Archaeological Chemistry

A Mark Pollard, Carl Heron, R D Gillard

• Publisher: Royal Society of Chemistry
• Year: 2015
• ISBN:
  9781782626114
• Edition: 2
• Topic: Naturwissenschaften
• Subtopic: Chemie

The application of chemistry within archaeology is an important and fascinating area. It allows the archaeologist to answer such questions as "what is this artefact made of?", "where did it come from?" and "how has it been changed through burial in the ground?", providing pointers to the earliest history of mankind. Archaeological Chemistry begins with a brief description of the goals and history of archaeological science, and the place of chemistry within it. It sets out the most widely used analytical techniques in archaeology and compares them in the light of relevant applications. The book includes an analysis of several specific archaeological investigations in which chemistry has been employed in tracing the origins of or in preserving artefacts. The choice of these investigations conforms to themes based on analytical techniques, and includes chapters on obsidian, ceramics, glass, metals and resins. Finally, it suggests a future role for chemical and biochemical applications in archaeology. Archaeological Chemistry enables scientists to tackle the fundamental issues of chemical change in the archaeological materials, in order to advance the study of the past. It will prove an essential companion to students in archaeological science and chemistry, field and museum archaeologists, and all those involved in conserving human artefacts.

CHAPTER 1

The Development of Archaeological Chemistry

1.1 INTRODUCTION

In its endeavour to understand human behaviour primarily through the material remains of past societies, archaeology has interacted more and more with the sciences of physics, chemistry, biology and of the Earth. In truth, it is a test to conjure the name of any scientific discipline which has not at one time or another provided information of direct use for the archaeologist (Pollard, 1995). Indeed, many would consider archaeology itself, a discipline which involves the systematic collection, evaluation and analysis of data and aims to model, test and theorize the nature of past human activity, to be a science. Furthermore, they might argue that it is possible to arrive at an objective understanding of past human behaviour, and in that sense archaeology is no different from other scientific disciplines, given the obvious differences in methodology. As Trigger (1988; 1) has reminded us, from a different perspective, archaeologists have a unique challenge:

‘Because archaeologists study the past, they are unable to observe human behaviour directly. Unlike historians, they also lack access to verbally encoded records of the past. Instead they must attempt to infer human behaviour and beliefs from the surviving remains of what people made and used before they can begin, like other social scientists, to explain phenomena.’

The claim that archaeology is a science is clearly not universally held. Many archaeologists suggest that the study of human behaviour in the past is restricted by science, with its apparent rigidity of scientific method and dubious claims of certainty, and must continue to reside with the humanities. Undoubtedly, archaeology is one of the few disciplines which straddles the gulf between the humanities and the sciences.

Archaeological Chemistry, Second Edition

By A. Mark Pollard and Carl Heron

© The Royal Society of Chemistry 2008

In our view, one of the fundamental enquiries in archaeology is the relationship between residues, artefacts, buildings and monuments, and human behaviour. From the period of production, use or modification of materials (whether natural or synthetic) to the time when traces are recovered by archaeologists, the material output of humans is altered by a plethora of physical, chemical and biological processes including those operating after deposition into the archaeological record. A significant part of the evidence is lost, displaced or altered significantly. Inferring the activities, motivations, ideas and beliefs of our ancestors from such a fragmentary record is no small task. In fact, it is a considerable challenge. Although there are notable exceptions, archaeology in the past 150 years has been transformed from a pastime pre-occupied with the embellishment of the contemporary world (or at least a minuscule portion of it) with treasure recovered from ‘lost civilizations’ (still a view which predominates in some media representations of archaeology, such as the cinema) to a discipline which relies on painstaking and systematic recovery of data followed by synthesis and interpretation. However, the development of archaeology has not been one uniform trajectory. There have been, and still are, numerous agendas which encompass the broad range of archaeological thought, and many uncertainties and disagreements concerning the direction of the discipline remain. Collectively, the sciences provide archaeology with numerous techniques and approaches to facilitate data analysis and interpretation, enhancing the opportunity to extract more information from the material record of past human activity. Specifically, chemistry has as much to offer as any other scientific discipline, if not more.

The sheer diversity of scientific analysis in archaeology renders a coherent and comprehensive summary intractable. Tite (1991) has packaged archaeological science rather neatly into the following areas:

• Physical and chemical dating methods which provide archaeology with absolute and relative chronologies.

• Artefact studies incorporating (i) provenance, (ii) technology, and (iii) use.

• Environmental approaches which provide information on past landscapes, climates, flora and fauna as well as diet, nutrition, health and pathology of people.

• Mathematical methods as tools for data treatment also encompassing the role of computers in handling, analysing and modelling the vast sources of data.

• Remote sensing applications comprising a battery of non-destructive techniques for the location and characterization of buried features at the regional, microregional and intrasite levels.

• Conservation science involving the study of decay processes and the development of new methods of conservation.

Although in this volume we focus on the interaction between chemistry and archaeology or archaeological chemistry, chemistry is relevant to most if not all of the areas proposed by Tite. For example, although many subsurface prospecting techniques rely on (geo)physical principles of measurement (such as localized variations in electrical resistance and small variations in Earth magnetism), geochemical prospection methods involving the determination of inorganic and biological markers of anthropogenic origin (i.e., chemical species arising as a direct consequence of human action) also have a role to play. Throughout this book, archaeological chemistry is viewed not as a straightforward application of routine methods to archaeological material but as a challenging field of enquiry, which requires a deep knowledge of the underlying principles in order to make a significant contribution.

1.2 EARLY INVESTIGATIONS

It would not be possible to write a history of chemistry without acknowledging the contribution of individuals such as Martin Heinrich Klaproth (1743–1817), Humphry Davy (1778–1829), Jöns Jakob Berzelius (1779–1848), Michael Faraday (1791–1867), Marcelin Berthelot (1827–1907) and Friedrich August von Kekulé (1829–1896). Yet these eminent scientists also figure in the early history of the scientific analysis of antiquities. Perhaps the primary motivation for their work was curiosity, which resulted from their dedication to the study and identification of matter and the way in which it is altered by chemical reaction. In addition to his significant contributions to analytical and mineralogical chemistry, Martin Heinrich Klaproth determined the approximate composition of some Greek and Roman coins, a number of other metal objects and a few pieces of Roman glass. Klaproth was a pioneer in gravimetry – the determination of elemental composition through the weighing of an insoluble product of a definite chemical reaction involving that element. His first paper entitled ‘Mémoire de numismatique docimastique’ was presented at the Royal Academy of Sciences and Belles-Lettres of Berlin on July 9th, 1795. The coins were either copper or copper alloy. In producing compositional data on ancient materials, Klaproth had first to devise workable quantitative schemes for the analysis of copper alloys and glass. His scheme for coins has been studied by Caley (1949; 242–43) and is summarized briefly below:

‘After the corrosion products had been removed from the surface of the metal to be analysed, a weighed sample was treated with ‘moderately concentrated’ nitric acid and the reaction mixture was allowed to stand overnight . . . the supernatant liquid was poured off and saved, and any undissolved metal or insoluble residue again treated with nitric acid . . . If tin was present as shown by the continued presence of a residue insoluble in nitric acid, this was collected on filter paper . . . (this) was simply dried in an oven and weighed . . . a parallel control experiment was made with a known weight of pure tin. It was found from this that 100 parts of dried residue contained 71 parts metallic tin, in other words the gravimetric factor was 0.71.

The filtrate from the separation of the tin was tested for silver by the addition of a saturated solution of sodium chloride to one portion and the introduction of a weighed copper plate into another.

Lead was separated from the solutions . . . by evaporation to a small volume. The separated lead sulfate was collected and either weighed as such or reduced to metallic lead in a crucible for direct weighing as metal.

(Copper) was determined as metal from the filtrate from the lead separation by placing in it a clean iron plate. The precipitated copper was then collected, dried and weighed.’

In addition to Klaproth’s pioneering work in quantitative analysis, he made a major contribution to mineralogical chemistry and discovered many elements in the process. His efforts did not go unrewarded, since he became Berlin’s first Professor of Chemistry.

In 1815, Humphry Davy published a paper on the examination of ancient pigments collected at Rome and Pompeii. In addition to reviewing evidence for natural pigments, he was also able to identify a synthetic pigment, later to be called Egyptian Blue, formed by fusing copper, silica and naturally occurring natron (sodium carbonate). A report by H. Diamond, published in the journal Archaeologia in 1867, includes a section on a Roman pottery glaze studied by Michael Faraday in which the presence of lead in the sample provided the first indications on chemical grounds of the use of lead glaze in antiquity. In addition to his significant contributions to modern chemistry during the first half of the 19th Century, Berzelius became interested in the composition of ancient bronzes. Similarly, Kekulé carried out analysis of an ancient sample of wood tar that may have comprised, in part, compounds with aromatic or benzene rings, the structure of which he subsequently proposed in 1865.

In addition to the diverse activities of these well-known scientists, efforts made by a number of other investigators during the 19th Century are worthy of note. Frequently they sought to examine ancient metal objects (Caley, 1949, 1951, 1967) with a view initially to understanding their composition and the technology needed to produce the artefacts, although other questions began to emerge. As these investigations continued, mostly in isolation from one another, prehistoric archaeology was making its first steps towards a systematic enquiry into the study and chronology of early materials. In 1819, Christian Thomsen assigned the artefacts in the Danish national collection into successive ages of stone, stone and copper, bronze, early iron and later iron. This relative chronology was based on comparisons of material type, decoration and the context of recovery, and it marked a major development in the study of ancient materials which prevails in archaeology today [see Trigger (2006; 121–129) for a more detailed consideration].

As early as the mid-19th Century, the Austrian scholar J.E. Wocel suggested that correlations in chemical composition could be used to ‘provenance’ or identify the source of archaeological materials and even to provide relative dates of manufacture and use. During the 1840s, C.C.T.C. Göbel, a chemist at the University of Dorpat (Tartu) in Estonia, began a study of large numbers of copper alloy artefacts from the Baltic region, comparing those recovered from excavations with known artefacts of prehistoric, Greek and Roman date. He concluded that the artefacts were probably Roman in origin. With the work of Göbel, scientific analysis progressed beyond the generation of analytical data on single specimens to, as Harbottle (1982; 14) has emphasized, ‘establishing a group chemical property.’ The French mineralogist Damour proposed that the geographical source of stone axes could be located by considering the density and chemical composition of a number of rock types, including jade and obsidian found ‘dans les monuments celtiques et chez les tribus sauvages’, as his papers of 1864 and 1866 were entitled (Caley, 1951; 66). Damour also exhorted archaeologists to work with specialists from other disciplines such as geology, zoology and palaeontology (Harbottle, 1982; 14). Perhaps he was aware of the interdisciplinary research programmes comprising zoologists, geologists and archaeologists then being carried out in Scandinavia on ancient shell mounds along the coast of Denmark (Klindt-Jensen, 1975; 71–73). Damour’s primary interest was jade. He proposed that prehistoric jade axes were fashioned from outcrops in the Mont Viso massif in northern Italy. It is only recently that his ideas have been confirmed by archaeological fieldwork (Pétrequin et al., 2006).

The appearance of the first appendices of chemical analysis and references to them in the text of a major excavation report represents the earliest significant collaboration between archaeologists and chemists. Examples include the analysis of four Assyrian bronzes and a sample of glass in Austen Henry Layard’s ‘Discoveries in the Ruins of Nineveh and Babylon’ published in 1853 and Heinrich Schliemann’s ‘Mycenae’ first published in 1878 (so distinguished was the publication of the Arno Press edition of 1880 that William Gladstone, the then British Prime Minister, wrote the preface!). The reports in the appendices of both these works were overseen by the metallurgist, John Percy, at the Royal School of Mines in London. Between 1861 and 1875, Percy wrote four major works on metallurgy which included significant sections on the early production and use of metals (Percy, 1861, 1864, 1870, 1875). These books remain important sources even today, because they contain first-hand descriptions of now lost metallurgical processes. Analysis of metal objects from Mycenae showed the extensive use of native gold and both copper and bronze, the latter used predominantly for weapons. Percy wrote in a letter to Schliemann dated August 10, 1877 that ‘Some of the results are, I think, both novel and important, in a metallurgical as well as archaeological point of view.’

The effort made by Otto Helm, an apothecary from Gdansk, Poland, to source amber towards the end of the 19th Century constitutes one of the earliest systematic applications of the natural sciences to archaeology. I...