Capabilities and limitations of Pb, Sr and Fe isotopic analysis of iron-rich slags: a case study on the medieval port at Hoeke (Belgium)

In this work, an analytical approach was developed for Pb, Sr, and Fe isotopic analysis of archaeological samples recovered from an iron work site by using multi-collector inductively coupled plasma – mass spectrometry (MC-ICP-MS). The sample types include slag, coal, clay and hammer scales, all obtained from an archaeological site at Hoeke (Belgium). Despite the wide concentration range of the target elements present in the samples and some sample manipulations necessarily performed outside of a clean laboratory facility, the analytical procedure yielded accurate and precise results for QA/QC standards while blank levels were negligible. Preliminary results concerning Pb, Sr and Fe isotope ratio variations in archaeological materials associated with iron working processes are provided. The samples revealed high variability in metal isotopic compositions, with the ²⁰⁸Pb/²⁰⁷Pb ratio ranging from 2.4261 to 2.4824, the ⁸⁷Sr/⁸⁶Sr ratio from 0.7100 to 0.7220, and δ⁵⁶Fe values from −0.34 to +0.08‰, which was tentatively attributed to the mixing of materials during the iron production process or variability within the source material. Also, contamination introduced by coal and furnace/hearth lining material could have contributed to the wide range of isotopic compositions observed. Because of the absence of information and data for primary ore samples to compare with, the provenance of the materials could not be established. The present study highlights the challenges in interpreting archaeological data, particularly in terms of the isotopic variability observed. It underscores the necessity of integrating analysis data with historical and archaeological knowledge. Further research, involving detailed analysis of these source materials combined with robust historical evidence, is essential to validate hypotheses concerning the origin of iron.