Ancient copper smelting techniques are investigateed by ⁵⁷ Fe-Mössbauer spectroscopy. Stress is laid upon the bronze-age process for smelting chalcopyrite as used in mass production throughout the eastern Alps, but Japanese and 16th-century slags have been included. The spectra are evaluated for partial pressures of oxygen in the atmosphere of the furnaces. The latter was, as a rule, highly oxidizing with respect to iron, whenever chalcopyrite was the main source of copper. As a consequence, the chalcopyrite smelting furnaces of the Alps, as well as some cases in Cyprus and Sardinia, must have had a construction more like open hearths than shaft furnaces. -- AATA