Out-of-equilibrium behaviour of copper-based alloys in industrial, artistic and historical gravity casting processes

Copper alloys and casting practices are of major importance in both fields of archaeometry and material sciences. Universally recognized as key technologies in humankind history, they have known a continuous use through times and still are intensively applied in our modern industry. However, a considerable part of the actual knowledge on the non-equilibrium behaviour of ancient copper alloys during casting processes, like bronze and brass, comes from empirical traditions more than from scientific researches. Furthermore, from an archaeological point of view, technological investigations are often limited by the fact that as-cast artefacts and metallurgical structures (moulds, furnaces, etc.) are rarely discovered on the same site. This Ph.D. research investigates the impact of different cooling conditions on the microstructural and mechanical features of as-cast copper-based alloys with industrial and archaeological interest in the aim to 1) better predict the characteristic of copper-based alloys after undergoing common casting processes, 2) better understand archaeological corpus in as-cast state in order to make hypothesis about the parameters of casting process. The present thesis is organized in 5 main parts: - The 1st chapter consists of an overview highlighting the importance of casting technology in both ancient and modern times. We will also present copper-based alloys as a key materials in several fields, form archaeological sciences to modern industry and artistic production. - The 2nd chapter presents the main mechanisms of solidification. Phase diagrams as well as a state of the art about the gathered knowledge on out-of-equilibrium curves for the considered copper based alloys are presented; - The 3rd chapter describes the methodology applied, as well as the materials investigated and the techniques used. The work is separated in two parts. The first one consists in gathering quantitative data on the solidification of experimental as-cast alloys (bronze, brass, gunmetal and leaded alloys) in order to find correlations between cooling conditions and final characteristics (macro and microstructure, macrohardness). A multi-technical methodology based on a multivariate approach (DoE, PCA) was built in order to study different scales of cooling rate (i.e. medium and near rapid), and the impact of different variables of process (i.e. alloy composition, superheat, mould material).The second part is based on the metallographic study of the archaeological corpus of Loyettes (France). The assemblage is composed by 69 as-cast axe-ingots coming from a Bronze Age deposit and presenting similar macroscopic characteristics. A reduced corpus of 20 objects have been selected and investigated in the intent to determine their microstructural features and some process parameters (composition of the alloy, assumptions on the mould material, etc.). - The 4th chapter presents the results obtained during the experimental casting tests and the archaeometrical investigation. A particular attention is paid on the filing of the model of experimental design and the application of these results assuming the casting technology applied on the Loyettes corpus as a reference. - The 5th chapter displays the final conclusions of the achieved works with some considerations for further studies. This study includes working collaborations with:  Prof. Roberto CABELLA, DISTAV – Università degli Studi di Genova, Italy  Prof. Cristina CARBONE, DISTAV – Università degli Studi di Genova, Italy  Dr. Fabien DELRIEU, DRAC Auvergne-Rhône-Alpes, Ministère de la Culture et de la Communication, France  Ph.D. Maréva GABILLOT, UMR 6298 ARTEHIS – Université de Bourgogne, France  Prof. Riccardo LEARDI, DIFAR – Università degli Studi di Genova, Italy  Dr. Daniel MACCIÒ, DCCI – Università degli Studi di Genova, Italy  Ph.D. Lorenzo MONTESANO, DIMI – Università degli Studi di Brescia, Italy  Dr. Eng. Marialaura TOCCI, DIMI – Università degli Studi di Brescia, Italy  Prof. Massimo VERDOYA, DISTAV – Università degli Studi di Genova, Italy