Ancient metallurgical waste found on the Greek island of Andros is giving researchers a closer look at ironworking during the Hellenistic period. A new study shows that slags excavated in the ancient city of Paleopolis date to the 4th and 3rd centuries BCE and came from the local refining and forging of iron, not from the first stage of ore smelting.
Andros, part of the Cyclades in the Aegean Sea, contains rich mineral deposits, including iron ores near Palaeopolis and the villages of Agios Petros and Mpatsi. These ores formed within marbles and schists and include minerals such as limonite, goethite, hematite, and siderite. Archaeologists have known for years that Paleopolis held signs of metalworking, with large accumulations of slag so common in one area that a nearby hill became known as Skouria, the Greek word for slag.
To better understand this activity, researchers analyzed 22 slag samples recovered from excavations in the agora, the ancient city’s central public area, along with five iron ore samples from the nearby Agios Petros mines, located around 10 kilometers northwest of Paleopolis.
Excavations in the agora have uncovered several furnaces from the 4th century BCE, three smelting pits, and small hearths built over layers of fired clay bricks. Archaeologists also found clay mold fragments, charcoal remains, pieces of iron and bronze, lead fragments, and tools made of bone or metal. A plaster model of a small lekythos, a vessel common in the period, was also recovered and appears to have been used in mold production.
Laboratory tests included X-ray diffraction, X-ray fluorescence, and electron microscopy. These methods identified several mineral phases typical of ironworking waste, including wüstite, fayalite, bustamite, tephroite, and related silicates. The chemical composition of the slag matched local iron ores, confirming that the raw materials came from Andros itself.
The ores stood out because they were naturally rich in manganese and barium. These elements lowered melting temperatures during iron production and improved slag flow, helping blacksmiths process iron more efficiently. Researchers found manganese minerals such as braunite, todorokite, and cryptomelane in some ore samples, along with traces of lead and zinc.
The slag chemistry also revealed unusually high phosphorus levels, much higher than those found in the raw ores. This suggests blacksmiths deliberately added calcium- and phosphorus-rich rocks, likely containing apatite, as extra flux material. Such additions would have improved metalworking by lowering melting points and making iron easier to refine.
One unusual result was the presence of very high thallium concentrations in some slag samples, reaching up to 899 parts per million. Researchers link this to the geological origin of the ore deposits. Similar thallium-rich iron oxides are known from hydrothermal systems in the Aegean region, including submarine volcanic environments near Milos and Santorini. This suggests the Andros ores formed in a deep marine volcanic setting.
Researchers used a mineral chemistry diagram comparing iron oxide, alumina, silica, calcium, and manganese to determine which metallurgical process created the waste. The results placed the samples firmly within the range of smithing slags rather than primary smelting slags.
This distinction matters. Primary smelting turns ore into raw iron blooms, while smithing refines that iron, removes impurities, and shapes finished tools or weapons. Because the slags were found in the urban center of Paleopolis rather than near the mines, the evidence points to workshops focused on manufacturing and repair.
The timing of this metalworking activity overlaps with the period after Alexander the Great’s death, when his successors fought for control of his empire. During these centuries, Andros was under Macedonian rule. Researchers suggest the island’s iron industry may have supported local economic or military needs, though the evidence does not yet allow a direct link to warfare.
Near the Agios Petros mines, archaeologists have also documented several Hellenistic stone towers dating to roughly the 3rd century BCE. Similar towers appear near mining districts on other Cycladic islands and likely served as watchtowers or protective structures.
The study paints a picture of Paleopolis as an active center of ironworking in the Hellenistic Aegean, using local mineral resources and practical metallurgical knowledge. Large parts of the ancient city remain unexcavated, leaving open the possibility that more workshops, tools, and industrial features are still buried beneath the site.
More information: Stamatakis, G., Palaiokrassa-Kopitsa, L., & Stamatakis, M. G. (2026). The ancient slags of Palaeopolis, Andros Island, Greece. Geochemical and mineralogical characterization and archaeometallurgical implications. Archaeological and Anthropological Sciences, 18(5). doi:10.1007/s12520-026-02463-z
Disclaimer: This website is a science-focused magazine that welcomes both academic and non-academic audiences. Comments are written by users and may include personal opinions or unverified claims. They do not necessarily reflect the views of our editorial team or rely on scientific evidence.
Comment Policy: We kindly ask all commenters to engage respectfully. Comments that contain offensive, insulting, degrading, discriminatory, or racist content will be automatically removed.