Ancient Brasses: Misconceptions and New Insights

Abstract

Prior to the mastering of metallic zinc production in the mid 19th century AD, brass making in Europe had been based on the so-called cementation process: within a (closed) vessel, gaseous zinc is produced by the carbothermic reduction of zinc ore at ca. 1000 °C and simultaneously diffuses into metallic copper. The ancient processes are only partially documented by archaeological finds (mainly ceramics), written records, and modern experimental reconstructions. Hence, very little is known about the exact operating conditions and the associated performances of the process. Moreover, the brass cementation process is surrounded in the archaeometric literature by a halo of misconceptions, including a hypothetical superior limit of zinc uptake and the dramatic influence of the purity of the copper. The authors performed in-door experimental simulations related to brass cementation for three years. Almost 300 different working conditions have been tested, thus allowing them to build up a better picture of the process. The aim of this paper is to present some aspects of this new picture, by discussing the influence of various key parameters on Zn content and recovery rates, (temperature, isothermal treatment, duration, initial Zn/Cu ratio, purity of the ore and of the copper, presence or not of a lid, etc.). These laboratory experiments will be compared with the preliminary results of field experiments using natural draft powered furnaces. For some particular working conditions close to those of an ancient workshop, both scale-effect and “working conditions variability effect” inherent to the field are highlighted