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Defining, mapping and assessing deterioration patterns in stone conservation projects
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Original article
Defining, mapping and assessing deterioration patterns in stone conservation projects José Delgado Rodrigues Nat. Lab. of Civil Engineering, Avenue Brasil, 101, 1700-066 Lisbon, Portugal
a r t i c l e
i n f o
Article history: Received 26 February 2014 Accepted 16 June 2014 Available online xxx Keywords: Deterioration patterns Mapping Deterioration assessment Stone conservation Black crusts Dry-laid stone constructions
a b s t r a c t Deterioration patterns are the visible consequences of the impact of environment factors on the stone objects. They depend on the type and severity of the external agents and on the type of substrate and its vulnerabilities. When properly understood, they may serve as key-indicators of the decay processes and of the possible causes of the observed damage. Correctly describing the deterioration patterns is an essential requisite when studying exposed stone objects, to understand the problems, to identify conservation needs and to define conservation actions. This paper discusses a few types of deterioration patterns to illustrate the need of choosing accurate definitions to describe them with the aim of reducing ambiguity when crossing the border between theory and practical application. The paper discusses a few deterioration patterns (detachment forms, black crusts and patina) that are currently found in conservation of built cultural heritage to highlight the importance of reducing the ambiguity that is frequently associated to such descriptive terms, aiming at better using them when acting in the passage from diagnostics to conservation actions. When mapping the spatial distribution of deterioration patterns, the most widely used methodologies are appropriate for scientific studies, but their usefulness to prepare and implement conservation interventions is much smaller. The paper proposes an innovative methodology to identify, describe and classify conservation problems and to prepare the documentation to support the tender documents in conservation interventions. A new methodology to help assessing the risk of structural instability and to help defining priorities in maintenance strategies in dry-stone laid constructions is presented. © 2014 Elsevier Masson SAS. All rights reserved.
1. Introduction Deterioration patterns are the visible component of the decay processes that are operating onsite. They reflect the interaction between the stone intrinsic components and the environment or extrinsic factors. They provide relevant information on the evolving decay mechanisms, but in general they are not univocal signatures of those processes. In complex cases, deterioration patterns alone may be insufficient to interpret the situation, and complementary data may be required to support more consistent diagnostics. The deterioration patterns are essential tools for the conservation scientist to work with, and constitute the basic units to define the conservation actions to be implemented. To fulfil these essential functions, they need to be conceptualised with this objective in mind, and have to be defined in precise terms, by using unambiguous terminology and by adopting concepts that might
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be informative for the diagnostics phase and be operative for the intervention action. Deterioration patterns, under this designation or similar others (erosion, alteration, degradation, decay forms) have a long tradition of use in the Earth Sciences disciplines, namely Geology and Geomorphology. The Building Industry borrowed some concepts from those disciplines and has introduced others that would better reflect the situation on stone ware buildings. Assimilation of terms and introduction of new ones are language and culture dependent, and it is not unexpected that any national or even regional communities would use slightly or even totally distinct vocabularies for describing the same deterioration problems. In the decade of 1970, an international move was initiated inside ICOMOS to establish a common terminology to describe deterioration patterns on built heritage. The Group Petrography, of the ICOMOS International Scientific Committee for Stone (ISCS), firstly headed by J. Ragot and subsequently by A. Arnold and D. Jeanette, made an enormous effort to gather, harmonise and define a certain number of terms taken from English, French and German scientific literature, and a set of highly valuable internal documents was prepared. However, the task of finding a common understanding
http://dx.doi.org/10.1016/j.culher.2014.06.007 1296-2074/© 2014 Elsevier Masson SAS. All rights reserved.
Please cite this article in press as: J. Delgado Rodrigues, Defining, mapping and assessing deterioration patterns in stone conservation projects, Journal of Cultural Heritage (2014), http://dx.doi.org/10.1016/j.culher.2014.06.007
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among the three languages showed to be harder than expected and no final document could be produced. Based on those internal documents, during a period of chairmanship of that Group, the author had the opportunity to prepare a personal selection of terms, which was published in the Group Newsletter [1]. A new impulse to that group was given in the decade of 2000, by Isabelle Pallot Frossard and Véronique Vergès-Belmin and a bilingual version of a glossary was produced and made accessible for personal downloading [2]. Meanwhile, the scientific community continued the effort to better understand decay phenomena and to establish more accurate terminologies to describe and map them. Some of them were typical authored [3] papers, while others were produced as national [4], or group [5,6] contributions. These documents share a common background to support their definitions: their basic descriptive units are chosen to correspond as closely as possible to a specific degradation process, thus trying to create unambiguous descriptors of the problems that call for the use of this kind of documents. In this perspective, such documents constitute an enormous progress in the field, and are strong pillars to build up a common understanding, since they use terminologies that can be shared by people with distinct backgrounds, or coming from different regions, or having different mother languages. At the scientific level, the ground is pretty well defined and in spite of improvements that can always be added, the existing elements constitute a solid basis for any professionals wishing to use others’, or share their own experience. In spite of this enormous progress, the passage from the scientific analysis of decay processes to the professional implementation of conservation actions is less developed and still needs additional improvements. The present paper addresses a few topics considered relevant to better translate scientific data into executable conservation actions. 2. Defining and using degradation terms When preparing a glossary of deterioration patterns and concepts, there is a natural tendency to adapt terms currently in use in the profession or in related disciplines. The result of the work of any task group very often ends up in a compromise, when the efforts to harmonise are not enough to produce a fully coherent set of terminological definitions. One frequent drawback of the standard terminologies is the ambiguity that some terms convey when trying to use the information to support the choice of the appropriate conservation actions. When this happens, the end-user must make an effort to eliminate the ambiguity by providing a precise context for each term or concept he/she is using. This situation will be herewith illustrated with three common terms used in stone conservation: detachment forms, black crust, and patina. 2.1. Detachment forms When a stone surface is losing mass, the form, size and shape of the detaching elements may vary from powder and very small grains to large chips and plaques. Their characteristics inform on the degradation processes that are causing damage and are an essential input in the preparation of the required conservation actions. A proper description is always to be recommended, but the information conveyed by the terms used in this description has not the same value for both objectives. To discriminate between blistering, contour scaling, peeling and spalling (when thin elements are detaching) is of relevant interest when interpreting the mechanisms and searching for damage causes, but it is of minor
interest when defining conservation actions. All forms correspond to a detachment of thin plaquettes, with high lateral continuity, and all of them may require similar gluing actions and sealing of the external contours. When degradation events are causing detachment, the size of the stone elements in risk of being lost is the critical parameter in the specification of the needed conservation actions. Thick elements (thick plaques, chips and similar) usually need to be glued and eventually securely fastened to the substrate. Thin plaques (plaquettes), contour scaling and similar also need to be glued, but nailing them to the substrate tends to be unfeasible and most likely unnecessary given the lightweight in cause. When detachment is produced as individual grains, the grain size matters, but also the type of substrate has to be taken into account: powdering and chalking, frequent in very porous limestones, tend to correspond to hopeless situations and little is usually feasible to counteract them directly; sand disintegration coming from high porosity sandstones tends to be very difficult to solve or it is even hopeless, while sand disintegration in granites (and also sugaring, in marbles) when surface grains are still in place may have chances to be properly glued back. The detachment of scales is an intermediate situation between individual grains and plaques/plaquettes and usually corresponds to situations of very difficult treatment. Larger scales may accept treatment similar to plaques and plaquettes, while smaller ones tend to configure virtually impossible endeavours. Therefore, for conservation purposes, the key terms with direct practical implications are: detachment as individual grains (powdering and sand disintegration), as scales (specify actual size and frequency), as plaques, and as plaquettes (Fig. 1). 2.2. Black crusts Black crusts are widespread features in urban environments and are particularly frequent in carbonate substrates. They result from the interaction of urban pollutants with the stone and produce deterioration patterns requiring very demanding solutions. When in advanced stages of evolution, black crusts tend to detach from the substrate bringing about the complete loss of the stone surface. The qualifying term black has no discussion, but the essential descriptive kernel – crust – is ambiguous and some accepted definitions may be of little value for practical purposes. The ISCS Glossary defines crust as “Generally coherent accumulation of materials on the surface. A crust may include exogenic deposits in combination with materials derived from the stone.” and the only connection to the substrate is the possibility of having materials derived from it. When detailing this definition, the same document states that a black crust is a “Kind of crust developing generally on areas protected against direct rainfall or water runoff in urban environment. Black crusts usually adhere firmly to the substrate.”, therefore considering that a strong adhesion is what matters in its relation with the substrate. The concept underlying these definitions considers that a black crust is in fact a deposit that may eventually have some interaction with the substrate. Translating this concept to practical actions, it is acceptable to infer that black crusts can be removed with appropriate cleaning methods, and in the precise terms applicable to a superficial deposit. However, the concept of black crust may be defined with a significantly different amplitude. In fact, the interaction of the environment pollutants, especially SO2 and its derivatives, may start from the very surface of the object, but it rapidly progresses inside, dissolving calcium carbonate and re-depositing calcium sulphate. The result is the transformation of a thin layer in the outermost part of the stone, creating a brittle and fragile zone, very frequently underlined by a powdering and friable layer rich in gypsum. The
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Fig. 1. Detachment types. Thickness and length of detaching elements are the main parameters of interest for the conservator-restorer and therefore should be the basis of classification and applied as precisely as possible. Plaques (> 0.5 cm in thickness) and plaquettes (< 0.5 cm), in the top, are to fix and seal individually, while powdering and scaling (bottom), only can be addressed globally, when consolidation is feasible.
presence of such a fragile zone is of paramount importance in stone conservation and it cannot be ignored or depreciated in any serious conservation action in areas where black crusts are present (Fig. 2). Transformations in the stone outermost layers are very frequent and may be due to natural causes (dissolution, deposition, porosity changes, etc.) or artificial actions (treatments). The term crust has been used to describe the occurrence of this transformed outer layer, and in the context here presented, the mentioned fragile zone is taken as a morphological equivalent of a crust and is named as such. The very frequent and well known situation of carbonate stones in sheltered areas exposed to urban environments can be described as follows: dark to very dark surfaces made of accumulated particles deposited over a fragile stone surface weakened by the transformation of calcite into gypsum; in some spots, the upper stone layer is detaching and a powdering white zone is crumbling below the black detaching layer. The set of these 3 components – black surface deposit + fragile zone + underlining powdery layer – constitutes a whole entity called black crust. Under this assumption, the ISCS definition is not appropriate to describe the entire set of degradation features. In fact, a black crust is the addition of two totally distinct components: a superficial deposit (black) plus an underlying fragile stone layer (crust). The consequences in terms of practical use are enormous. Removal of black crusts, considered under this concept, is no more acceptable. Proper cleaning of black crusts implies that only the black component (the superficial deposit) is removed, leaving the crust component (which in fact is frequently white) untouched.
Knowing the extreme difficulty that the cleaning and stabilisation of areas affected by black crusts usually present to conservator-restorers, it seems quite illogical to describe such a stone degradation feature with terms that convey the false impression that black crusts can be removed as any other superficial deposit. 2.3. Patina This term is so often used in the stone conservation field as a multipurpose descriptor that it is usually taken as an obvious and unequivocal concept. In reality, it is exactly the opposite: a too vague and ill-defined concept, a descriptor used to name forms of different natures, distinct origins, and distinct conservation meanings. In the ISCS Glossary, the term patina is defined as: “Chromatic modification of the material, generally resulting from natural or artificial ageing and not involving in most cases visible surface deterioration.” Leaving aside the term generally, for its apparent lack of specific content (what else can ageing be other than natural or artificial?), we shall retain that patina is perceived as a colour change and that surface deterioration is not involved or perceived as such (most cases is, here, more confusing than clarifying and could well be avoided). However, the two terms accepted as subtypes of patina in this Glossary, iron rich patina and oxalate patina, do not help to better define the main term, and even contribute with some additional ambiguity. In fact, iron rich patinas are real surface deposits (at least most of them) and oxalate patinas are
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Fig. 2. Examples of black crusts in limestone substrates in sheltered areas. The four situations fit in the general paradigm of black crust. They contain a superficial dark deposit, a fragile crust in the outermost part of the stone and a powdery zone below the crust. The bottom pictures (C–D) were taken in laser cleaning test spots.
transformations of the substrate and therefore they could be called as crusts (in the sense given in the present paper). Both sub-types integrate very significant transformations and it is hard to conceive how they can match the basic principle of “. . . not involving. . . visible surface deterioration”. In stone conservation, the term noble patina is sometimes quoted and in general readers understand what is meant with it. In spite of being understood, the concept remains mostly an undefinable one because its very essence lies on its standing on an edge itself, it is a borderline concept, thus lacking enough substance from one or from both sides to build up a coherent definition. In fact, when an external component interacts with a stone surface the consequences that may happen at fall in one of these categories: • deposit, when the cause of perceived visual modification can be attributed to something existing on top of the surface; • crust, when the result is a modification of the stone itself. When we use the term patina (here taken as a synonym of noble patina) it is meant that we are referring to something that is happening at the surface (perceived as a colour change) but that we are unable to ascertain whether it is a deposit or a crust. In spite of the lack of a specific definition, the term patina still has a relevant place in the field of stone conservation, especially in the practical side of the profession. Although a certain degree of ignorance is implied in its use (to be a deposit or a crust, that is the question), when properly used, the term signifies that whatever has been imposed on the surface is certainly gentle, weak, or incipient and therefore it describes a comfortable situation when the preservation of the stone surface is the objective (Fig. 3). Its usefulness is well demonstrated when the cleaning of stone surfaces is considered. Cleaning is an operation used to eliminate components that are alien to the stone itself, and the process is stopped when the elimination reaches a level considered as
adequate for the objective and safe in terms of preserving the stone surface intact. The definition of this level is not always straightforward and may involve significant uncertainties. When the cleaning method is well mastered, the cleaning levels can be progressively incremented and stopped close to the stone surface. When a level can be defined as having a patina that does not disfigure the image of the object (noble patina), the operator can be ensured that something alien is still on top of the surface and no damage was caused by the cleaning method. So, in spite of being unable to define patina, when this condition is reached we know that it corresponds to a right and harmless level to stop cleaning. In this context, the term patina constitutes a valuable concept in stone conservation. 3. From patterns to entity mapping The understanding of stone deterioration processes is based on the systematic observation and study of field data, preferably supplemented with in-depth analytical investigation. Lab research felt a substantial progress with the boom of novel and sophisticated instruments that largely incremented the research capacity, allowing researchers in stone conservation to incorporate the knowledge coming from other sciences and going more and more in-depth in the search to understand and to describe decay phenomena. In conservation practice, progress has been much slower, but the appearance of new non-destructive tests (NDT) and novel uses of imaging technologies are contributing to raise the level of field interventions and to reduce the risks introduced by insufficiently supported decisions. Whatever the lab sophistication and the extension and depth of the field survey, the passage from the research step to the practical implementation of solutions always has to go through an integration step in which the research data are combined and synthesised and the outcome is confronted with the reality of the object under consideration. The amount and depth of the analytical data, on one
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Fig. 3. Patina and its common neighbours in limestone substrates. A. Aspect of a cleaning test with pressurized water (left). The whitish stripes correspond to areas rinsed by rainwater runoff; they are clean or slightly colonized by greyish microorganisms. The black deposits seen in the upper part were cleaned and have left what we can call a brownish patina, seen in the lower part of the photo. None of the areas display the real color of the stone substrate. B. Laser cleaning test area (right). The removal of the surface deposit in an area with visible occurrences of black crusts leaves the surface with a yellowish patina. Whatever the origin and composition of this patina might be, it signifies that the stone surface is not yet hit and therefore this stage may be defined as a harmless cleaning level.
side, and the complexity of the real situations, on the other side, make this confrontation a very challenging operation, and make it the key-action in any conservation intervention. It is a component of the decision-making process and any serious approach to the conservation of stone surfaces needs to look at this step as a crucial phase of the intervention process. Mapping of degradation features and of any other kind of anomalies is a very powerful tool to help the building up of the synthesis between lab and field data and to help transforming research data into useful practical notions that will become inductors of the adequate conservation actions. In this assumption, mapping is not a goal itself, and rather it has to be organised to help reaching the implementation of the major objectives of the intervention project. Mapping includes any spatial representation of numeric data, visual impressions, symbolic translators of data or events, and of actions carried or to be carried out. Accepting as granted the paramount importance of this working tool, this paper will discuss in more detail a novel use of mapping specifically developed to improve the dialogue between researchers, tutelage actors and the contractors, aiming at better clarifying the object of the intervention project and at minimising the risks of slippage in the budget costs. The usual mapping procedure employed in the preparation of conservation interventions uses deterioration patterns as mapping units. Biological colonisation, black crusts, fractures, open joints, past repairs, etc. are examples of mapping units currently found in the literature. Several authors have elaborated comprehensive schemes to map field data (Fitzner [3]) and some glossaries even suggest symbols to unambiguously map the identified units (NORMAL, LNEC). This mapping methodology is especially useful to identify decay patterns and, through them, to help to interpret the acting degradation processes and to identify the degradation causes. A differential distribution of biological colonisation may be due to different exposure conditions, but a localised and elongated concentration of green algae may point out to the existence of a leakage source nearby. Typically, this is the kind of use (of essential importance, it should be said) given to this thematic mapping methodology. This same kind of maps has been used to identify conservation needs, to prepare tender documents and to estimate intervention costs. In scientific literature, the roles and interactions between heritage tutelage, conservation scientists and entrepreneurs are rarely tackled, in spite of the obvious fact that any deflation in each one’s role and any biasing in the interrelationships may result
in serious impairment of the conservation intervention protocol, ending up in risky situations to the heritage item. One reason to cause undesired outcomes is the lack of clarity of the conservation actions that are supposed to be carried out in the object. The use of thematic maps to estimate the labour involvement may be a source of confusion that may be subsequently taken by contractors to extend the contract limits. Experiences with this type of contractors moved the author to search for a different way of mapping anomalies and conservation needs, aiming at reducing the uncertainties in the preparation of tender documents and at facilitating the negotiations when selecting the contractor. This new mapping system, especially conceived to be used in architectural heritage conservation, is described in the following section. Mapping of conservation entities: heritage items requiring intensive conservation care frequently exhibit complex situations that imply difficulties in the identification of the what to do, how much to do and where to do lists, that are essential to prepare a complete and fair arrangement between owner and contractor. Some actions are difficult to be exactly quantified (how much of the soiling is loose dirt? How much of it are black crusts? How many fractures are there to seal? Etc.), and others occur as superimposed anomalies (fragments to be stabilised and fractures to be sealed in areas also needing to be cleaned, etc.) (Fig. 4). Competent conservator-restorers look at the object from a holistic approach and tend to split the object in areas requiring homogeneous treatments and for each identified type a specific plan of actions is designed and implemented. This same concept was taken into consideration when searching for a mapping methodology more friendly for the tutelage and for the contractor. It consists in the identification of homogeneous units in which the conservation needs can be identified and described accurately enough. These units are subsequently characterised and their position in the object is to be unambiguously identified. The case study presented below illustrates this concept, but briefly one may say that a column shaft and the corresponding capital may correspond to distinct units when needing different conservation actions, a decorated area and the nearby rough wall are to be considered distinct units as well, etc. (Fig. 5). This mapping method is not contradictory to the usual pattern mapping and is by no means mutually exclusive. Both are relevant instruments in the preparation of conservation interventions and therefore are to be taken more as complementary rather than as alternatives. Pattern mapping is more adequate to identify the alteration patterns and to understand the decay mechanisms, while
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Fig. 4. Examples of complex situations in a limestone substrate. The superposition of several deterioration processes and different severity degrees (left) and the high three-dimensionality of the object (right) make it impossible to individually locate and quantify each degradation form to support tender documents.
the entity mapping is better suited to serve as a preparatory tool when launching a competition to implement a conservation intervention and to select the contractor and to negotiate the contract terms. A possible programme to start a conservation intervention could be:
• • • •
define the objectives and constraints of the intervention; identify the conservation problems; describe, characterise and map the deterioration patterns; interpret the collected data, identify the degradation causes and define the appropriate conservation actions; • divide the object into entities according to the similitude of the conservation needs they require;
• describe each unit as detailed as feasible in terms of the problems that affect each of them, and typify the conservation actions expected to be applied to solve the problems.
From the author’s personal experience, in competitions launched under this concept, competitors should be asked to describe their own approach to solve the problems identified in each conservation entity and this approach should be given a large weight in the selection algorithm. This option obliges each competitor to carefully analyse the object, thus limiting largely the possibility of misinterpretations and undervaluation of the conservation needs and working difficulties. Extra costs are strongly minimised and the work budget has much better chances to be kept within the defined limits.
Fig. 5. Examples of spatial definitions of the entity-mapping units in Arco da Rua Augusta, in Lisbon. The units represent areas with a certain homogeneity in terms of morphology, decay problems and conservation needs. For the preparation of the intervention, the entire arch was sub-divided in 51 entity units.
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Fig. 6. Aqueduct of Segovia. View of the multi-arch stretch and detail of the dry-stone masonry.
4. Defining and mapping stability indicators in a dry-stone built monument Deterioration of stone elements in any built object usually implies some impairment of the surface condition and may entail relevant losses in terms of object significance. In specific circumstances, when the stone element is a direct component of the structural framework, any impairment of the element integrity may have implications that go far beyond the significance and may represent severe threats to the structural stability of the entire object. To identify damage signs and to assess their risk is of paramount importance in these situations. Structures made in dry-stone are particularly sensitive to structural unbalances and any deterioration progress needs to be carefully assessed in terms of the influence it may have in creating or incrementing unbalanced loads. A pilot study carried out on the Segovia Aqueduct provided an opportunity to develop and test a methodology based on several risk indicators. These indicators constitute the basis to classify the
elements in terms of intrinsic risk and to identify the areas with priority needs of intervention actions. The highlights of that methodology are described and illustrated in the following section. Methodology to identify and assess the stability risk: the Aqueduct of Segovia is an impressive structure built in the Roman times, around the I century AD, having undergone some relevant reconstruction interventions in the XV and XVI centuries. In its urban course, the Aqueduct assumes the form of a bridge, the “bridge-aqueduct”, with an elegant multi-arch, two-level structure spanning for more than 700 m (Fig. 6). The Aqueduct is built of large stone blocks stacked in a drymasonry style. The stone material includes a few varieties of granites, medium to coarse-grained, quarried in the city and in its vicinity. The most widespread degradation form affecting the Aqueduct is sand disintegration. Some blocks are slightly to moderately rounded, while others are severely affected, and it is highly
Fig. 7. Treatment of the reported data. Mapping of the highest ratings of each class (left); grouping of any ratings of 4 and 5, rating 3 and the sum of the major 4 units (right).
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Fig. 8. Synopsis of the obtained results in terms of the priorities of the conservation needs. Pink dots represent the elements with first priority needs and in yellow those with second priority needs.
probable that the most vulnerable ones have disappeared long ago and replaced with new ones. All these credibly original blocks suffer from moderate to severe mass loss. Decay of the stone components leads to a progressive weakening of the block strength and the mass loss induces a progressive rounding of the blocks with potential unbalancing of the carried loads. A monument of this size and age has permanent conservation needs and requires a robust methodology to identify what to do, where and when, and for such a sensitive construction this is not a simple endeavour. To find a simple and assertive methodology to identify such conservation needs is thus of particular relevance here. Besides the study of several other aspects concerning the degradation factors and forms, a specific analysis of the factors that may contribute to stability problems was carried out [7]. A specific glossary was prepared to identify and map deterioration patterns. These vary from disintegration patterns, to rounding of blocks, presence of fractures, etc. The field inspection and reporting were made block by block, and a subsequent deskwork was done to synthesise the information and to develop a risk assessment path. The basic working tool was the glossary. Besides reporting any aspect deemed as potentially informative, the basic descriptive classes of deterioration patterns were: D – disintegration patterns; S – shape of the block, having the “perfect” original cuboid shape as reference; F – presence of fractures; I – presence of past interventions. For each class, blocks were given a rating value from 0 to 5 according to the perceived severity for each item that affect them. All data were reported directly in individual prospects of the Aqueduct. Disintegration is a typical deterioration process in granite materials and it is particularly noticeable in the original Roman blocks. Besides implying the reduction of the block overall strength disintegration leads to the progressive rounding of blocks and may induce unbalance of the carried loads. This situation was
reflected in the methodology through the shape factor (S). For the risk assessment, fractures (F) that cross the entire block were given high ratings, since they may constitute signs of unbalancing of loads and of potential safety problems. The presence of past interventions (I) was considered a relevant indicator and these occurrences were integrated in the assessment. The Aqueduct was restored a few decades ago and some invasive actions were made, such as the gluing of fractures, deep consolidation through injection holes and shape reconstructions. These types of actions presuppose that the blocks had to be in a very bad condition to justify such intrusive actions. The blocks with this past history have to be monitored and assessed for their present condition, reason why they were taken as a risk indicator. Treatment of data was carried out with the objective of identifying situations with distinct safety risks, seen exclusively from the point of view of the condition of the individual blocks. Several trials were made, namely by considering each mapping unit individually, by associating two of them, or by summing up all the major four classes (Fig. 7). The fieldwork was carried out by professional conservatorrestorers following a protocol presented and discussed beforehand with them. The field reports were subsequently analysed and synthetic maps were prepared to facilitate their interpretation and the assessment of the overall significance for the structural safety of the Aqueduct. The results of the fieldwork were firstly ranked and lower values eliminated from the graphic representation to facilitate their interpretation, as exemplified on Fig. 7. Subsequently these data were jointly analysed with the aim of identifying the priority situations in terms of conservation needs. This has implied the definition of a series of criteria to weight the different classes trying to find a result representative of the real situation. For instance, the simultaneous occurrence of any two ratings 4 or 5 as well as one such a rating plus a sum of 8 of all parameters would imply first priority needs,
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while a single 4 or 5 would include that block in a second priority group. These two levels of conservation needs were applied to the entire pilot area, as illustrated on Fig. 8. 5. Conclusions The study of deterioration patterns is currently used in the diagnostic phases to describe and characterise acting decay mechanisms and to identify the responsible degradation causes. Both are essential elements to help finding conservation needs and to plan intervention actions. The way that deterioration patterns are defined and mapped has not the same meaning in a context of scientific research, to identify the mechanisms and causes of decay, or when the planning and implementation of a conservation intervention is concerned. The individual identification of deterioration patterns and their precise characterisation and mapping are the most appropriate procedures when identifying mechanisms and causes because deterioration patterns are good fingerprints directly connected to their immediate past genetic context. Working tools such as the ICOMOS Glossary and mapping methodologies such as the one recommended by Fitzner et al. are appropriate to achieve the mentioned objectives. When preparing information to be handled on to practical implementation, these methodologies show some flaws, namely when definitions are not sufficiently unambiguous and when mapping options do not match the practical reasoning and the negotiation and contractual needs. When placed in the crossing step between diagnostics and implementation phases, the conservation scientist needs to have a critical attitude as regards data gathered and documents prepared in the diagnostics phases, and to aim at preparing elements that adequately suit the subsequent phases of negotiation of contracts and implementation of conservation actions. To do it, some of the used concepts might need to be improved or redefined as illustrated here with the terms used to describe detachment forms, black crusts and patina. The methodology used to map deterioration patterns is also susceptible to be better adapted to fit this objective, namely by moving from mapping of individual deterioration patterns to comprehensive units having reasonably homogeneous problems and conservation needs. A novel methodology to reach this desideratum is proposed and illustrated in the paper. Its successful implementation in one major monument in Lisbon attests its applicability. The conservation problems of a dry-stone masonry monument, such as the Segovia Aqueduct, are directly related to what affects each individual block and therefore they require to be tackled at the exact level of these elementary construction elements. The
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potential influence of deterioration patterns in the overall stability of the structure of the Aqueduct was assessed by means of a detailed study of every single block and by selecting a certain number of degradation features with higher impact in the strength reduction (namely the occurrence of sand disintegration forms) and in the unbalancing of the carried loads (namely the rounding of blocks). The case study here presented describes the essential methodology working steps and illustrates how the identification of priorities was achieved. The study was carried out in a pilot area and the site reports show that the methodology can be easily extended to the entire structure. Substantial savings in future conservation interventions are expected to be obtained with the clear identification of actions to be implemented and a better delimitation of areas to be intervened. Acknowledgements The author acknowledges the exchange of opinions with José Aguiar and Nuno Proenc¸a on the mapping methodology and Sandra Alves for her diligence in its implementation in Arco da Rua Augusta. Atelier 15 coordinated the conservation intervention project. The study on the Segovia Aqueduct was sponsored by World Monuments Fund Spain. Thanks are due to Pablo Longoria and Norma Barbacci for their confidence and support. The fieldwork was diligently and professionally carried out by Carlos Sanz and a team of Nova Conservac¸ão, Lda. The author thanks Marisa Tabasso, A. Elena Charola, Dória Costa and M. João Revez for their comments on the manuscript. References [1] J. Delgado Rodrigues, “Proposal for a terminology of stone decay forms on monuments”. ICOMOS Stone Committee Newsletter, 1/1991, 1991 (Unpublished intern document). [2] ICOMOS, “Illustrated glossary on stone deterioration patterns”, ICOMOS International Scientific Committee for Stone, 2008 (ISBN: 978-2-918086-00-0). [3] B. Fitzner, K. Heinrichs, “Damage diagnosis on stone monuments – weathering forms, damage categories and damage indices”, in: R. Prikryl, H.A. Viles (Eds.), Understanding and Managing Stone Decay, Proc. Int. Conf. SWAPNET 2001, Charles University in Prague, The Karolinum Press, 2002, pp. 11–56. [4] 1/88. Normal, “Alterazioni macroscopiche dei material lapidei: lessico”. “Macroscopic alteration of stone materials: glossary”, Comas Graphica, Rome, 1990 (36 p.). [5] F.M.A. Henriques, J. Delgado Rodrigues, L. Aires-Barros, N. Proenc¸a, “Materiais pétreos e similares: terminologia das formas de alterac¸ão e degradac¸ão”. ICTInformac¸ao Técnica, Patologia e Reabilitac¸ão das Construc¸ões, LNEC, ITPRC 2, Lisbon, 2004 (39 p.). [6] J. Ordaz, R.M. Esbert, “Glosario de términos relacionados con el deterioro de las piedras de construcción”, Mater. Construc. 209 (1988) 39–45. [7] J. Delgado Rodrigues, “Segovia Aqueduct. Degradation forms, condition survey and conservation needs”, 2013 (Unpublished internal report for World Monuments Fund Spain).
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Original article
Defining, mapping and assessing deterioration patterns in stone conservation projects José Delgado Rodrigues Nat. Lab. of Civil Engineering, Avenue Brasil, 101, 1700-066 Lisbon, Portugal
a r t i c l e
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Article history: Received 26 February 2014 Accepted 16 June 2014 Available online xxx Keywords: Deterioration patterns Mapping Deterioration assessment Stone conservation Black crusts Dry-laid stone constructions
a b s t r a c t Deterioration patterns are the visible consequences of the impact of environment factors on the stone objects. They depend on the type and severity of the external agents and on the type of substrate and its vulnerabilities. When properly understood, they may serve as key-indicators of the decay processes and of the possible causes of the observed damage. Correctly describing the deterioration patterns is an essential requisite when studying exposed stone objects, to understand the problems, to identify conservation needs and to define conservation actions. This paper discusses a few types of deterioration patterns to illustrate the need of choosing accurate definitions to describe them with the aim of reducing ambiguity when crossing the border between theory and practical application. The paper discusses a few deterioration patterns (detachment forms, black crusts and patina) that are currently found in conservation of built cultural heritage to highlight the importance of reducing the ambiguity that is frequently associated to such descriptive terms, aiming at better using them when acting in the passage from diagnostics to conservation actions. When mapping the spatial distribution of deterioration patterns, the most widely used methodologies are appropriate for scientific studies, but their usefulness to prepare and implement conservation interventions is much smaller. The paper proposes an innovative methodology to identify, describe and classify conservation problems and to prepare the documentation to support the tender documents in conservation interventions. A new methodology to help assessing the risk of structural instability and to help defining priorities in maintenance strategies in dry-stone laid constructions is presented. © 2014 Elsevier Masson SAS. All rights reserved.
1. Introduction Deterioration patterns are the visible component of the decay processes that are operating onsite. They reflect the interaction between the stone intrinsic components and the environment or extrinsic factors. They provide relevant information on the evolving decay mechanisms, but in general they are not univocal signatures of those processes. In complex cases, deterioration patterns alone may be insufficient to interpret the situation, and complementary data may be required to support more consistent diagnostics. The deterioration patterns are essential tools for the conservation scientist to work with, and constitute the basic units to define the conservation actions to be implemented. To fulfil these essential functions, they need to be conceptualised with this objective in mind, and have to be defined in precise terms, by using unambiguous terminology and by adopting concepts that might
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be informative for the diagnostics phase and be operative for the intervention action. Deterioration patterns, under this designation or similar others (erosion, alteration, degradation, decay forms) have a long tradition of use in the Earth Sciences disciplines, namely Geology and Geomorphology. The Building Industry borrowed some concepts from those disciplines and has introduced others that would better reflect the situation on stone ware buildings. Assimilation of terms and introduction of new ones are language and culture dependent, and it is not unexpected that any national or even regional communities would use slightly or even totally distinct vocabularies for describing the same deterioration problems. In the decade of 1970, an international move was initiated inside ICOMOS to establish a common terminology to describe deterioration patterns on built heritage. The Group Petrography, of the ICOMOS International Scientific Committee for Stone (ISCS), firstly headed by J. Ragot and subsequently by A. Arnold and D. Jeanette, made an enormous effort to gather, harmonise and define a certain number of terms taken from English, French and German scientific literature, and a set of highly valuable internal documents was prepared. However, the task of finding a common understanding
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among the three languages showed to be harder than expected and no final document could be produced. Based on those internal documents, during a period of chairmanship of that Group, the author had the opportunity to prepare a personal selection of terms, which was published in the Group Newsletter [1]. A new impulse to that group was given in the decade of 2000, by Isabelle Pallot Frossard and Véronique Vergès-Belmin and a bilingual version of a glossary was produced and made accessible for personal downloading [2]. Meanwhile, the scientific community continued the effort to better understand decay phenomena and to establish more accurate terminologies to describe and map them. Some of them were typical authored [3] papers, while others were produced as national [4], or group [5,6] contributions. These documents share a common background to support their definitions: their basic descriptive units are chosen to correspond as closely as possible to a specific degradation process, thus trying to create unambiguous descriptors of the problems that call for the use of this kind of documents. In this perspective, such documents constitute an enormous progress in the field, and are strong pillars to build up a common understanding, since they use terminologies that can be shared by people with distinct backgrounds, or coming from different regions, or having different mother languages. At the scientific level, the ground is pretty well defined and in spite of improvements that can always be added, the existing elements constitute a solid basis for any professionals wishing to use others’, or share their own experience. In spite of this enormous progress, the passage from the scientific analysis of decay processes to the professional implementation of conservation actions is less developed and still needs additional improvements. The present paper addresses a few topics considered relevant to better translate scientific data into executable conservation actions. 2. Defining and using degradation terms When preparing a glossary of deterioration patterns and concepts, there is a natural tendency to adapt terms currently in use in the profession or in related disciplines. The result of the work of any task group very often ends up in a compromise, when the efforts to harmonise are not enough to produce a fully coherent set of terminological definitions. One frequent drawback of the standard terminologies is the ambiguity that some terms convey when trying to use the information to support the choice of the appropriate conservation actions. When this happens, the end-user must make an effort to eliminate the ambiguity by providing a precise context for each term or concept he/she is using. This situation will be herewith illustrated with three common terms used in stone conservation: detachment forms, black crust, and patina. 2.1. Detachment forms When a stone surface is losing mass, the form, size and shape of the detaching elements may vary from powder and very small grains to large chips and plaques. Their characteristics inform on the degradation processes that are causing damage and are an essential input in the preparation of the required conservation actions. A proper description is always to be recommended, but the information conveyed by the terms used in this description has not the same value for both objectives. To discriminate between blistering, contour scaling, peeling and spalling (when thin elements are detaching) is of relevant interest when interpreting the mechanisms and searching for damage causes, but it is of minor
interest when defining conservation actions. All forms correspond to a detachment of thin plaquettes, with high lateral continuity, and all of them may require similar gluing actions and sealing of the external contours. When degradation events are causing detachment, the size of the stone elements in risk of being lost is the critical parameter in the specification of the needed conservation actions. Thick elements (thick plaques, chips and similar) usually need to be glued and eventually securely fastened to the substrate. Thin plaques (plaquettes), contour scaling and similar also need to be glued, but nailing them to the substrate tends to be unfeasible and most likely unnecessary given the lightweight in cause. When detachment is produced as individual grains, the grain size matters, but also the type of substrate has to be taken into account: powdering and chalking, frequent in very porous limestones, tend to correspond to hopeless situations and little is usually feasible to counteract them directly; sand disintegration coming from high porosity sandstones tends to be very difficult to solve or it is even hopeless, while sand disintegration in granites (and also sugaring, in marbles) when surface grains are still in place may have chances to be properly glued back. The detachment of scales is an intermediate situation between individual grains and plaques/plaquettes and usually corresponds to situations of very difficult treatment. Larger scales may accept treatment similar to plaques and plaquettes, while smaller ones tend to configure virtually impossible endeavours. Therefore, for conservation purposes, the key terms with direct practical implications are: detachment as individual grains (powdering and sand disintegration), as scales (specify actual size and frequency), as plaques, and as plaquettes (Fig. 1). 2.2. Black crusts Black crusts are widespread features in urban environments and are particularly frequent in carbonate substrates. They result from the interaction of urban pollutants with the stone and produce deterioration patterns requiring very demanding solutions. When in advanced stages of evolution, black crusts tend to detach from the substrate bringing about the complete loss of the stone surface. The qualifying term black has no discussion, but the essential descriptive kernel – crust – is ambiguous and some accepted definitions may be of little value for practical purposes. The ISCS Glossary defines crust as “Generally coherent accumulation of materials on the surface. A crust may include exogenic deposits in combination with materials derived from the stone.” and the only connection to the substrate is the possibility of having materials derived from it. When detailing this definition, the same document states that a black crust is a “Kind of crust developing generally on areas protected against direct rainfall or water runoff in urban environment. Black crusts usually adhere firmly to the substrate.”, therefore considering that a strong adhesion is what matters in its relation with the substrate. The concept underlying these definitions considers that a black crust is in fact a deposit that may eventually have some interaction with the substrate. Translating this concept to practical actions, it is acceptable to infer that black crusts can be removed with appropriate cleaning methods, and in the precise terms applicable to a superficial deposit. However, the concept of black crust may be defined with a significantly different amplitude. In fact, the interaction of the environment pollutants, especially SO2 and its derivatives, may start from the very surface of the object, but it rapidly progresses inside, dissolving calcium carbonate and re-depositing calcium sulphate. The result is the transformation of a thin layer in the outermost part of the stone, creating a brittle and fragile zone, very frequently underlined by a powdering and friable layer rich in gypsum. The
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Fig. 1. Detachment types. Thickness and length of detaching elements are the main parameters of interest for the conservator-restorer and therefore should be the basis of classification and applied as precisely as possible. Plaques (> 0.5 cm in thickness) and plaquettes (< 0.5 cm), in the top, are to fix and seal individually, while powdering and scaling (bottom), only can be addressed globally, when consolidation is feasible.
presence of such a fragile zone is of paramount importance in stone conservation and it cannot be ignored or depreciated in any serious conservation action in areas where black crusts are present (Fig. 2). Transformations in the stone outermost layers are very frequent and may be due to natural causes (dissolution, deposition, porosity changes, etc.) or artificial actions (treatments). The term crust has been used to describe the occurrence of this transformed outer layer, and in the context here presented, the mentioned fragile zone is taken as a morphological equivalent of a crust and is named as such. The very frequent and well known situation of carbonate stones in sheltered areas exposed to urban environments can be described as follows: dark to very dark surfaces made of accumulated particles deposited over a fragile stone surface weakened by the transformation of calcite into gypsum; in some spots, the upper stone layer is detaching and a powdering white zone is crumbling below the black detaching layer. The set of these 3 components – black surface deposit + fragile zone + underlining powdery layer – constitutes a whole entity called black crust. Under this assumption, the ISCS definition is not appropriate to describe the entire set of degradation features. In fact, a black crust is the addition of two totally distinct components: a superficial deposit (black) plus an underlying fragile stone layer (crust). The consequences in terms of practical use are enormous. Removal of black crusts, considered under this concept, is no more acceptable. Proper cleaning of black crusts implies that only the black component (the superficial deposit) is removed, leaving the crust component (which in fact is frequently white) untouched.
Knowing the extreme difficulty that the cleaning and stabilisation of areas affected by black crusts usually present to conservator-restorers, it seems quite illogical to describe such a stone degradation feature with terms that convey the false impression that black crusts can be removed as any other superficial deposit. 2.3. Patina This term is so often used in the stone conservation field as a multipurpose descriptor that it is usually taken as an obvious and unequivocal concept. In reality, it is exactly the opposite: a too vague and ill-defined concept, a descriptor used to name forms of different natures, distinct origins, and distinct conservation meanings. In the ISCS Glossary, the term patina is defined as: “Chromatic modification of the material, generally resulting from natural or artificial ageing and not involving in most cases visible surface deterioration.” Leaving aside the term generally, for its apparent lack of specific content (what else can ageing be other than natural or artificial?), we shall retain that patina is perceived as a colour change and that surface deterioration is not involved or perceived as such (most cases is, here, more confusing than clarifying and could well be avoided). However, the two terms accepted as subtypes of patina in this Glossary, iron rich patina and oxalate patina, do not help to better define the main term, and even contribute with some additional ambiguity. In fact, iron rich patinas are real surface deposits (at least most of them) and oxalate patinas are
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Fig. 2. Examples of black crusts in limestone substrates in sheltered areas. The four situations fit in the general paradigm of black crust. They contain a superficial dark deposit, a fragile crust in the outermost part of the stone and a powdery zone below the crust. The bottom pictures (C–D) were taken in laser cleaning test spots.
transformations of the substrate and therefore they could be called as crusts (in the sense given in the present paper). Both sub-types integrate very significant transformations and it is hard to conceive how they can match the basic principle of “. . . not involving. . . visible surface deterioration”. In stone conservation, the term noble patina is sometimes quoted and in general readers understand what is meant with it. In spite of being understood, the concept remains mostly an undefinable one because its very essence lies on its standing on an edge itself, it is a borderline concept, thus lacking enough substance from one or from both sides to build up a coherent definition. In fact, when an external component interacts with a stone surface the consequences that may happen at fall in one of these categories: • deposit, when the cause of perceived visual modification can be attributed to something existing on top of the surface; • crust, when the result is a modification of the stone itself. When we use the term patina (here taken as a synonym of noble patina) it is meant that we are referring to something that is happening at the surface (perceived as a colour change) but that we are unable to ascertain whether it is a deposit or a crust. In spite of the lack of a specific definition, the term patina still has a relevant place in the field of stone conservation, especially in the practical side of the profession. Although a certain degree of ignorance is implied in its use (to be a deposit or a crust, that is the question), when properly used, the term signifies that whatever has been imposed on the surface is certainly gentle, weak, or incipient and therefore it describes a comfortable situation when the preservation of the stone surface is the objective (Fig. 3). Its usefulness is well demonstrated when the cleaning of stone surfaces is considered. Cleaning is an operation used to eliminate components that are alien to the stone itself, and the process is stopped when the elimination reaches a level considered as
adequate for the objective and safe in terms of preserving the stone surface intact. The definition of this level is not always straightforward and may involve significant uncertainties. When the cleaning method is well mastered, the cleaning levels can be progressively incremented and stopped close to the stone surface. When a level can be defined as having a patina that does not disfigure the image of the object (noble patina), the operator can be ensured that something alien is still on top of the surface and no damage was caused by the cleaning method. So, in spite of being unable to define patina, when this condition is reached we know that it corresponds to a right and harmless level to stop cleaning. In this context, the term patina constitutes a valuable concept in stone conservation. 3. From patterns to entity mapping The understanding of stone deterioration processes is based on the systematic observation and study of field data, preferably supplemented with in-depth analytical investigation. Lab research felt a substantial progress with the boom of novel and sophisticated instruments that largely incremented the research capacity, allowing researchers in stone conservation to incorporate the knowledge coming from other sciences and going more and more in-depth in the search to understand and to describe decay phenomena. In conservation practice, progress has been much slower, but the appearance of new non-destructive tests (NDT) and novel uses of imaging technologies are contributing to raise the level of field interventions and to reduce the risks introduced by insufficiently supported decisions. Whatever the lab sophistication and the extension and depth of the field survey, the passage from the research step to the practical implementation of solutions always has to go through an integration step in which the research data are combined and synthesised and the outcome is confronted with the reality of the object under consideration. The amount and depth of the analytical data, on one
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Fig. 3. Patina and its common neighbours in limestone substrates. A. Aspect of a cleaning test with pressurized water (left). The whitish stripes correspond to areas rinsed by rainwater runoff; they are clean or slightly colonized by greyish microorganisms. The black deposits seen in the upper part were cleaned and have left what we can call a brownish patina, seen in the lower part of the photo. None of the areas display the real color of the stone substrate. B. Laser cleaning test area (right). The removal of the surface deposit in an area with visible occurrences of black crusts leaves the surface with a yellowish patina. Whatever the origin and composition of this patina might be, it signifies that the stone surface is not yet hit and therefore this stage may be defined as a harmless cleaning level.
side, and the complexity of the real situations, on the other side, make this confrontation a very challenging operation, and make it the key-action in any conservation intervention. It is a component of the decision-making process and any serious approach to the conservation of stone surfaces needs to look at this step as a crucial phase of the intervention process. Mapping of degradation features and of any other kind of anomalies is a very powerful tool to help the building up of the synthesis between lab and field data and to help transforming research data into useful practical notions that will become inductors of the adequate conservation actions. In this assumption, mapping is not a goal itself, and rather it has to be organised to help reaching the implementation of the major objectives of the intervention project. Mapping includes any spatial representation of numeric data, visual impressions, symbolic translators of data or events, and of actions carried or to be carried out. Accepting as granted the paramount importance of this working tool, this paper will discuss in more detail a novel use of mapping specifically developed to improve the dialogue between researchers, tutelage actors and the contractors, aiming at better clarifying the object of the intervention project and at minimising the risks of slippage in the budget costs. The usual mapping procedure employed in the preparation of conservation interventions uses deterioration patterns as mapping units. Biological colonisation, black crusts, fractures, open joints, past repairs, etc. are examples of mapping units currently found in the literature. Several authors have elaborated comprehensive schemes to map field data (Fitzner [3]) and some glossaries even suggest symbols to unambiguously map the identified units (NORMAL, LNEC). This mapping methodology is especially useful to identify decay patterns and, through them, to help to interpret the acting degradation processes and to identify the degradation causes. A differential distribution of biological colonisation may be due to different exposure conditions, but a localised and elongated concentration of green algae may point out to the existence of a leakage source nearby. Typically, this is the kind of use (of essential importance, it should be said) given to this thematic mapping methodology. This same kind of maps has been used to identify conservation needs, to prepare tender documents and to estimate intervention costs. In scientific literature, the roles and interactions between heritage tutelage, conservation scientists and entrepreneurs are rarely tackled, in spite of the obvious fact that any deflation in each one’s role and any biasing in the interrelationships may result
in serious impairment of the conservation intervention protocol, ending up in risky situations to the heritage item. One reason to cause undesired outcomes is the lack of clarity of the conservation actions that are supposed to be carried out in the object. The use of thematic maps to estimate the labour involvement may be a source of confusion that may be subsequently taken by contractors to extend the contract limits. Experiences with this type of contractors moved the author to search for a different way of mapping anomalies and conservation needs, aiming at reducing the uncertainties in the preparation of tender documents and at facilitating the negotiations when selecting the contractor. This new mapping system, especially conceived to be used in architectural heritage conservation, is described in the following section. Mapping of conservation entities: heritage items requiring intensive conservation care frequently exhibit complex situations that imply difficulties in the identification of the what to do, how much to do and where to do lists, that are essential to prepare a complete and fair arrangement between owner and contractor. Some actions are difficult to be exactly quantified (how much of the soiling is loose dirt? How much of it are black crusts? How many fractures are there to seal? Etc.), and others occur as superimposed anomalies (fragments to be stabilised and fractures to be sealed in areas also needing to be cleaned, etc.) (Fig. 4). Competent conservator-restorers look at the object from a holistic approach and tend to split the object in areas requiring homogeneous treatments and for each identified type a specific plan of actions is designed and implemented. This same concept was taken into consideration when searching for a mapping methodology more friendly for the tutelage and for the contractor. It consists in the identification of homogeneous units in which the conservation needs can be identified and described accurately enough. These units are subsequently characterised and their position in the object is to be unambiguously identified. The case study presented below illustrates this concept, but briefly one may say that a column shaft and the corresponding capital may correspond to distinct units when needing different conservation actions, a decorated area and the nearby rough wall are to be considered distinct units as well, etc. (Fig. 5). This mapping method is not contradictory to the usual pattern mapping and is by no means mutually exclusive. Both are relevant instruments in the preparation of conservation interventions and therefore are to be taken more as complementary rather than as alternatives. Pattern mapping is more adequate to identify the alteration patterns and to understand the decay mechanisms, while
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Fig. 4. Examples of complex situations in a limestone substrate. The superposition of several deterioration processes and different severity degrees (left) and the high three-dimensionality of the object (right) make it impossible to individually locate and quantify each degradation form to support tender documents.
the entity mapping is better suited to serve as a preparatory tool when launching a competition to implement a conservation intervention and to select the contractor and to negotiate the contract terms. A possible programme to start a conservation intervention could be:
• • • •
define the objectives and constraints of the intervention; identify the conservation problems; describe, characterise and map the deterioration patterns; interpret the collected data, identify the degradation causes and define the appropriate conservation actions; • divide the object into entities according to the similitude of the conservation needs they require;
• describe each unit as detailed as feasible in terms of the problems that affect each of them, and typify the conservation actions expected to be applied to solve the problems.
From the author’s personal experience, in competitions launched under this concept, competitors should be asked to describe their own approach to solve the problems identified in each conservation entity and this approach should be given a large weight in the selection algorithm. This option obliges each competitor to carefully analyse the object, thus limiting largely the possibility of misinterpretations and undervaluation of the conservation needs and working difficulties. Extra costs are strongly minimised and the work budget has much better chances to be kept within the defined limits.
Fig. 5. Examples of spatial definitions of the entity-mapping units in Arco da Rua Augusta, in Lisbon. The units represent areas with a certain homogeneity in terms of morphology, decay problems and conservation needs. For the preparation of the intervention, the entire arch was sub-divided in 51 entity units.
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Fig. 6. Aqueduct of Segovia. View of the multi-arch stretch and detail of the dry-stone masonry.
4. Defining and mapping stability indicators in a dry-stone built monument Deterioration of stone elements in any built object usually implies some impairment of the surface condition and may entail relevant losses in terms of object significance. In specific circumstances, when the stone element is a direct component of the structural framework, any impairment of the element integrity may have implications that go far beyond the significance and may represent severe threats to the structural stability of the entire object. To identify damage signs and to assess their risk is of paramount importance in these situations. Structures made in dry-stone are particularly sensitive to structural unbalances and any deterioration progress needs to be carefully assessed in terms of the influence it may have in creating or incrementing unbalanced loads. A pilot study carried out on the Segovia Aqueduct provided an opportunity to develop and test a methodology based on several risk indicators. These indicators constitute the basis to classify the
elements in terms of intrinsic risk and to identify the areas with priority needs of intervention actions. The highlights of that methodology are described and illustrated in the following section. Methodology to identify and assess the stability risk: the Aqueduct of Segovia is an impressive structure built in the Roman times, around the I century AD, having undergone some relevant reconstruction interventions in the XV and XVI centuries. In its urban course, the Aqueduct assumes the form of a bridge, the “bridge-aqueduct”, with an elegant multi-arch, two-level structure spanning for more than 700 m (Fig. 6). The Aqueduct is built of large stone blocks stacked in a drymasonry style. The stone material includes a few varieties of granites, medium to coarse-grained, quarried in the city and in its vicinity. The most widespread degradation form affecting the Aqueduct is sand disintegration. Some blocks are slightly to moderately rounded, while others are severely affected, and it is highly
Fig. 7. Treatment of the reported data. Mapping of the highest ratings of each class (left); grouping of any ratings of 4 and 5, rating 3 and the sum of the major 4 units (right).
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Fig. 8. Synopsis of the obtained results in terms of the priorities of the conservation needs. Pink dots represent the elements with first priority needs and in yellow those with second priority needs.
probable that the most vulnerable ones have disappeared long ago and replaced with new ones. All these credibly original blocks suffer from moderate to severe mass loss. Decay of the stone components leads to a progressive weakening of the block strength and the mass loss induces a progressive rounding of the blocks with potential unbalancing of the carried loads. A monument of this size and age has permanent conservation needs and requires a robust methodology to identify what to do, where and when, and for such a sensitive construction this is not a simple endeavour. To find a simple and assertive methodology to identify such conservation needs is thus of particular relevance here. Besides the study of several other aspects concerning the degradation factors and forms, a specific analysis of the factors that may contribute to stability problems was carried out [7]. A specific glossary was prepared to identify and map deterioration patterns. These vary from disintegration patterns, to rounding of blocks, presence of fractures, etc. The field inspection and reporting were made block by block, and a subsequent deskwork was done to synthesise the information and to develop a risk assessment path. The basic working tool was the glossary. Besides reporting any aspect deemed as potentially informative, the basic descriptive classes of deterioration patterns were: D – disintegration patterns; S – shape of the block, having the “perfect” original cuboid shape as reference; F – presence of fractures; I – presence of past interventions. For each class, blocks were given a rating value from 0 to 5 according to the perceived severity for each item that affect them. All data were reported directly in individual prospects of the Aqueduct. Disintegration is a typical deterioration process in granite materials and it is particularly noticeable in the original Roman blocks. Besides implying the reduction of the block overall strength disintegration leads to the progressive rounding of blocks and may induce unbalance of the carried loads. This situation was
reflected in the methodology through the shape factor (S). For the risk assessment, fractures (F) that cross the entire block were given high ratings, since they may constitute signs of unbalancing of loads and of potential safety problems. The presence of past interventions (I) was considered a relevant indicator and these occurrences were integrated in the assessment. The Aqueduct was restored a few decades ago and some invasive actions were made, such as the gluing of fractures, deep consolidation through injection holes and shape reconstructions. These types of actions presuppose that the blocks had to be in a very bad condition to justify such intrusive actions. The blocks with this past history have to be monitored and assessed for their present condition, reason why they were taken as a risk indicator. Treatment of data was carried out with the objective of identifying situations with distinct safety risks, seen exclusively from the point of view of the condition of the individual blocks. Several trials were made, namely by considering each mapping unit individually, by associating two of them, or by summing up all the major four classes (Fig. 7). The fieldwork was carried out by professional conservatorrestorers following a protocol presented and discussed beforehand with them. The field reports were subsequently analysed and synthetic maps were prepared to facilitate their interpretation and the assessment of the overall significance for the structural safety of the Aqueduct. The results of the fieldwork were firstly ranked and lower values eliminated from the graphic representation to facilitate their interpretation, as exemplified on Fig. 7. Subsequently these data were jointly analysed with the aim of identifying the priority situations in terms of conservation needs. This has implied the definition of a series of criteria to weight the different classes trying to find a result representative of the real situation. For instance, the simultaneous occurrence of any two ratings 4 or 5 as well as one such a rating plus a sum of 8 of all parameters would imply first priority needs,
Please cite this article in press as: J. Delgado Rodrigues, Defining, mapping and assessing deterioration patterns in stone conservation projects, Journal of Cultural Heritage (2014), http://dx.doi.org/10.1016/j.culher.2014.06.007
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while a single 4 or 5 would include that block in a second priority group. These two levels of conservation needs were applied to the entire pilot area, as illustrated on Fig. 8. 5. Conclusions The study of deterioration patterns is currently used in the diagnostic phases to describe and characterise acting decay mechanisms and to identify the responsible degradation causes. Both are essential elements to help finding conservation needs and to plan intervention actions. The way that deterioration patterns are defined and mapped has not the same meaning in a context of scientific research, to identify the mechanisms and causes of decay, or when the planning and implementation of a conservation intervention is concerned. The individual identification of deterioration patterns and their precise characterisation and mapping are the most appropriate procedures when identifying mechanisms and causes because deterioration patterns are good fingerprints directly connected to their immediate past genetic context. Working tools such as the ICOMOS Glossary and mapping methodologies such as the one recommended by Fitzner et al. are appropriate to achieve the mentioned objectives. When preparing information to be handled on to practical implementation, these methodologies show some flaws, namely when definitions are not sufficiently unambiguous and when mapping options do not match the practical reasoning and the negotiation and contractual needs. When placed in the crossing step between diagnostics and implementation phases, the conservation scientist needs to have a critical attitude as regards data gathered and documents prepared in the diagnostics phases, and to aim at preparing elements that adequately suit the subsequent phases of negotiation of contracts and implementation of conservation actions. To do it, some of the used concepts might need to be improved or redefined as illustrated here with the terms used to describe detachment forms, black crusts and patina. The methodology used to map deterioration patterns is also susceptible to be better adapted to fit this objective, namely by moving from mapping of individual deterioration patterns to comprehensive units having reasonably homogeneous problems and conservation needs. A novel methodology to reach this desideratum is proposed and illustrated in the paper. Its successful implementation in one major monument in Lisbon attests its applicability. The conservation problems of a dry-stone masonry monument, such as the Segovia Aqueduct, are directly related to what affects each individual block and therefore they require to be tackled at the exact level of these elementary construction elements. The
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potential influence of deterioration patterns in the overall stability of the structure of the Aqueduct was assessed by means of a detailed study of every single block and by selecting a certain number of degradation features with higher impact in the strength reduction (namely the occurrence of sand disintegration forms) and in the unbalancing of the carried loads (namely the rounding of blocks). The case study here presented describes the essential methodology working steps and illustrates how the identification of priorities was achieved. The study was carried out in a pilot area and the site reports show that the methodology can be easily extended to the entire structure. Substantial savings in future conservation interventions are expected to be obtained with the clear identification of actions to be implemented and a better delimitation of areas to be intervened. Acknowledgements The author acknowledges the exchange of opinions with José Aguiar and Nuno Proenc¸a on the mapping methodology and Sandra Alves for her diligence in its implementation in Arco da Rua Augusta. Atelier 15 coordinated the conservation intervention project. The study on the Segovia Aqueduct was sponsored by World Monuments Fund Spain. Thanks are due to Pablo Longoria and Norma Barbacci for their confidence and support. The fieldwork was diligently and professionally carried out by Carlos Sanz and a team of Nova Conservac¸ão, Lda. The author thanks Marisa Tabasso, A. Elena Charola, Dória Costa and M. João Revez for their comments on the manuscript. References [1] J. Delgado Rodrigues, “Proposal for a terminology of stone decay forms on monuments”. ICOMOS Stone Committee Newsletter, 1/1991, 1991 (Unpublished intern document). [2] ICOMOS, “Illustrated glossary on stone deterioration patterns”, ICOMOS International Scientific Committee for Stone, 2008 (ISBN: 978-2-918086-00-0). [3] B. Fitzner, K. Heinrichs, “Damage diagnosis on stone monuments – weathering forms, damage categories and damage indices”, in: R. Prikryl, H.A. Viles (Eds.), Understanding and Managing Stone Decay, Proc. Int. Conf. SWAPNET 2001, Charles University in Prague, The Karolinum Press, 2002, pp. 11–56. [4] 1/88. Normal, “Alterazioni macroscopiche dei material lapidei: lessico”. “Macroscopic alteration of stone materials: glossary”, Comas Graphica, Rome, 1990 (36 p.). [5] F.M.A. Henriques, J. Delgado Rodrigues, L. Aires-Barros, N. Proenc¸a, “Materiais pétreos e similares: terminologia das formas de alterac¸ão e degradac¸ão”. ICTInformac¸ao Técnica, Patologia e Reabilitac¸ão das Construc¸ões, LNEC, ITPRC 2, Lisbon, 2004 (39 p.). [6] J. Ordaz, R.M. Esbert, “Glosario de términos relacionados con el deterioro de las piedras de construcción”, Mater. Construc. 209 (1988) 39–45. [7] J. Delgado Rodrigues, “Segovia Aqueduct. Degradation forms, condition survey and conservation needs”, 2013 (Unpublished internal report for World Monuments Fund Spain).
Please cite this article in press as: J. Delgado Rodrigues, Defining, mapping and assessing deterioration patterns in stone conservation projects, Journal of Cultural Heritage (2014), http://dx.doi.org/10.1016/j.culher.2014.06.007