Benzonelli, Agnese; (2020) Technological traditions and trajectories in the production of the black bronze alloys. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This PhD presents a combined archaeometric and archaeological study of artificially patinated black copper alloys. Its particular focus is a type of patination of special archaeological interest: “black bronzes”, a class of alloys reportedly used for high-status objects from ancient Egypt to modern China and including, among others, the famous Corinthian bronzes and Japanese \textit{shakud\=o}. These alloys were generated by adding precious metals to copper before treating it with a variety of techniques to develop a fine and durable black patina. For the first time, all the scientific and historical information concerning black bronzes was compiled into a single database, which highlighted significant research gaps regarding the processes of patina formation, the reverse engineering of archaeological black bronzes, and the invention and transmission of this technology. In order to address these questions, an experimental project was designed and executed. A series of 27 black bronze alloys with controlled contents of tin, gold, and silver was produced and treated using four different procedures: chemical patination by means of solutions that mimic those used in traditional methods, Chinese patination using perspiration, thermal patination, and simulation of natural corrosion. The resulting patinas were examined with a range of analytical techniques including colorimetry, XRD, SEM-EDX, pXRF, UV-VIS, and Raman spectroscopy. The results of the project provide important information on composition, microstructure, colour, appearance, and durability of the different patinas, and allow an understanding of the relationship between production technology, alloy composition, and their physical-chemical characteristics. This investigation of the patinas allowed for the discrimination of characteristic features for each technology. This experimental dataset was used as a reference for the study and interpretation of archaeological black bronzes. Egyptian, Roman, Greek, Anglo-Saxon, Chinese and Japanese artefacts from a range of museums were analysed and compared with the experimental results, providing insight into the procedures and materials used by the different cultures. The comparison of the experimental data with the results of the legacy and new analyses of archaeological artefacts confirmed the similarities between the Chinese and Japanese patinas. It was also possible to explain the reasons behind the technical choices made in the selection of alloy preparation and composition, solution ingredients, and patination processes, all aimed to obtain a unique and durable blue-black colour. These procedures ensured that patina growth was minimal and caused the depletion of copper and the enrichment of the surface in precious metals, both which are essential for the phenomenon of surface plasmon resonance to take place. This phenomenon, which explains the colour of black bronzes, can only be achieved with nanoparticles on precious metals and it explains the reason behind the addition of gold. The black layer on Egyptian, Roman, and Anglo-Saxon artefacts appears thicker, rougher, and visibly different from those observed on Chinese and Japanese objects, as well as those produced experimentally. These alloys contain tin (not present on Chinese and Japanese artefacts) together with variable concentrations of gold and a large variety of other alloying elements. Though it appears that the patinas achieved on Anlgo-Saxon artefacts are influenced by the high-tin content, it cannot be confirmed that the gold was intentionally added for patination, or whether what we see today results from the recycling of copper scraps, including gildings, with a natural patina developed in the burial environment. Similar concerns can be raised for the Egyptian and Roman artefacts examined, hence calling for a major reconsideration of our accepted knowledge regarding the origins and transmission of black bronze technologies. In sum, the analysis of Eastern artefacts, together with the information from the written sources, confirm the presence of a well-defined intentional tradition and controlled recipes used to produce black bronze alloys in these two cultures, and it is coherent with knowledge transmission between both. Conversely, the attribution of Western artefacts to the class of intentional patinated artefacts is questionable. Even if we accept some of the artefacts as intentional black bronzes, the variability suggests the lack of a standardised tradition using established and accepted recipes. This makes it very unlikely that all the black bronze artefacts derive from a single invention as previously proposed, and therefore calls into question the existence of 'black bronzes' as a time-deep, well-defined class of alloys.

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