A technical study of the three exceptional figures adorning the Monument of the Heart of Duke Anne de Montmorency commissioned in 1571 and attributed to the sculptors Barthélémy Prieur and Martin Lefort was carried out. This study includes X-radiography, alloy analysis, characterisation of the refractory core, visual observation of the interior and the exterior of the sculptures, as well as endoscopic examination of the interior. The objective of this study was to fill in further details regarding the fabrication of the figures: where and by whom they were made and how they were cast. In particular, the question of whether the foundryman Nicolas Péron carried out the casting of all three bronzes is addressed here. The results show significant consistency between the three figures, which were cast using a technique commonly employed during the sixteenth century in France, known as the lasagna process. Yet, several specific details linked to the monument commission – the use of chaplets and a sandy-clay mixture defined by a specific grain size distribution – shed light on the working relationship between artist and founder. While the conclusions of this study confirm Prieur’s leadership over the technical aspects of his sculptures, Lefort seems to have been less involved in the casting process and to have relied on the expertise of both Prieur and Péron. Furthermore, the results contribute to a better understanding of the casting of large bronzes during the second half of the sixteenth century by providing key information on the relationship between artists and founders.