The frequent occurrence of iron slag in excavation layers of the Hellenistic to Byzantine city of Sagalassos (SW Turkey), indicates continuous iron working from the 1st to 7th century AD. The slag samples are identified as smithing hearth bottoms and are found in dump fills, building foundations and road constructions. Though several smelting sites are present in the 1800 km² large territory of the city, the chemical signature of the smelting slag does not correspond to the characteristics of the smithing slag found in the city itself. The smithing slag does offer insight in the smithing techniques applied, which in turn provides information about the objects produced and production chain or “chaîne opératoire” followed for iron production in Sagalassos. Textural and mineralogical analyses reveal two main smithing techniques were applied throughout the 1st to 7th century AD. The most commonly applied smithing technique is used for the production of simple bulk materials, like hammers or anvils. Microscopic textures in the slag show mainly mechanical deformation at relatively high temperatures. The second, more sophisticated smithing technique shows the blacksmith treated the iron at different temperatures. The presence of high lime contents indicates the use of a flux and/or a protective agent against oxidation. This technique was applied to make more complex objects or objects with cutting edges. Based on the reconstruction of the smithing techniques, the iron production of Sagalassos produced mainly everyday bulk material. The current study demonstrates the importance of using the mineralogy of smithing slag in reconstructing the technological evolution of iron production. Microtextural and mineralogical studies and comparison with modern smithing techniques offer greater insight in the complex world of Roman blacksmithing.