Ancient Iron Metals Tracing by Iron Isotopes Analysis

Provenance studies of iron artefacts have become an important topic in archaeology to better understand the socioeconomic organization of ancient societies. Elemental and isotopic tracing methods used so far for iron metal provenance studies showed some limitations, and the development of new additional tracers are needed. Since the last decade, the rise of cutting edge analytical techniques allows for the development of new isotopic tools for this purpose. The present study explores for the first time the use of iron isotopes analyses as a potential method for ancient iron metal tracing. Ore, slag and metal samples from two experimental reconstitutions of iron ore reduction by bloomery process were collected. Their Fe isotope compositions were measured by Multi Collector e Inductively Coupled Plasma e Mass Spectrometry (MC-ICP-MS) to assess the possible impact of smelting on the Fe isotope composition of the metal produced. Our results show that the iron isotope compositions of the slag and metal are for 8 out of 9 samples analyzed undistinguishable from that of the starting ores. This suggests that overall, no significant Fe isotope fractionation occurs along the chaîne op eratoire of iron bars production, even if slight isotopic differences might be found in blooms before refinement. This fact, combined with the natural isotopic variability of iron ores, as reported in the literature, may allow the use of Fe isotopes as a relevant tracer for archaeological iron metals. This new tracing approach offers many perspectives for provenance studies. The combination of elemental and Fe isotope analyses should thus be useful to validate origin hypotheses of ancient iron artefacts.

Archaeometry, 2021

Fe isotopes were used to determine the origin of iron bars from Les Saintes-Maries-de-laMer Roman shipwrecks, which was a major archaeological finding at the end of the 20th century in France. Their Fe isotope composition was measured by multi collectorinductively coupled plasma-mass spectrometry (MC-ICP-MS) after chemical Fe purification. The results allowed us to suggest provenances that were compared with those based on trace element analyses of slag inclusions. For most of the bars, we validate the provenance hypotheses previously proposed. Two bar groups originate from the Montagne Noire metallurgical district (south-west France), whereas a third group comes from another source, not clearly identified so far. In this context of Roman iron production, we argue for a non-spatially segmented production, where bars were manufactured close to smelting sites. Combined trace element and Fe isotope analyses on the same objects provide crucial information about the nature of their ore source. The elemental heterogeneity, positive Eu anomaly and Fe isotopic homogeneity of several bars were inherited from gossan-type ores, whereas the negative Eu anomaly and variable Fe isotopes signature of others most likely correspond to sedimentary iron ores. This study demonstrates that combined trace element and Fe isotopes analyses in a well-defined archaeological context is a promising approach for provenance studies of iron metals of archaeological interest.

Archaeometry, 2003

Knowledge of the iron trade in ancient times rests on determining the source of the objects recovered during archaeological excavations. Unfortunately, attributing the origin is not always possible using archaeological tools alone. Trace element data of the ore and the archaeological material (reduction and reheating slags, entrapped slag inclusions from the bloom, bar iron and iron blades) from two Gallo-Roman iron-making centres, Les Martys (Montagne Noire) and Les Ferrys (Loiret), were determined using ICP-AES/MS and LA-ICP-MS. The results highlight a continuity of composition that makes it possible to establish a link between the initial ore and the semi-finished iron products.

A B S T R A C T Determining the provenance of non-ferrous metals is a major issue in archaeology and history. The classical method using Pb isotopes shows several limitations, and new isotopic tracers need to be developed in archae-ometallurgy. Recently, Fe isotopes were found to be promising for iron metal tracing. In this context, we evaluated the pertinence of Fe isotopes as a new tool for provenance studies of non-ferrous metals. We collected slag and furnace lining samples from the Imiter silver mine in Morocco (Anti-Atlas), where many archaeological remains attest ancient lead-silver production. Their mineralogical composition was investigated by optical microscopic observations, SEM-EDS and EPMA. Besides, we measured both Pb and Fe isotope compositions of these samples by high resolution MC-ICP-MS after wet sample decomposition and purification chemistry. The occurrence of Ag, Pb, Cu and Ag/Pb metallic beads in the slag samples and their high Fe content suggests that these metallurgical materials result from the exploitation of Ag-rich polymetallic ores associated with gossan formations. Isotopic analyses of these slags highlighted their heterogeneous Pb isotope compositions and homogeneous Fe isotope composition. This points to the exploitation of two distinct ore sources with different Pb but similar Fe isotope compositions, despite the Fe isotope variability often encountered in metallic ores worldwide. Hence, Fe isotopes provide a complementary approach to trace lead-silver products from Imiter. Overall, the combination of Pb and Fe isotope analyses constitutes a promising method for further provenance investigations of non-ferrous metals.

Archaeology, New Approaches in Theory and Techniques, 2012

Os isotopic ratios and trace element approaches were used to compare the signatures of ore and slag from different potential production sites located in eastern France and South-West Germany with the signature of artefacts from the end of the first Iron Age. A set of 31 artefacts was tested, consisting of bipyramidal semi-products, chariot tires, blooms and other commodities. The complementarity of the two approaches is demonstrated. Bipartite bipyramidal semi-products made by assembling two crude masses of distinct origins are evidenced suggesting the existence of intermediate producing centres assembling products from different origins. Only the provenance of blooms and wheel-tires could be established as local. Two spheres of metal circulation were evidenced: prestige and local. Bipyramidal semi-products and chariot tires belong to different long distance distribution networks.

PLOS ONE

The innovation of iron production is often considered one of the greatest technological advances in human history. A reliable provenancing method for iron is instrumental for the reconstruction of economic, social and geo-political aspects of iron production and use in antiquity. Although the potential of osmium isotopes analysis for this purpose has been previously suggested, here we present for the first time the results of osmium isotope analysis of ores, bloom and metal obtained from a set of systematic, bloomery iron-smelting experiments, utilizing selected ores from the Southern Levant. The results show that the 187 Os/ 188 Os ratio is preserved from ore to metal, with no isotopic fractionation. In addition, enrichment/depletion of osmium content was observed in the transition from ore to metal and from ore to slag. This observation has potential significance for our ability to differentiate between the various processes and sheds light on the suitability of various production remains for this method, which emerges as a robust and promising tool for the provenancing of archaeological ferrous metals.