archaeometallurgy

Background

Early iron metallurgy of the first millennium BCE is often described either as technically immature or as a sharp break from Bronze Age practice. Both views are too coarse. Our new study of a bloomery steel chisel from Rocha do Vigio shows a more gradual development (Asmus et al 2026).

A composite mesoscopic image of the sampled bloomery steel chisel-tip. © 2025 Bastian Asmus

The artefact

The object was made for a bloomery iron and is dated to the ninth century BCE.  In an earlier study, we characterised the metal as bloomery steel and determined its carbon content. Quantifying carbon in hardened material is only partly reliable. For this reason, the first study focused on the body of the tool. The cutting edge was not examined at that stage (Araque Gonzalez et al 2023).

Based on a carbon content of about 0.5 wt% C and the early date of the artefact, we later investigated the tip itself. The aim was to assess whether, and to what extent, it had been thermally treated of it was  hardened at all.

Bloomery steel: Microstructure and hardness

Metallography of the cutting edge shows a homogeneous and very fine pearlitic to pearlitic-bainitic microstructure. Ferrite is present only in small amounts. Martensite is absent. This points to accelerated cooling, but not to full quenching in the modern sense.

Secondary electron image of the chisel tip close to the cutting edge, showing mostly very fine pearlite, with some upper bainite in between the feathery colonies of the fine pearlite. Image: Asmus.

Vickers microhardness measurements show a moderate hardness gradient between the softer body and the refined tip. The values are consistent with controlled thermal treatment during forging. There is no indication that maximum hardness was the goal.

Alloy chemistry

The bloomery steel is low in manganese, as expected for early iron. Its hardenability therefore differs strongly from that of modern steels. Many commonly used transformation models are based on modern reference compositions. Our results show that these models are only of limited use for early iron artefacts. More accurate transformation data for low-Mn systems are – surprisingly – still lacking.

Production context

Slags from the site confirm local primary iron production. The chisel is part of a regional metallurgical practice. It is not an imported or exceptional object.

Taken together, the evidence points to a deliberate transfer of Bronze Age thermal working strategies to iron. Early iron metallurgy in this case reflects continuity of skill rather than a technological breakthrough.

The full article is available here:

https://doi.org/10.1016/j.jmrt.2026.01.091

References

Image width 200 µm, PPL. Medieval copper smelting slag.
The first thing to do is to establish the number of different phases present in the sample. In this case there are five different phases.

If you managed to follow so far, you have now reached part seven part of the slag microscopy course. After sample prep, with find documentation, cutting, mounting, grinding, lapping and polishing we are now going to have a look at the tool to be used for the next sessions: the polarising reflected light microscope, also referred to as an ore microscope. Continue reading

Fig 1: The use of scientific image processing software allows to quantify the area proportion of each phase in optical micrographs. This is a two step process. The original micrograph is converted to a “threshold map” by modifying the colour channels of the source. The resulting black and white image is analysed for their respective area proportions. The count mask is then produced after quantification and may be used to verify which inclusions have been counted.

I used this method during my  PhD thesis to approximate the chemical composition based on a micrograph  .

A traditional method for the quantification of an alloying element in another is the estimation of the carbon content of a steel sample. The area of carbon inclusions is estimated by comparison with known standards, or better by measuring them. Area proportions are believed to represent volume proportions and need to be multiplied with the density ρ to calculate wt% proportions. Continue reading