Abstract

This issue of Elements celebrates the diverse contributions that the Earth sciences have made to characterizing, interpreting, conserving, and valorizing cultural heritage. Archaeometry and conservation science are connected to the geosciences at different levels. Earth scientists possess a profound perception of the complexity of natural materials, they have the necessary knowledge of the ancient and recent geological and physicochemical processes acting on natural materials and on the artifacts produced by human activities, and they master most of the techniques useful to investigate our common heritage. Therefore, Earth scientists can greatly contribute towards a better understanding and preservation of our past.

INTRODUCTION

Our cultural heritage derives from the development and diversification of human cultures and abilities. Scientific investigation of cultural heritage has several important aims: (1) increasing our knowledge and understanding of the tangible heritage; (2) assembling information, including physical evidence, in order to add cultural value to the heritage, including the “intangible” values; and (3) helping to define the conditions, limits, risks, and potential for sustainable conservation and management of human heritage in general.

In the words of Renfrew and Bahn (2008), “Archaeology deals with the full spectrum of human experience.” This makes the discipline both fascinating and rather complex. It is clear that archaeology must use the diverse methods and techniques developed in other disciplines, from A to Z (from “architecture to zoology,” in the words of Pollard (1995)). The need for scientific inquisition becomes more important the more we penetrate into prehistory, where no written accounts are available and even the interpretation of the material evidence is often problematic. The wide area of scientific research supporting archaeology is now generally called archaeometry, which relies on scientific training, dedicated research institutions, and so on. In this way, archaeometry differs from conservation science, which is related to fine arts, restoration, and museums. However, a current trend is to combine archaeological excavations and conservation plans (Agnew and Bridgland 2006). It is worth noting that the analytical tools and approaches of modern archaeometry are similar to those used in mineralogy, geochemistry, and materials science. There is a necessity to take a shared and multidisciplinary approach when studying cultural heritage, especially when addressing complex materials problems, because of the long training time required by scientists to use the required sophisticated techniques and to make the most efficient use of human and financial resources (Pollard and Bray 2007; Artioli 2010).

Within this framework, the geosciences offer a wide range of competencies and specific knowledge of natural materials across a wide range of spatial and temporal scales. Intrinsic to the geosciences are an understanding of geological and geochemical processes, the distribution of mineral resources, the evolution of a landscape, and the perception of time and dating methods. The impact of past human activities on the geological environment and on natural processes is important for understanding the context of cultural history. Geosciences are fundamental to perceiving the fine interplay between human society and natural resources. Climate, earthquakes, agriculture, mining, fossilization, glaciations, planetary cycles, pyrotechnology, human evolution, diagenetic processes, may at first glance seem unrelated: but all are pieces of the same puzzle—the history of mankind and human societies on Earth. Nature and humans influence each other.

This introductory article will use a few selected examples to convey the need for, and the potential of, geoscience research in archaeometry and in conservation studies.

ARCHAEOLOGICAL SITES: FROM SPACE TO MOLECULES

Few scientists can adequately perceive how chemical and physical processes operate over a wide range of scales. Most people working with materials have a good perception of the nano- and microscales (the scale of atoms, molecules, and crystal structures), and some have a grasp of the textural and structural aspects at the mesoscale (extended defects, textured materials, composite materials). Others see matter as a continuum with specific properties at the macroscale, in order to turn materials into devices and tools. The choice of analytical scale is important because, depending on the technique, we may obtain different experimental responses and information on the same object. Each discipline and each specialist has a special perception of the sample and its context (Fig. 1).

Further, even at the same scale we may select a variety of techniques, adopting different probes and analyzing specific signals. Most techniques are complementary and may be used together in order to produce integrated results.

At the large scale (tens of meters to kilometers), we can use remote sensing (multispectral/hyperspectral imaging), satellite imagery, or ground-penetrating radar. During area surveys, the characterization of the local architectural remains, buried structures, landscape morphology, hydrology, transportation routes, the soil potential for agriculture, and so on, helps to understand the archaeological site in the territorial context.

At the scale of a field survey (meters), geophysical methods are the most useful to map subsurface features or penetrate architectural structures (Sala et al. 2016 this issue), and this information is often conveniently combined with a 3-D visualization of the site by geographic information system (GIS) referencing, photogrammetric reconstructions, or laser scan maps.

At the scale of the objects and human remains (submeter) materials science comes into play. A number of portable probes (X-ray fluorescence spectrometers, X-ray diffraction probes, Fourier transform infrared spectrometers, etc.) may be used to retrieve chemical or mineralogical information before, during, and after an excavation. These techniques may help in understanding the stratigraphy and recognizing the different materials unearthed during the excavation process. Ideally, using these techniques means that no information is lost, better excavation strategies can be developed, and appropriate samples can be selected for subsequent laboratory analyses.

AN OBJECT'S TIME LINE: THERMODYNAMICS VERSUS KINETICS

Earth is an extremely dynamic planet, even at the human time scale. The living environment, the climate, the landscape, the hazards, the natural resources are all constantly changing. Human cultures also change through very complex interrelationships between natural constraints and social needs. Art and archaeology, our cultural heritage, are the tangible signs of such change. To understand the driving forces behind cultural and social evolution and to define the pace of that change are two of the more fascinating tasks of cultural heritage investigations. At the grand scale, civilizations rise and fall because of the resource availability, changing environmental conditions, or technological developments. Scientifically investigating cultural heritage can be linked to a two basic questions: “What happened?” and, “How, why, and when did they do it?”

At the small scale of materials, everything humans do is bound to alter the thermodynamics and/or kinetics of natural processes: ore extraction, metal reduction, woodworking, farming and soil exploitation, architecture, transport—they all substantially alter the dynamics of natural change. As Earth scientists, we strive to have a good understanding of the thermodynamics and the kinetics of processes from the grand scale of plate tectonics to the electronic scale of oxidation/reduction reactions. We need to understand changes in cultural heritage in the context of such physicochemical timescales. Most human actions represent a fight against thermodynamics, which is why we need energy (food, fuel) for all our activities. If we follow the life cycle of a material, it can be described as an alternation of slow and fast processes (Artioli and Angelini 2011). Natural processes (such as phase transformation, alteration, diffusion) try to bring a material into thermodynamic equilibrium and are considered slow on the human timescale. In contrast, most human actions (pyrotechnology of production, i.e. high-temperature technology, transport, conservation) are fast on the same scale but commonly produce abrupt physicochemical changes that take matter into highly unstable or metastable states. There is no way out: most of our achievements (tools, buildings) and ourselves are metastable.

Most of the studies on cultural heritage materials focus on understanding their time line (Gould 1987), either on their past (archaeometry) or their future (conservation). In archaeometry, we strive to place a given human object (or activity) on an absolute or relative timescale, often carefully developed through the scrupulous application of physicochemical dating methods such as dendrochronology, radiocarbon methods, thermoluminescence, and optically stimulated luminescence (Wagner 1998; Athanassas and Wagner 2016 this issue). For conservation, we try to understand the kinetics of change (alteration, degradation, diffusion, phase transformation, etc.) and define the best conditions to slow or stop any degradation/alteration processes in order to save the heritage material (the building, the archaeological site, etc.) for future generations. Restoration and conservation aim to increase the stability of the materials, so increasing their longevity, provided that environmental conditions and thermodynamic parameters do not change.

Unfortunately, despite all our efforts, sometimes the lack of culture itself acts as an unexpected player in the conservation game. Tragically, this is now happening on a large scale in the Middle East, where cultural heritage itself is under attack by cultureless humans: in 2011, the Bamiyan Buddha statues (Fig. 2) were destroyed; in 2015 the Assyrian Nimrud and the Syrian Pamira sites were attacked and heavily damaged. Such actions substantially increase the kinetics of degradation.

PROVENANCE AND AUTHENTICITY: GEOCHEMICAL AND ISOTOPIC TRACERS

Investigation of the chemical, mineralogical, and isotopic compositions of cultural heritage materials yields a wealth of information. Not only do the analytical data allow the nature of the compounds and their origin to be defined, but they also offer important clues on (a) the provenance of an object, (b) the chemical classification in which to group an object, and (c) the authenticity of an object.

Determining provenance is an important aspect of archaeometric investigations. Identifying the nature and characteristics of an object leads to deductions on the origin of the raw materials used to produce it, where it was manufactured, and the routes by which it was traded or exchanged. In most instances, the simple typology of the object is not sufficient to define reliable groups and parameters of identification. In ancient and modern times, humans have systematically copied and imitated objects that are aesthetically pleasing and/or functional. Thus, it is often impossible to discriminate between imported ceramics and those locally produced using typology and morphology alone. This is true for prehistoric pottery, Romano-Celtic sigillata, or Renaissance and Chinese porcelains (Heimann and Maggetti 2014). Even expert discrimination based on purely stylistic parameters can lead to serious errors, but mineralogical or chemical parameters commonly allow the definition of reliable reference groups.

The same is true for many other materials, including metals and glass. Compositional and isotopic parameters are usually employed to discriminate the ore sources used for metal extraction (Rehren and Pernicka 2008). For most metals (copper, bronze, lead, silver) the lead isotope ratios carrying the age signature of the exploited ore deposit is the most important tracer employed in determining a metal's provenance (Gale and Stos-Gale 2000). Extensive lead isotope databases exist for most Old World ore deposits, and they are systematically used to compare the lead isotope signature of ancient objects. When geological deposits are coeval, discrimination is further refined based on a suite of minor, trace, or rare ultra-trace elements present in the mineral assemblage and affine to the main metallic elements during the reduction and refinement processes, e.g. the suite of chalcophile elements that follow copper during slagging operations—Sb, Ag, Se, Ni, Co, Bi, As. For other metals, different compositional and isotopic signatures have been attempted for determining provenance of the metal: for example, the tungsten–tin signature for iron (Benvenuti et al. 2013).

Archaeological discoveries coupled with the substantial amount of archaeometallurgical work in the last few decades are such that our picture of early metallurgy is changing substantially. The standard view of early metallurgy is that it developed in the Near East and then moved slowly through the Balkans and the Mediterranean to the rest of the Old World (Fig. 3). This view is now being seriously challenged.

Glass can also be shaped and worked (or reworked) easily, and imitations are frequent. Therefore, archaeological and historical glass, and glassy materials such as faience objects, are extremely difficult to group into the appropriate glass type and provenance without appropriate analysis. Typical examples are prehistoric faience (beads, small objects such as scarabs, pendants, ushabti, mostly of Egyptian production) and protohistoric glass beads, both of which generally have a few fairly standardized typologies, despite being widespread across the Mediterranean world and beyond. Similar problems exist with Roman mosaic tesserae, which were easily transported, reworked, mixed, and imitated. The chemical composition of a glass, the presence of any crystalline impurities, and isotopic tracers (Rehren 2000; Degryse et al. 2009; Henderson 2013) are all used as discriminating parameters to identify different glass productions and the origin of the materials.

A few words now on authentication. The aesthetic, stylistic, and chemical evolution of the material in time provides information that can be used to interpret technological and artistic changes throughout human history. These data can also be used to recognize forged antiquities and art works. Authenticity tests, based on laboratory data, expose artistic, stylistic, and physicochemical features (including direct absolute dates) of an artifact that are inconsistent with the supposed age of that artefact (Craddock 2007). Extensive scientific analysis of certified works by an artist may also help in the attribution task, to the point that connoisseurship can lead into the realm of technical art history. Sometimes, discriminating between fakes and originals is very difficult, and several techniques are needed to produce a reasonable answer. The case of the beautiful cast bronze Benin plaques, which are frequently counterfeited, is a good example of complex attribution: surface corrosion features, metal alloy composition, and 210Pb signatures (Keisch et al. 1967) have to be measured in order to reach reliable conclusions about authenticity (Pernicka et al. 2008). Sometimes a suite of impurities in a metal can be directly related to the method of production, as in the case of chemically versus electrolytically separated aluminum (Bourgarit and Plateau 2007).

IDENTIFYING AN ARTISTS' MATERIALS

Ever since man started being artistically creative, natural minerals, rocks, and organic substances have formed the physical basis of this creative expression, either in their raw form or when artificially transformed by heat and chemical reactions. Pigments for painting and coloring, and rocks, metals, glass, and wood for sculptures and decorations are examples.

The identification and characterization of the materials of art is important for art history, authentication, and conservation. With respect to modern pigments and dyes, natural minerals as pigments have a limited palette: black, white, and different shades of ochre, red, and brown. However, these basic pigments have been used to express inner feeling, belief, and communication since the dawn of humankind (Figs. 4and5). Investigation of the pigments, their binders, and the way that they were used gives us insight into the inspiring moments when early humans projected their thoughts onto a rock surface. The scientific investigation of pigments often reveals unexpected sides to ancient artistic technologies.

Walter et al. (1999) conducted a systematic study of ancient Egyptian cosmetics using synchrotron radiation to demonstrate that many cosmetic samples were a mixture of ground natural minerals (galena) and synthetically precipitated crystal powders (laurionite), thereby proving the chemical ability of ancient Egyptians. Another fascinating story is the one related to the Maya blue pigment (Fig. 6), which is a very stable compound used by the Maya to paint murals, ceramic surfaces and other objects; it was also possibly used ceremonially. Maya blue pigment is very resistant to alteration and chemical attack and it has attracted the interest of a number of scientists because of its structural complexity: it is a true organo-clay hybrid material, where indigo molecules adsorb on the surface, and perhaps enter the channels, of one of the polymorphs of the clay mineral palygorskite (Chiari et al. 2008; Sánchez del Rio et al. 2008). This brilliant blue pigment was prepared by ancient Mayans with a specific recipe that involved boiling the clay together with the indigo plant Indigo suffruticosa (Reyes-Valerio 1993).

Other blue pigments used in antiquity are the Egyptian blue (cuprorivaite) and lapis (lapis lazuli, or lazurite), whose modern counterpart is synthetic ultramarine. Both pigments tell many fascinating stories: first, of long journeys, because the only source of lazurite exploited in antiquity was the Afghan Badakhshan deposit; second, these are rare pigments used for artistic expression—lapis was very expensive and only the most famous and well-paid painters could afford it, including Michelangelo, Masaccio, Titian, and Vermeer, the latter being a real fan of lapis lazuli; third, local ancient imitations—the origin of synthetic cuprorivaite in ancient Egypt is assumed to be replacement for scarce supply of lapis, and their recipe continued to be used in the Roman world and beyond (Bredal-Jørgensen et al. 2011); fourth, modern processing—the synthesis of modern ultramarine was the result of a prize that the French Societé pour l'Encouragement d'Industrie offered in 1824 for the artificial production of the precious color. Jean Baptiste Guimet (1826) and Christian Gmelin (1828) both derived processes to produce ultramarine. While Guimet kept his process a secret, Gmelin published his results and became the originator of the “artificial ultramarine” industry.

GEOSCIENCE AND CULTURAL HERITAGE

This issue of Elements is designed to stimulate a wider involvement by geoscientists in cultural heritage research. Geoscientists commonly possess a good knowledge of physicochemical processes over a range of space and time scales. The perception of the complexity of materials, the interaction between human activities and natural processes, and the availability and properties of geo-resources are important requisites for heritage studies. Furthermore, the geosciences (including materials science) are at the forefront of experimental and instrumental developments in different research areas because of the stringent requirements that are posed by the complex problems they are faced with. For example, the use of neutron beams for textural analysis was developed to investigate rock textures. In the field of microanalysis, the use of narrow and focused beams (including synchrotron radiation) for investigating matter has been historically pioneered by geoscientists (Quartieri 2015) and is now an essential tool for other disciplines (Janssens et al. 2016 this issue). It is not by chance that most laboratory instruments using microbeams (ion probes, electron microprobes, high resolution transmission electron microscopes) are installed in geoscience laboratories, as well as in materials science labs that work on complex synthetic samples. Similarly, the noninvasive 3-D imaging of objects offered by computed tomography is a rapidly growing field with applications for heritage material, as shown by Gueriau et al. (2016 this issue). The holistic imaging of objects, coupled with spectroscopic mapping, is bound to become a major tool for characterizing art and archaeological objects in the future and will be a great aid in establishing restoration protocols. Such imaging can be thought of as the modern 3-D equivalent of the neutron autoradiographic techniques developed for paintings decades ago. The large-scale extension of 2-D and 3-D imaging is demonstrated by the survey of archaeological sites and buildings by geophysical methods as described by Sala et al. (2016 this issue).

Another field where geosciences is at the forefront of conceptual and technical achievements is the dating of (geologically) recent events: many techniques developed as alternatives to radiocarbon dating (optically stimulated luminescence, thermoluminescence, cosmic nuclides, etc.) are being actively pursued to understand recent geological processes, such as volcanic eruptions, sedimentary processes, and landslides. Athanassas and Wagner (2016 this issue) show how these techniques may be important for archaeometric purposes. Finally, the use of isotopes as tracers and clocks of geological events is, of course, a stronghold of geochemistry and petrology. The high accuracy and precision needed in geological mass spectrometry has implications for the interpretation of cultural heritage materials, the reconstruction of human activities and trades, and the authentication of art works.

The scientific investigation of cultural heritage should not just be the mere application of techniques and methods that were developed for other purposes. Instead, the geosciences should join with those in the cultural heritage sector to develop and optimize the required concepts and experiments to mutually benefit both fields. And in the future, we expect that the links between cultural heritage science and the geosciences will get stronger and that much exciting research will follow.

ACKNOWLEDGMENTS

We thank Gordon Brown and Jodi Rosso for encouragement and editorial assistance. S. Quartieri is grateful to R. Arletti and G. Vezzalini for the longstanding and fruitful collaboration in the scientific studies of cultural heritage. G. Artioli would like to thank all students and coworkers for enthusiasm with archaeometric work, despite institutional barriers and scarcity of funds.