The aim of this paper is to contribute to the understanding of the organisation of the manufacture and circulation of iron bars and the possible bar standardisation of a given set of artefacts in order to throw more light on commercial patterns during Antiquity. A set of 48 iron based metal bars originating from the Roman shipwrecks at Les Saintes-Maries-de-la-Mer (France) has been studied using comprehensive metallographic observations, ranging from macroscopic to microscopic scales and slag inclusion (SI) analyses. A comparison of the results allows one to distinguish different metal qualities (inclusions, pores, welding, carbon content) that may be linked to morphological types, which shows the possible standardisation of this set of artefacts. Moreover, SI analyses allow distinguishing various origins for the Primary Pieces of Metal, thereby throwing light on a specific organisation of the iron bar production line during the late Roman period.