Archaeological chemistry

Introduction

Archaeological chemistry belongs to the broader research field of archaeometry, which represents the application of various scientific analytical techniques to archaeological artefacts. These can include physics, chemistry, biology, palaeoanthropology, mathematics, computer science, etc.

The application of archaeological chemistry has shifted the traditional archaeological research approach away from determination of artefacts’ typologies and dating, into the sphere of determining the preservation, variety and origin of organic molecules.

Many archaeological artefacts and sediments are porous and absorbent (pottery, bones, textiles, soil), which represents an excellent environment for trapping these molecules and slowing down their degradation during the post-depositional period. With the application of analytical chemistry, these can then be related back to previous vessel use, ancient diet, trade and economy.

Although this interdisciplinary research began more than 25 years ago, the archaeological audience is slowly getting familiar with its scope and potential, despite the fact that, according to the recently published Analytical Chemistry in Archaeology (Pollard et al. 2007), approximately 99.9 % of the human past is out of history’s reach. Due to the nature of archaeological excavation, which is always a destructive process, the need for complementary research is therefore essential in order to extract the most information, which can in turn enhance archaeological interpretations.

The basic analytical approach, adopted by the Organic Geochemistry Unit (OGU), relies upon the identification of preserved molecules (biomarkers); matching their distribution to the compounds present in organisms that were most likely to have been exploited in the past. The OGU’s involvement in archaeology is multi-layered and covers a broad spectrum of artefacts and materials submitted for analysis.

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Organic residue analysis of archaeological pottery

Lipid residues of cooking and the processing of other organic commodities have been found to survive in archaeological pottery vessels as components of surface and absorbed residues for several thousand years. Following extraction, using a combination of modern analytical techniques, including: high temperature-gas chromatography (HTGC), GC/mass spectrometry (GC/MS) and GC-combustion-isotope ratio MS (GC-C-IRMS), the components of the lipid extracts of such residues can be identified and quantified. Characterisation of lipid extracts to commodity type is only possible through detailed knowledge of diagnostic compounds and their associated degradation products formed during vessel use or burial. An increasing range of commodities is being detected in pottery vessels, including animal products (meat and milk), leafy vegetables, specific plant oils and beeswax.

Animal fats are by far the most common residue identified from archaeological pottery with the use of compound-specific stable carbon isotope analysis allowing detailed characterisation of their source. GC-C-IRMS enables the carbon stable isotope (δ13C) values of individual compounds to be determined. We have found that the δ13C values for the principal fatty acids (C16:0 and C18:0) are crucial in distinguishing between different animal fats, e.g. ruminant and non-ruminant adipose fats and dairy fats, as well as in the identification of the mixing of commodities.

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Investigating the diet of ancient people

Another very abundant and important archaeological artefact is human osteological material, which survives predominantly as remains of burials or as accidental preservation (e.g. glacier bodies). Human bones also display a very porous surface, which enables the entrapment and preservation of organic molecules, which can then be extracted with organic solvents and analysed, using a combination of techniques, such as gas chromatography, GC-mass spectrometry (MS), and more importantly GC-combustion-isotope ratio MS (GC/C/IRMS) and liquid chromatography IRMS (LC/IRMS) for the measurement of stable isotopes ratios of carbon (δ13C) and/or nitrogen (δ15N). Archaeological plant remains, like charred seeds, can also provide insights into their exploitation and revealing the ancient practices of manuring the domesticated crops.

The most routinely analysed compounds are amino acids in bone collagen, which can be used for bulk and/or compound specific isotopic measurements that enables us to unravel the palaeodietary signals (terrestrial vs. marine input) and reconstruct the past food webs.

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Analysis of ancient sediments and soils

Ancient sediments can offer satisfactory environment for the preservation of a wide range of organic molecules under favourable conditions, like waterlogged or extremely arid deposits. Classes of compounds, like sterols and bile acids from these anthropologically modified sediments provide and interesting insight into the world of past agricultural activities (manuring), waste water disposal or reveal traces of burial practices and ritual activities. The latter was put to test, when ancient sediments, originating from the royal Syrian tomb of Qatna, where analysed and remains of the purple dye pigments revealed in connection to the microscopic fossilised textiles.

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Mummies and characterizing the embalming agents

The world of the ancient Egyptians has been under the scientific scrutiny for a long time. Although most of the domestic activities, engineering work and Egyptian culture can be reconstructed by using the historical and archaeological data, the same cannot be said for the process of mummification. Without any substantial early historical reports (the earliest being Herodotus), the chemical analysis is the only means to identify the complex mixtures of natural products that were used in the process. The biomarkers, present in either tissue samples, bone surfaces or bandages, show that animal fats were used as an inexpensive base, to which more exotic substances were added to enhance the smell, prevent microbial degradation and change the plasticity, such as myrrh, frankincense, pistachio resin, beeswax etc.

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Image of some archaeological pots
The basic analytical approach, adopted by the Organic Geochemistry Unit, relies upon the identification of preserved molecules (biomarkers); matching their distribution to the compounds present in organisms that were most likely to have been exploited in the past. The OGU's involvement in archaeology is multi-layered and covers a broad spectrum of artefacts and materials submitted for analysis.
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