Archeological chemistry

(archeology) The application of chemical techniques to the study of the material remains of the cultures of historical or prehistorical peoples, for example, to ascertain the age or composition of such remains.

The application of chemical techniques to the study of archeological finds, natural or anthropogenic, in order to ascertain their composition or age. Traditional chemical analysis uses wet methods, in which a sample is brought into solution and its components are assayed by precipitation or titration. These methods were applied to ancient coins as early as the late eighteenth centuary. The obvious need to minimize damage to an irreplaceable object spurred the development of microchemical techniques. Modern analysis relies on instrumental methods that require only very small samples or are entirely nondestructive. Although these methods rely on physical phenomena rather than chemical transformation, all procedures that are capable of the qualitative and quantitative determination of the atomic or molecular composition of the object under study are usually included under the broad heading of archeological chemistry.

Various analytical methods are utilized in archeological chemistry, including optical emission spectography, atomic absorption spectroscopy, inductively coupled plasma, neutron activation analysis, x-ray fluorescence spectrometry, electron microprobe analysis, proton-induced x-ray emission, Auger electron spectroscopy, and x-ray photoelectron spectroscopy. It should be noted that these methods of analysis are not competing but complementary. The choice of method depends on the nature of the object, on the elements to be determined, and on the accuracy required. See also Activation analysis; Atomic spectrometry; Auger effect; Proton-induced x-ray emission (PIXE); Radioisotope; X-ray fluorescence analysis.

Organic materials constitute only a small portion of archeological finds, but since they include such basic necessities as food, drink, and clothing, they have the potential of revealing much about past life. Because they consist of covalently bound, complex, and sensitive molecules, their study requires special methods of analysis. Organic archeometry is the newest and most rapidly expanding field of archeological chemistry. Organic dyes have long been determined qualitatively and quantitatively by absorption spectroscopy in the visible and ultraviolet ranges. The extension into the infrared range allows not only the identification of organic materials by visual or computer-aided comparison of infrared spectra (“fingerprinting”) but also some structural interpretation. Since organic residues typically consist of mixtures of dozens or even hundreds of individual compounds, the progress of organic archeometry has crucially depended on the development of chromatographic separation procedures. These include column chromatography, paper and thin-layer chromatography, gas chromatography, with or without prior pyrolysis, and liquid chromatography. All of these techniques not only separate mixtures into individual components but permit their identification if the rate at which they travel through the chromatographic substrate, the retention time, can be matched to those of authentic reference compounds.

Another method that is gaining use in organic archeometry is nuclear magnetic resonance spectrometry (NMR), which detects a limited number of atomic nuclei, among them ordinary hydrogen, carbon-13, nitrogen-15, fluorine-19, and phosphorus-31, by their simultaneous interaction with an external magnetic field and a radio-frequency field. See also Gas chromatography; Infrared spectroscopy; Mass spectrometry; Nuclear magnetic resonance (NMR); Spectroscopy.

The determination of the chemical composition of an archeological find is not an end in itself, but provides the archeologist with factual evidence not otherwise obtainable and touching on many aspects of early human life. The changing elemental composition of coins detects progressive debasement and reveals economic history and fiscal policy. The metals added to copper to make bronze and brass outline the history and spread of technology. The foodstuffs consumed are indicators of the advent and progress of agriculture and animal husbandry. Together, all these paint a picture of prehistoric social, cultural, and economic stratification. The composition of an object also offers clues to its geographic origin, which may be far from the excavation site. This provides evidence of trade and exchange in commodities and raw materials. See also Prehistoric technology.

While the most widely used methods for dating archeological material—radioactive decay, thermoluminescence, and archeomagnetism—deal with physical processes, three depend on the progress of conventional chemical reactions. (1) Amino acid dating uses the rate of racemization of optically active organic molecules. (2) Hydration dating measures the thickness of the weathering layer produced by the action of water on natural and artificial glass, including obsidian and flint. (3) NFU dating of bone relies on the loss of nitrogen (N) from the organic collagen component and on the uptake of fluorine (F) and uranium (U) by the inorganic hydroxyapatite component. Like all nonnuclear chemical reactions, these changes are a function not only of time but also of temperature, of acidity and, in the case of fluorine and uranium uptake, of the concentrations of these elements in the surrounding soil. Chemical methods cannot produce absolute dates unless these other variables are known or can be estimated reasonably closely. They are, however, useful in establishing relative ages of finds within a single site in which the depositional characteristics are likely to have been uniform. See also Amino acid dating; Archeological chronology; Chemical microscopy; Dating methods; Paleomagnetism; Racemization; Radiocarbon dating.