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Stone Conservation
Principles and Practice
Edited by Alison Henry
Managing Editor: Jill Pearce
Chapter One
Introduction
Most of the world's greatest buildings as well as many of its humbler architectural gems are built of stone, so it is no wonder that the subject of stone conservation continues to arouse great interest amongst conservation professionals, practitioners and the general public.
The science of stone conservation spans three centuries: the nineteenth-century saw attempts to consolidate stone using waxes and Baryta water; during the twentieth century lime-based techniques were refined, and synthetic consolidants and sophisticated cleaning methods were developed; today we have at our disposal such techniques as the use of calcite-forming bacteria, hydroxylating conversion treatment and advanced laser and poultice cleaning.
But architectural stone conservation is not just a science; it is an art and a craft skill, and for many people therein lies its attraction. Underpinning all aspects of this multi-disciplinary activity is a decision-making process that is crucial to successful conservation. In a world of rapidly developing scientific techniques and new materials, it is all too easy to get carried away with enthusiasm for actually doing the practical work. Too little thought is given to its aims and objectives and the effect that the work might have on the significance of the building as a whole. This book aims to redress the balance by presenting stone conservation as a process that starts long before the first specification is written or the first spatula is wielded on site.
The emphasis is on stonework, both plain and decorative, in an architectural context. Whilst an appreciation of the geology of building stone types and an understanding of the mechanisms of stone decay are fundamental to successful stone conservation, these are not covered in depth in this volume for want of space.
The philosophical principles on which stone conservation is founded can be traced back to antiquity. Peter Burman and Michael Drury provide a fascinating account of the development, through the centuries, of conservation thinking and craft skills, paying particular attention to re-evaluating the role of the great nineteenth-century British architects. They argue that the work of the 'restoring' architects was not as black-and-white as is often portrayed: many of these villains of the conservation movement in fact carried out sympathetic repairs to many buildings, particularly churches, but were constrained by the contexts in which they were working, which often demanded radical interventions in buildings that were close to ruin.
Michael Drury develops the themes of restoration, conservation and preservation, which is a challenging task given the generally ambiguous use of these words today. How often, for example, do we hear someone talk of restoration when what they really mean (according to the British Standard definitions) is in fact conservation? And the picture is clouded even further if one considers the choices to be made when deciding to treat individual stones: at what point does restoration or preservation (of individual stones) stop and conservation (of the building) begin? You might be tempted to think that terminology is not important so long as we, the professionals and practitioners, know what we mean. But terminology is important if we are to counter the generally negative connotations of conservation amongst the general public, who often confuse conservation with preservation and see it as something that is stultifying and restrictive. The concept of conservation as a proactive means of managing the inevitable change in the historic environment is something that escapes the majority of people.
Michael Drury goes on to use the example of the conservation of the West Front of Salisbury Cathedral to illustrate how these philosophical ideas influence decision making, and how they can be used to develop a conservation policy for a project. However, it is not only outstanding buildings that deserve such an approach; the humblest of projects will benefit from the same thought processes, even if they are not formalized into a written document.
Jerry Sampson, who, I believe, must surely be descended from Sherlock Homes, explains how, with a detective's eye an enormous amount of information about the history and development of a building can be gleaned from its fabric. This information, combined with the results of documentary research, can help provide an understanding of the building, which then allows informed judgements about its significance to be made. This is an essential pre-requisite of any conservation project: only by appreciating the value of something can we truly determine whether the interventions we make are beneficial or harmful. We might clean, repair, and consolidate stone in the name of conservation, and in the process wipe out evidence of part of a building's history because we did not appreciate, for example, that there were fragments of polychromy surviving beneath the lichen growth or carbon deposits that we removed. The significance of a building lies not only in the physical condition of its constituent parts but also in its history and development, and many buildings still have much to tell us if only we would use our eyes (or those of an experienced buildings' archaeologist) before starting work.
The same principles apply in understanding the nature of the substrate and the causes of its deterioration; it is often unwise to attempt to treat a symptom until the cause of a problem is fully understood. Nick Durnan and Colin Muir explain the range of survey, monitoring, testing and analysis that might be needed as part of the decision-making process long before work starts on site.
The subject of stone consolidation has probably aroused more interest and controversy than any other aspect of stone conservation. Being able to spray a consolidant onto crumbling stone and thereby halt further decay sounds so simple, and yet is fraught with problems. When I speak to members of the general public, I often find that they are incredulous that there is no simple, universal means of consolidating stone in this way. It is a measure of how rapidly things change in conservation that twenty years ago silanes seemingly held such promise for stone consolidation, yet today their use is limited to a few special cases. Clifford Price's chapter provides an overview of the development of stone consolidants that takes us into the 'post-silane' era of the early twenty-first century. He suggests that in the future we should move away from trying to develop long-lasting panaceas, and that we should instead aim to develop methods that would meet very specific needs, even if perhaps only for a relatively short period of time. As things change fast in the world of science and conservation it is conceivable that new techniques will be developed that will enable further (and possibly better) treatment of the stone in the future. An interesting example of this, although not specifically related to consolidation, is the large number of rusting iron reinforcements that have been removed from stone buildings in the past and replaced with stainless steel, often involving major cutting out and stone replacement; in many similar situations today, such a problem might well be dealt with by cathodic protection with practically no stone replacement required – a solution unheard of twenty years ago.
Another area in which public aspiration and practical reality often fail to meet is in the cleaning of stonework. Kyle Normandin and Deborah Slaton's chapter starts by explaining that masonry cleaning of historic buildings must distinguish between soiling, which is detrimental to the building (either because it conceals structural problems or because it actively contributes to stone decay), and benign patinas, which contribute to the character and sense of age of a building. They emphasize the need to understand the nature of the substrate and the causes of any soiling before moving to trials of possible cleaning methods. The full range of current masonry cleaning systems is summarized. Although written from a North American perspective, the systems currently used in the UK are fully covered. The authors provide a useful methodology for selecting a cleaning technique as well as a matrix summarizing stone types and possible cleaning methods. Whilst many new methods have appeared in recent years, the concluding section contains a decades-old caution that still applies: use the gentlest possible method to achieve the desired level of cleaning.
When existing stones are severely decayed, the decision may be made to replace them in whole or in part. Ewan Hyslop considers the decision-making process involved in such cases, highlighting possible alternatives to replacement. Even if it is decided, after due deliberation, that stone is to be replaced, finding a suitable match can be difficult, especially if the quarry that supplied the original stone is no longer open, or if the specific beds of stone from which it was obtained are worked out. He explains how petrographic analysis can be used to characterize the original stone, and to find replacement stones that will not only initially look similar to the original stone, but will perform and weather similarly too. Sometimes, procuring a suitable stone means re-opening abandoned quarries. This is usually met with howls of opposition from local residents fearful of noise, dust and heavy lorries. However, in many cases extraction of small quantities of building stone from not much more than a hole in the ground in the corner of a field can go on practically unnoticed, with the occasional load of stone moved to a shed in a farmyard by a tractor and trailer. Such quarries can provide distinctive local stone for distinctive local buildings. We need to decide whether we would prefer a small number of 'super-quarries' restricted to the backyards of an unfortunate few, but causing significant environmental impact and exporting tons of stone well beyond their region of production.
Moving on to consideration of individual stone types, the chapters by Nick Durnan, Colin Muir and Jonathan Kemp provide a conservator's perspective on the conservation of limestone, sandstone and marble. With a wealth of practical experience between them, they illustrate the specific approaches and techniques suitable for these different stone types. They show how the more traditional methods of cleaning, repair and consolidation are being augmented by newer methods, always bearing in mind the underlying need to fully establish the precise nature of the stone in question and the causes of its deterioration.
Christopher Weeks, in his chapter on the conservation of polychrome stone, stresses the importance of initial survey work to establish the presence and extent of paint on external architectural stonework. It is perhaps significant that as the call for thorough investigation prior to starting any conservation work is being increasingly heeded, remnants of paint schemes are coming to light on buildings where no one ever previously considered looking for them. In an earlier chapter, Jerry Sampson mentions the example of Beaminster Church in Dorset, where paint fragments were unexpectedly discovered on the quatrefoil band of the tower. As more such discoveries are made, both in the UK and abroad, the need to understand fully the special requirements for cleaning and consolidating polychrome and the implications that the presence of polychrome has for the conservation of the underlying stone, must increase. This chapter is an excellent start in this direction.
David Carrington presents a surprisingly dynamic picture of the current state of church monument conservation. Again, we come back to the need to understand the processes acting on a monument in its wider context before considering remedial action. All too often decisions have been made that have considered a monument in isolation, rather than considering whether or not decay might be reduced or arrested by improving the environment in which the monument is located. Indeed, the result of many past interventions was to provide barriers and coatings to physically isolate a monument from its environment. He cites examples of monuments which had been completely dismantled and rebuilt on a lead damp-proof membrane, but which continued to deteriorate because of a leaking roof. He also suggests that conservators in this field are increasingly considering alternatives to damp-proof membranes and surface coatings, such as improved ventilation and drainage.
Graveyard memorials have been attracting much attention in recent years, partly as a result of increasing interest in family history research, and partly due to some serious accidents, including fatalities, caused by the collapse of unstable memorials. For many graveyard managers, health and safety in the graveyard has become a pressing concern. There is often a tendency to react swiftly in the face of perceived threats, and as a result many graveyard memorials have been made safe by laying them flat on the ground. Advice on alternative methods of improving safety has been published by Historic Scotland, and this is summarized in Chris Daniels' chapter. Raising the profile of graveyard memorials has also had the benefit of increasing funding for their repair, with some local authorities and dioceses running grant schemes for the repair of memorials of architectural or historic interest. Although the amount of money available is minute in proportion to the need, successful projects can serve as exemplars and may help to bring in funds from private sources. The specific issues pertaining to these structures are explained. These are effectively situated in the unusual environment of an unprotected 'museum' with twenty four-hour unrestricted access and offer hopelessly inadequate funding for maintenance. The range of techniques and materials currently in use for conservation of such graveyard memorials are also described.
Various recurring themes emerge from these chapters, particularly the need for thorough investigation and assessment of significance before work starts, and for a thorough understanding of the nature of the material being treated, its environmental context and the causes of its decay. However, the process of stone conservation is not complete until the details of the treatment carried out have been documented and lodged with the appropriate authorities. In this way we can learn in future from both successful and unsuccessful projects.
It hardly needs stating that well-trained and highly skilled conservators are essential for successful stone conservation. Through their workmanship, the efforts of architects, archaeologists, geologists and other consultants come to fruition. Sadly, whilst there are numerous stonemasonry courses in the UK, opportunities for stonework conservation training are limited, and many conservators learn on the job. While such training may be sufficient to help them develop practical craft skills, it is unlikely to produce conservators with the knowledge required to make informed judgements about how to treat a particular piece of stone or an entire building. The situation is not helped by the lack of recognized qualifications in stone conservation. The accreditation scheme for conservators in the UK promoted by the Institute of Conservation is a step in the right direction, but it needs to be underpinned by more training opportunities. But no matter what stage we are at in our conservation careers, whether practical or professional, we all have more to learn. Ongoing education is the key to successful stone conservation, and it is hoped that this book will make a positive contribution to that process.
Stone Conservation: Principles and Practice
