Archaeological Chemistry

A Mark Pollard, Carl Heron, Ruth Ann Armitage

• Publisher: Royal Society of Chemistry
• Year: 2020
• ISBN:
  9781839162992
• Edition: 3
• Topic: Scienze sociali
• Subtopic: Archeologia

The use of chemistry in archaeology can help archaeologists answer questions about the nature and origin of the many organic and inorganic finds recovered through excavation, providing valuable information about the social history of humankind. This textbook tackles the fundamental issues in chemical studies of archaeological materials. Examining the most widely used analytical techniques in archaeology, the third edition of this comprehensive textbook features a new chapter on proteomics, capturing significant developments in protein recognition for dating and characterisation. The textbook has been updated to encompass the latest developments in the field. The textbook explores several archaeological investigations in which chemistry has been employed in tracing the origins of or in studying artefacts, and includes chapters on obsidian, ceramics, glass, metals and resins. It is an essential companion to students in archaeological science and chemistry, as well as to archaeologists, and those involved in conserving human artefacts.

1 The Development of Archaeological Chemistry

1.1 Introduction

In its endeavour to understand human behaviour primarily through a study of the material remains of past societies, archaeology has interacted from its very beginnings 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 been of direct use to archaeology.¹ Indeed, many would consider archaeology itself, a discipline which involves the systematic collection, evaluation and analysis of data, and which aims to model, test and theorise 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. However, as Trigger² (p. 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.’

There is clearly a difference of emphasis here, and one which is primarily focused on contrasting the methods used (often scientific) against the desired outcomes (largely cultural). Whether or not one regards archaeology of itself as a science, or as a humanities-based discipline which calls upon scientific methods to aid in the collection and interpretation of evidence, there can be little doubt that archaeology and the natural sciences have been intertwined since their very inception in the 17th century. Undoubtedly, archaeology is one of the few disciplines which comprehensively straddles the humanities and the sciences, so famously described by C. P. Snow as ‘the two cultures’.³ Debate now tends to focus on the multifaceted nature of archaeology, drawing unashamedly on a whole range of other disciplines, and not privileging one form of knowledge above any other.⁴ It is tempting to note that when archaeological chemistry started in the late 18th century (as described below), the world was such that an educated person could be expected to be conversant with all aspects of science, in addition to literature and, especially, the classics. Regrettably this is now much more difficult, and so the essence of good archaeology is open, respectful, meaningful and iterative dialogue across the many disciplinary boundaries involved.⁵

Archaeology in the last 300 years has largely been transformed from a pastime of the nobility and the educated upper classes, often simply preoccupied with the embellishment of the contemporary world with treasure recovered from ‘lost civilisations’, into an academic discipline which relies on painstaking and systematic recovery of data followed by careful synthesis and interpretation. (Unfortunately, however, the former preoccupation can still be seen in some media representations of archaeology!) In our view, the fundamental characteristic of archaeology is the creation of an understanding of the many relationships between residues, artefacts, buildings, monuments, landscapes, and past human behaviour. It is this last aspect—the inference of human behaviour from material culture—which distinguishes archaeological from ‘heritage science’, which is more interested in the objects themselves, their preservation, and their potential role in modern society as part of the ‘heritage industry’. 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 may be lost, displaced or altered significantly. Inferring the activities, motivations, ideas and beliefs of our ancestors from such a fragmentary record is a considerable but interesting challenge, and one which shares many characteristics with forensic science, which also seeks to reconstruct events and motives from material remains, albeit usually over a shorter timescale.

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 natural 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. Chemistry has as much to offer as any other scientific discipline, if not more.

The sheer diversity of scientific analysis in archaeology renders a simple coherent and comprehensive summary intractable. Tite⁶ 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 the diet, nutrition, health and diseases 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 characterisation of buried features at the regional, micro-regional and intra-site levels.
• Conservation science involving the study of decay processes and the development of new methods of conservation.

Although in this volume we focus specifically on the interaction between chemistry and archaeology, or archaeological chemistry, chemistry is relevant to most if not all of the areas identified by Tite. For example, although many subsurface prospecting techniques rely on (geo-) physical principles of measurement (such as localised 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. However, throughout this book, archaeological chemistry is viewed not as a straightforward application of routine chemical methods to archaeological material, but as a challenging field of enquiry, which requires a deep knowledge of the underlying principles in both archaeology and chemistry in order to make a significant contribution.

1.2 Early Chemical Investigations of Archaeological Material

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 prominently in the early history of the scientific analysis of antiquities. They appear to have considered the investigation of archaeological artefacts as simply part of the wider scientific exploration of the natural world which characterised the European Renaissance. The contents of the ‘cabinets of curiosities’ created by the aristocracy and, from the 17th century onwards, public museums, must partly have formed the basis for this explosion of interest, together with the development of the techniques of analytical chemistry ‘by the humid method’, i.e., wet chemistry and gravimetric methods of analysis, attributable to Torbern Bergman (1735–1784) at the University of Uppsala, Sweden around 1770.⁷

It has become traditional to assign the earliest analysis of archaeological metal to Martin Heinrich Klaproth, citing his detailed record of the gravimetric analysis of six Greek and nine Roman copper alloy coins, as well as a careful description of each coin.⁸ This paper, entitled Mémoire de numismatique docimastique, was presented at the Royal Academy of Sciences and Belles-Lettres of Berlin on July 9th, 1795, but was not published until 1798 (in a volume dated 1792–3). Further research, however, has suggested that he was not actually the first ‘archaeometallurgist’.⁹ This honour appears to go to Michel Jean Jérome Dizé (1764–1852), who, in 1790,¹⁰ published the analyses of eight copper alloy coins, given to him by M. L'abbé Antoine Mongez (1747–1835). These consisted of five Roman coins (dating to after the Emperor Nero), one Greek (‘de Syracuse’) and two ‘Gaulish’ coins. His analyses were not as comprehensive as those of Klaproth a few years later, however, since they reported only the amount of tin present in the alloy.

Klaproth's scheme for analysing copper, tin, lead and silver in copper coins has been studied by Caley¹¹ (pp. 242–43) and can be summarised as:

‘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 f...