Analysis of Ancient Slag Inclusion-Metal Systems as a Method to Disclose Processing Thermo-chemical Parameters: The Case Study of a Medieval Lombard Steel Bar from Northern Italy

Abstract

An archaeometallurgical characterization of a VI to VII century Lombard steel bar from Ponte di Val Gabbia I site (northern Italy) was performed to reconstruct its manufacturing process and extract processing thermochemical parameters (i.e., temperature and oxygen chemical potential) from the slag inclusion-metal systems. Metallographic observations by optical microscopy and Vickers microhardness measurements indicated that the steel bar was forged from a heterogeneous steel lump, most probably an iron bloom, which was cooled in the air after the forging process. The chemical composition of non-metallic inclusions was determined by scanning electron microscopy coupled with X-ray dispersive spectroscopy. A multivariate statistical analysis of the smelting-derived non-metallic inclusions chemistry allowed to conclude that the steel bar was obtained by the direct or bloomery method. Under the hypothesis of local near-equilibrium conditions for the slag inclusion-metal systems, a thermodynamic-based strategy was adopted to estimate the firing conditions in the bloomery furnace in terms of both temperature and oxygen chemical potential of the reducing gas atmosphere. In particular, the computed temperatures are in the range of 1003 °C to 1171 °C, whereas the values of oxygen chemical potential vary between − 447.47 and − 385.79 kJ/mol.

Exploring the Manufacturing Process of a Renaissance Breach Pike

“…In this respect, it is worth noting that an inverse correlation between the iron oxide concentration in smelting slag inclusions and the carbon content in the metal matrix has been frequently reported for ancient heterogeneous ferrous alloys produced through direct and indirect iron-making processes [19]. This phenomenon indicates that some SI-metal systems likely approach thermodynamic equilibrium [6,20]. Moreover, it was assumed that the iron oxide content in SI and the carbon concentration in the metal matrix are mainly governed by the gas phase generated from the reaction of air oxygen with charcoal in bloomery or fining furnaces.…”

“…In this research work, the experimental protocol specifically tailored by the authors for the archeometallurgical examination of other historical ferrous artefacts was applied [6][7][8][9]. In particular, each section was prepared following a standard metallographic procedure consisting of grinding with SiC abrasive papers (grades from 600 to 1200) and polishing with polycrystalline diamond pastes (3 µm and 1 µm).…”

“…The thermodynamic-based model developed and described in detail by the authors in previous research work [6] was employed to estimate the temperature (T) and oxygen chemical potential (µ O2 ) of the gas phase equilibrated with each SI-metal system (i.e., SI along with the surrounding metallic phase). These thermochemical parameters can provide an indication of the iron smelting working conditions.…”

“…Given this scenario, this study aims to provide new deep insights into the iron-making technology and forging techniques employed in the production of breach pikes. To achieve this goal, a breach pike donated from the private collection of the "The Martial Arts Museum" in Botticino (Brescia, northern Italy) was carefully analyzed following a state-ofthe-art methodology based on a metallography and slag inclusions (SI) analysis [6][7][8][9][10][11].…”

Archaeometallurgical Characterization of Two Lombard Early Medieval Bloomery Slags from Ponte di Val Gabbia I Site (Northern Italy)

“…Namely, the smelting temperature is usually estimated by projecting the slag bulk chemical composition on chemistry and mineralogy-based phase 2 of 15 diagrams [7][8][9]. Moreover, geothermometers and software for thermodynamic modeling, such as Rhyolite-MELTS, have proven to be useful and promising tools for high-resolution slag crystallization temperature reconstruction [10][11][12][13][14]. On the other hand, the smelting redox conditions were often inferred from the assemblage of slag mineralogical phases, using as references the oxygen buffer curves (e.g., Quartz-Fayalite-Magnetite oxygen buffer) in the oxygen fugacity-temperature space [15,16].…”