In 2016-2017, at Combaillaux (Hérault, France), the preventive excavation carried out by the Inrap allowed the study of an exceptional set of three slag pit furnace with multiple use related to the direct iron ore reduction in 700 m2 open area. All three are based on the same model: an oblong access pit leading to a circular excavated heating chamber with a natural ventilation system activated by 5 vents arranged around the bottom of the chimney. Around the furnace, some twenty structures reflect the organization of work within the workshop. The workshop is close to several contemporary protohistoric settlements: 17 km north-east of Sextantio (Castelnau-le-Lez), 15 km north of the Gardies oppidum and 27 km from the Lattara harbour. The site would have been frequented from the beginning of the 5th century BC to the end of the 3rd century BC, but the metallurgical activity would have functioned for less than one century and would have produced between 15 and 20 tons of iron according to estimates. Today, Combaillaux Les Érables site is the oldest evidence of an iron production workshop in southern Gaul. Chemical analyses of reduction slags have revealed a specific chemical signature that can now be tracked.