Analytical Strategies for Cultural Heritage Materials and their Degradation

Juan Manuel Madariaga, Juan Manuel Madariaga

• Publisher: Royal Society of Chemistry
• Year: 2021
• ISBN:
  9781788019224
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Analytic Chemistry

Reviewing the analytical strategies used in the study of cultural heritage assets such as movable artworks and archaeological items, and immovable objects like mural paintings, archaeological sites and historical buildings, this book pays particular attention to analytical methodology. It is not always necessary to use new and sophisticated instrumentation, what is important is how the instruments are used to obtain reliable, reproducible and repetitive results in view of the problems to be solved. The book considers the influence of the environment on the conservation state including degradation and how modern analytical methods have improved the analysis of materials. It emphasizes multi-method approaches on a range of materials, an approach that is of keen interest to those working in conservation practice.

Primarily aimed at final year undergraduate study and masters level students, it would also be useful as supplementary reading for postgraduates and academics who require analytical techniques to enhance their research.

Part 1

Introduction to Analytical Strategies for Cultural Heritage

CHAPTER 1

Introduction to Analytical Strategies for Cultural Heritage

JUAN MANUEL MADARIAGA^{*a, b}

^a Department of Analytical Chemistry, University of the Basque Country (UPV/EHU), P.O. Box 644, 48080 Bilbao, Spain, ^b UNESCO Chair on Cultural Landscapes and Heritage, University of the Basque Country (UPV/EHU), P.O. Box 450, 01006 Vitoria-Gasteiz, Spain
^*E-mail: [email protected]

This introductory chapter outlines the aims of the book, which is intended to be an introductory book for undergraduate students in chemistry or at the master's level for whom a sound knowledge of analytical chemistry applied to the study of cultural heritage needs to be acquired. Details the content of the different chapters are given, together with a brief review of other techniques not considered in the various chapters.

1.1 Aims and Scope of This Book

The aim of this book is to review the analytical strategies used in the past and to describe new methods proposed more recently for the study of the materials used in cultural heritage assets. Both movable (artworks and archaeological items) and immovable (mural paintings, archaeological sites, historical built heritage) cultural heritage assets with significant importance have been key examples of the roles of different professionals and how a multidisciplinary view is demanded nowadays.

First, the analytical techniques used to ascertain the elemental and molecular characterization of such materials are described in Parts 2 and 3, respectively. Subsequently, the different multianalytical strategies available to characterize materials (classical and modern) are described in Part 4 for different important examples of cultural heritage assets that people all around the world can appreciate.

This structure of the book is intended to consider first the individual analytical techniques and coupled methodologies, both at the laboratory level and in the field, and then the specific problems related to the different materials involved (metals, pigments, binders, inorganic mortars, stones, modern organic materials, etc.) that require attention when dealing with cultural heritage assets.

Taking into account that the degradation of cultural heritage materials starts at the moment the artwork is finished and exposed, each chapter of the book considers both the nature of the original materials and the degradation products that can be produced as a function of the reactivity of such materials with the surrounding environment. When possible, chemical modelling is included as a tool to ascertain the sources of impacts that degrade the materials used in the manufacture of the artwork, archaeological object or historical building.

1.2 Role of Analytical Chemistry in Cultural Heritage Projects

The field of science related to cultural heritage (CH) is vast and complex, encompassing analytical and physical chemistry, biology, engineering, materials science, etc. Sampling, quality assurance, simulation/modelling and chemometrics are usually far from the capabilities of restorers and daily practitioners. Fortunately, analytical chemistry is nowadays a science that is playing a more important and critic role in assisting the work of other professionals in the field of CH.

Analytical science can now provide answers to questions about the conservation state of CH assets that are under study or rehabilitation. This is mostly done by merging the characterization of original and deterioration-related compounds with the aim of ascertaining the decay pathways, because most of these degradations are due to chemical reactions between the original compounds and chemicals present in the environment surrounding the artwork, the archaeological remains or the historical building under study.

To obtain the required information to reach these goals, we as analytical chemists have an important role in the multidisciplinary teams involved with a project of rehabilitation or conservation. We must impress upon other professionals that we are not in a project to perform analyses; unfortunately, there are many restorers who reduce the problem to executing a number of analyses to be included in a report, and this is starting to be mandatory in practically all the administrations in charge of CH preservation. This dynamic must be broken, changing the past concept of “perform standard analyses” to a more realistic “design dedicated analytical strategies to solve the particular problem”.

We do not perform chemical analyses: technicians do this. Our role as analytical chemists is first to define the extent and nature, together with the other professionals, of the “problem to be solved” in the project. If the project is complex, we must encourage defining small, partial objectives that are more easily affordable. Once this has been clearly stated, we start our specialized work as analytical chemists.

First, we must select the most suitable analytical methodologies to obtain the knowledge required to reach each partial objective. If the small problem of the partial objective is similar to others reported in the literature, we can take advantage of that past expertise developed by other team(s). However, often this do not happen and we cannot find any previous similar problem described with sufficient detail in the literature. For such cases, we need to design dedicated analytical methodologies that must be validated before starting to apply them to the CH objects. This is important, because if we have success we will contribute to the advance of knowledge if we publish our new development and the results obtained.

At this stage of the book, an important aspect should be emphasized. Even if one belongs to a private institution, the projects around CH conservation issues are partially, if not totally, funded with public money. Part of that money will certainly come to the author's institution and therefore we must publish and disseminate our developments, findings and results. These are not property of the author's institution unless the institution has provided all the expenses with its own non-public resources. Fortunately, the number of private institutions that publish their results on CH projects is increasing year by year, demonstrating their commitment to society.

1.3 Analytical Strategies in Cultural Heritage

Our starting point in any CH project, after the problem to be solved has been clearly identified and the partial and global objectives have been defined, is to think about the most suitable set of analytical methods to be used, that is, what will be the most appropriate analytical strategy to achieve the global objective?

Any analytical methodology requires the fulfilment of different steps, after the definition of the problem to be solved, namely:

• homogeneity of the sample(s) under analysis;
• representativeness of the samples being analysed with respect to the whole object;
• accuracy and precision required;
• constraints around the sample to be analysed;
• selection of a suitable method;
• quality control and quality assurance of the process;
• critical analysis of the results, including their overall uncertainty;
• chemometric analysis of the data and chemical modelling.

These are not always taken into account by non-analytical chemists using modern analytical instrumentation “at the touch of a button”. This can be observed in some papers published in non-analytical chemistry journals and in some manuscripts submitted to such journals that cannot go ahead in the review process owing to the lack of descriptions of adequate analytical procedures.

1.3.1 Homogeneity of the Sample(s) Under Analysis

In CH, there is a common concept of homogeneity: all the artworks, objects and buildings are heterogeneous. For example, painted artworks (easel paintings, wall paintings, rock art paintings, historical wallpapers, polychromed sculptures, etc.) have several layers intentionally applied by the artist during their production. Buildings have an external patina resulting from the action of the environment on the original stones, mortar, balconies, roofs, etc. Archaeological objects also have patinas due to the action of the burial (soils or sediments) on the original materials.

If the object or asset has been subjected to past restoration processes, new layers are present on top of the original ones. Moreover, if the decay due to those interactions with the (indoor and outdoor) environment, where the object or asset is located, has progressed without any preventive action being taken, new compounds can be formed as sub-efflorescences (new chemical compounds inside the pores of the bulk) or efflorescences (new chemical compounds on the surface of the bulk). The detail of the wall painting shown in Figure 1.1 reflects signs of vandalism, detachments and the effect of past restoration.

Figure 1.1Detail of the head in a Renaissance wall painting where signs of vandalism are observed, in addition to some detachments promoted probably by the action of sub-efflorescence. This wall painting was analysed in situ using a set of first-generation portable instruments.

To sum up, if the CH element has suffered a natural impact (earthquakes, fires, floods, etc.) or a human impact (industrial processes, bad agricultural practices, vandalism, graffiti, stolen, etc.), new chemical compounds should be expected to be present. Natural impacts have been more and more frequently observed in recent years and will continue to increase as climate change progresses, especially for those CH elements located near rivers or the sea. However, human impacts are also increasing, especially dangerous being the consequences of bad agricultural practices where the excess of chemicals used is passed to rivers and ground waters, ultimately reaching the foundations of historical buildings and transporting soluble acids, cations and anions to the walls by capillary processes.

All of these complex processes affect the homogeneity of CH objects and their samples. Visual and photographic inspection, together with the past expertise of the professionals involved in the project, will give us a first idea of the inhomogeneity, which can be considered at the end as the sum of different homogeneities (each layer in a painting could be homogeneous but the sum of the layers gives us the inhomogeneity). This first analysis conditions the number of individual samples that should be analysed to characterize the CH element properly. Also, this first analysis must be made at the time of defining the problem to be solved by the project, because the different homogeneities will give us the number and extent of the particular objectives.

1.3.2 Representativeness of the Samples Analysed with Respect to the Whole Cultural Heritage Element

Are you sure that your sample represents the overall problem? This is the classical question in the field of analytical chemistry. However, this may not be questioned by other professionals in the field of CH because “a sample is a sample and represents a particular universe that is systematically observed”. We have observed this controversy too many times, but our responsibility as analytical chemists is to be aware about representativeness.

If we do not take representative samples, the overall conclusions that we obtain at the end of the project will not cover all the different degradation processes that are really present in the CH element as a whole. As an example, Figure 1.2 shows three public sculptures made of CorTen steel, two exposed outdoors and one displayed inside a museum, which suggest a high degree of homogeneity, guaranteeing the representativeness of the samples. However, the first analyses performed with portable instrumentation showed in all three cases the presence of different patinas as a function of orientation, which led to inhomogeneous surfaces at the millimetric level, conditioning the representativeness if sampling does not cover all the different surfaces.

Figure 1.2Three CorTen steel sculptures seeming apparently homogeneous due to the same bulk but which are really inhomogeneous at the surface level due to the different patinas.

To guarantee representativeness, we must define a number of actions that must be included in the overall set of quality control and quality assessment actions of the project, because the absence of representativeness affects both the accuracy and precision of the final results.

1.3.3 Accuracy and Precision Required

The use of the terms accuracy and precision in analytical science generally refers to quantitative data, but in the field of CH not only quantitative data are used but also qualitative information about what compounds are present in a given layer of a multilayered object under study. For quantitative data, the treatments (statistical tests) for accuracy and precision are the same as in other fields where analytical chemistry is involved and the same rules should be used to select one analytical method or another as a function of the required accuracy and precision, something that is set in the step of “defining the problem”. In this section, we must focus on the qualitative information relating to CH materials.

Usually, qualitative work is based in the interpretation of the spectral responses of one or several spectroscopic analytical techniques, so how can we define accuracy and precision? The answer is not unique, because it depends on the number and kind of spectroscopic techniques used. We are going to consider some possibilities, proposing how we can define and apply accuracy and precision. These proposals are based on the experience we have accumulated in the last 20 years working in the field of analytical chemistry for CH. Accuracy must be understood as certainty in the presence of a given element or chemical compound. Precision must be understood as the variability in the form and position of the bands/peaks characteristic of a given element or compound. Depending on the spectroscopic techniques used, we could have three possibilities.

For the case of using only one spectroscopic technique, a chemical element or compound is considered to be present if the unknown spectrum has at least two characteristic bands/peaks of the element (for techniques of elemental analysis) or compound (for techniques of molecular analysis) and those two signals appear more than five ...

Table of contents

• Cover
• Halftitle
• Series Editors
• Title
• Copyright
• Preface
• Contents
• Part 1 Introduction to Analytical Strategies for Cultural Heritage
• Part 2 Non-destructive Elemental Characterization of Cultural Heritage Materials
• Part 3 The Molecular Characterization of Cultural Heritage Materials
• Part 4 The Multianalytical Approach to Cultural Heritage Materials and its Degradations
• Subject Index