but important perspective, framed in the context of climate change, is crucial to understanding the length of time stone may remain damp at depth following a period of prolonged precipitation. The nature and speed of drying is also relevant in quantifying time of wetness of both surfaces and the interior of building stones. These ideas related to time of wetness have implications for decay processes, specifically how a prolonged time of deep wetness may re-focus the emphasis of salt weathering in natural building stones toward chemical action. Literature on chemical change is discussed, suggesting that chemical change occurring during periods of prolonged wetness is likely to be significant in itself, with implications for weakening the stone (in terms of, for example, cement dissolution or grain boundary weakening) and exacerbating physical damage from salt crystallization when blocks finally dry out.

Chapter 4.1 – Student Learning Styles

Abstract The existing literature in Earth science fieldwork demonstrates a particular concern to optimize the learning environment during fieldtrips and to encourage as much as possible the interactivity that should be inherent to fieldwork. Most of the studies on fieldwork learning focus on strategies during the fieldtrip, whereas only a few focus on the preparation, and yet this stage is crucial for students to get maximum benefit from the fieldwork experience. A wide recognition exists of the impact of students’ preferred learning styles in education outcomes; the aim of this study is to explore the impact that preferred learning styles have on the students’ perceptions of their performance during fieldwork and its preparation. Sixty undergraduate students participated, filling in a questionnaire with five sections (learning styles, two sections on their perception of novelty spaces in geography fieldwork, social behavior and their evaluation of fieldwork as a learning experience). The implications of this for educational practice are then examined in the conclusions, seeking to establish the best ways to interact with different learner types, for the best outcome.

A legacy of mistreatment: conceptualizing the decay of medieval sandstones in NE Ireland

Abstract Sandstone is commonly used as a building material in medieval monuments throughout NE Ireland. This paper explores the reasons why, and the ways in which, medieval sandstone monuments decay in the temperate Atlantic maritime environment of the north Antrim coast, using Bonamargy Friary, Ballycastle, as a case study. Monumental stone decay is placed in the context of inheritance and sensitivity to change, that is, the ability or inability of a sandstone to absorb change as a result of the past stress events it has experienced. A consideration of the combined impact of background environmental factors (such as salt accumulation, temperature cycles, frost, chemical alteration, soiling of the surface, changes in surface morphology and biological colonization) and ‘exceptional’ factors (such as lime rendering, fire, climate change, abandonment and conservation intervention) is used to formulate alternative decay pathways of the sandstones identified at the Friary. Discussion focuses on the value of identifying conceptual event sequences such as the cumulative impact of past events, individual and combined, to produce recognizable decay features seen in the present day. The possible impact of future climate change on the decay of medieval sandstone monuments is discussed.

Changing climate, changing process: implications for salt transportation and weathering within building sandstones in the UK

Salt weathering is a crucial process that brings about a change in stone, from the scale of landscapes to stone outcrops and natural building stone façades. It is acknowledged that salt weathering is controlled by fluctuations in temperature and moisture, where repeated oscillations in these parameters can cause re-crystallisation, hydration/de-hydration of salts, bringing about stone surface loss in the form of, for example, granular disaggregation, scaling, and multiple flaking. However, this ‘traditional’ view of how salt weathering proceeds may need to be re-evaluated in the light of current and future climatic trends. Indeed, there is considerable scope for the investigation of consequences of climate change on geomorphological processes in general. Building on contemporary research on the ‘deep wetting’ of natural building stones, it is proposed that (as stone may be wetter for longer), ion diffusion may become a more prominent mechanism for the mixing of molecular constituents, and a shift in focus from physical damage to chemical change is suggested. Data from ion diffusion cell experiments are presented for three different sandstone types, demonstrating that salts may diffuse through porous stone relatively rapidly (in comparison to, for example, dense concrete). Pore water from stones undergoing diffusion experiments was extracted and analysed. Factors controlling ion diffusion relating to ‘time of wetness’ within stones are discussed, (continued saturation, connectivity of pores, mineralogy, behaviour of salts, sedimentary structure), and potential changes in system dynamics as a result of climate change are addressed. System inputs may change in terms of increased moisture input, translating into a greater depth of wetting front. Salts are likely to be ‘stored’ differently in stones, with salt being in solution for longer periods (during prolonged winter wetness). This has myriad implications in terms of the movement of ions by diffusion and the potential for chemical change in the stone (especially in more mobile constituents), leading to a weakening of the stone matrix/grain boundary cementing. The ‘output’ may be mobilisation and precipitation of elements leading to, for example, uneven cementing in the stone. This reduced strength of the stone, or compromised ability of the stone to absorb stress, is likely to make crystallisation a more efficacious mechanism of decay when it does occur. Thus, a delay in the onset of crystallisation while stonework is wet does not preclude exaggerated or accelerated material loss when it finally happens.

An holistic approach to the assessment of stone decay: Bonamargy Friary, Northern Ireland

Abstract Decay mapping and condition assessment have proved to be useful tools in understanding stone decay and identification of remedial action. In this paper an holistic strategy is taken to the study of façade decay at the medieval Bonamargy Friary, on the north Antrim coast, Northern Ireland. After lithology and decay forms are mapped, interrelationships between decay form, stone type and environment are identified and quantified. This is accomplished through analysis of the spatial association of decay forms, and is used to inform our understanding of decay processes and environmental and lithological controls on those processes. This approach is combined with the application of the UAS (‘Unit’, ‘Area’, ‘Spread’) staging system developed by Warke et al. that is based upon a ‘whole-building’ approach to the assessment of stone condition, the spread to decay and a staged approach to conservation intervention. The case study demonstrates how the combination of these approaches improves our understanding of the factors that control stone decay whilst providing a clearer understanding of the cumulative impact of combined decay mechanisms.

Low temperature conditions in building sandstone: the role of extreme events in temperate environments

Data on rock temperatures has previously been collected to characterise typical diurnal regimes, and more recently to describe short-term variability in extreme locations. However, there is also the case that little is understood concerning the impact of extreme events in otherwise temperate environments. Internal stone temperatures (5 cm) collected during the atypical cold extreme experienced, throughout the UK, in December 2010 show a difference between ambient air temperatures and aspect-related thermal differences, particularly concerning temperature lows and the influence of radiative heating. In this case, debris release was not visible; however, laboratory simulations have shown that under such conditions, surface loss does not necessarily negate the occurrence of internal stone modifications. This preparatory sequence of change demonstrates that surface loss is not the result of one process, but rather many operating over time to sufficiently decrease stone strength to facilitate obvious damage.

Understanding the Long-Term Survival of Sandstone in Medieval Ecclesiastical Structures: Northern Ireland and Western Scotland

Much of the worlds, and especially Europes, built heritage has been constructed using natural stone. Historically, it is a building material that is both aesthetically pleasing and durable. However, contrary to common perception, stone is not immutable, and many of our valued stone-built historic structures show obvious signs of age and decay. Advanced decay leads to the irrevocable loss of irreplaceable cultural heritage. Because of this, much effort has gone into understanding natural stone decay and the ways in which it may be managed. This focus on decay has meant that the reasons why certain stones survive are rarely examined. The long-term survival of stone over many centuries is a very complex problem—for example, factors that might be expected to cause decay can sometimes increase durability, depending on a specific combination of history, environment and stone character. To begin to understand this, a clear picture of history (events that have had impact on stone in the past), environment (the related processes that the stone is exposed to) and stone character (both material and form) is crucial. This chapter begins by emphasising the importance of each of these factors in understanding the long-term pattern of decay and survival of medieval sandstones. Following this, case studies are presented on the use and performance of sandstones in two medieval ecclesiastical structures in Northern Ireland and western Scotland—Bonamargy Friary and Iona Abbey, respectively