Measuring the relative isotopic abundances of lead to characterize its isotopic composition has become a frequently used analytical method in determining the provenance of archaeological finds. In conjunction with diagnostic evaluations of an object's stylistic attributes and method of fabrication, the isotopic data can be used to address archaeological and art historical questions of authenticity, as well as to study the production and distribution of metal. Lead isotope analysis can provide information about the sources of the materials used and their trade patterns, information with valuable socioeconomic and political implications. The technique is applicable to a wide range of lead-containing materials such as glass, glazes, metals (copper, silver, bronzes, brasses, and pewter), paint pigments, and lead ores.
The technique has traditionally been used as an analytical tool in geological studies to measure the geochronological age of the Earth or to determine the age of an ore deposit. Elemental lead is composed of four stable isotopes whose masses are 208, 207, 206, and 204. Stable isotopes differ from each other only by the number of neutrons in their atomic nucleus, referred to as their mass number. Three of the four isotopes of lead (Pb 208, Pb 207, and Pb 206) are derived in part as the stable end products from the radiogenic decay of thorium 232, uranium 235, and uranium 238, respectively, and in part from primordial lead created at the time of the Earth's formation. The fourth isotope (Pb 204) exists in its original form (at the earth's beginning) and is dependent only on the amount originally present in the formation of the deposit. Thus, the isotopic composition of lead in ores is dependent upon the geochemical constituents of the mineralization and its age and emplacement history. These variances in the isotopic composition of lead create a distinct “isotopic signature” in ore deposits containing lead.
Because the differences in isotopic composition are very small, a measurement of high precision and high accuracy is required. Chemical-separation techniques under clean conditions, involving acid dissolution, ion-exchange chromatography, and anodic electrodeposition are used to extract and purify the lead. Thermal ionization mass spectrometry is the analytical tool generally used to measure the relative isotopic abundances in naturally occurring elements. This involves the introduction of a sample into a mass spectrometer, where it is thermally ionized, accelerated by electrostatic forces through a magnetic field, and separated according to mass. The various masses are then electronically detected and measured.
Archaeological evidence has demonstrated that the isotopic signature present in the ore is preserved in the object, unaffected by the object's metallurgical history or subsequent weathering. In theory, this unique isotopic composition can be used to identify the object's geographic source or to eliminate others. However, the method has its limitations: mixing and overlapping are factors that pose complications in interpretating the data. In antiquity, lead was frequently salvaged, remelted, and reused. During this recycling, certain materials could have been mixed, thus erasing their original lead signatures. The overlapping factor arises from the fact that ores in different regions, sometimes widely separated geographically, can have similar isotopic compositions if formed at the same time under geologically similar conditions. Still used in conjunction with other analytical methods, such as trace-element analysis and multi-variate and probability statistical analysis, lead isotope analysis has proven to be an important technique in archaeology.
[See also Analytical Techniques.]
- Faure, Gunter. Principles of Isotope Geology. New York, 1986.
- Gale, N. H. “Lead Isotope Studies Applied to Provenance Studies: A Brief Review.” In Archaeometry: Proceedings of the 25th International Symposium, edited by Yannis Maniatis, pp. 496–502. Amsterdam, 1989.
- Gulson, Brian L. Lead Isotopes in Mineral Exploration. New York, 1986.
- Russell, Richard D., and R. M. Farquhar. Lead Isotopes in Geology. New York, 1960.
- Sayre, E. V., et al. “Statistical Evaluation of the Presently Accumulated Lead Isotope Data from Anatolia and Surrounding Regions.” Archaeometry 34.1 (1992): 73–105. Covers the debate on the use of multivariate and probability statistical analysis, methodology, selection of data, treatment of outliers, etc. For continuing discussion, see Archaeometry 34.2 (1992): 311–336, and 35.2 (1993): 241–263.
Emile C. Joel