Microscopic study on some Iron Age bronze objects from Western Iran

Background: A microscopic study on ancient metals was established to identify chemical composition and microstructure of archaeological tin bronze objects. For this purpose, ten small bronze pieces from Baba Jilan Iron Age site, western Iran, were selected and studied by optical microscopy (OM) and scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM–EDS) methods. Results: The results showed that all objects were made of variable tin containing binary copper-tin alloy. It may be related to the application of an uncontrolled procedure to make bronze alloy. In some samples, lead and arsenic are detected as major elements, but they can be considered rather as impurities from the original ores. Different microstructural features such as elongated Cu–S inclusions as well as Pb globules are visible in the bronze matrix. Based on the metallographic examination, the bronze pieces are shaped by cycles of cold-working and annealing. Conclusion: The microstructure of Baba Jilan bronze objects is similar to other Iron Age bronze artefacts from different regions of Iran. In fact, process of bronze production in the Iron Age of Iran was to produce bronze alloy with different amounts of tin and without any third alloying element, using copper sulphidic or a mixture of copper sulphidic and oxidic ores to smelting copper, and applying working and heat treatment to shape and manufacture thin sheet objects.