Fire Assay, Cupellation and the Dissemination of Technical Knowledge in Post-Medieval Europe
This PhD thesis focuses on the study of assaying practices in the post-medieval Europe (c. 14th-17th centuries), especially on silver assaying. Modern analytical chemistry is an offspring of fire assay and cupellation, a method used to quantify the amount of precious metals such as silver and gold in ores, ingots or coins. Small-scale cupellation was used to determine quantitatively the richness of metalliferous ores, and for quality control in metals trade and coin minting. The stringent requirements in the efficiency and accuracy of these quantitative processes are reflected not only in the amount and purity of the silver available in post-medieval Europe, but also in the evolving procedures and materials used to carry out cupellation.
This thesis combines a critical evaluation of historical texts describing assaying practice with the analytical study of contemporary archaeological remains, and experimental replications. Particular emphasis is placed on the manufacture of assay vessels (scorifiers and cupels) and the efficiency of the assay methods. The primary case studies include the Royal Mint and Jesuit College in Kutná Hora (Czech Republic), the Jáchymov Mint (Czech Republic) and the Porto Mint (Portugal), but samples from Middelburg (Belgium) and Paris (France) are also examined. The analytical techniques employed include portable X-ray fluorescence, optical microscopy, inductively coupled plasma mass spectrometry/atomic emission spectroscopy, and scanning electron microscopy with energy dispersive spectrometry.
Compared with ceramic crucibles, made industrially and traded over long distances, cupels were more likely made by the practitioners and therefore show higher variability. The analyses of cupels from the sites have identified three different recipes: bone ash, wood ash, and a mixture of bone ash and wood ash, which is the dominant recipe. These are all recorded in historical sources, but the analytical study coupled with the new experiments allows further insight into regional manufacturing differences, possible reasons behind choices, and the effect of cupel materials on manufacture and performance. While cupels from the Porto Mint are made of a mixture of bone ash and wood ash in similar proportions, cupels from the Czech Republic are richer in wood ash. Using silver loss in cupels as a proxy for efficiency, the operations in the mints appear more standardised than in other workshops. The experimental project helped assess the performance of different cupel recipes and furnace structures reported in historical and archaeological data. Though bone ash is recognised today as ideal material for cupels, the experiments revealed challenges in obtaining reproducible results with these materials when the cupels are made by inexpert users. Scorifiers in this study were used for testing ores. The experiments demonstrated the difficulties in fully refining silver using ceramic scorifiers, hence helping explain the reasons for the historic transition to ash-based assaying vessels.
Overall, the reconstruction of assaying practices through archaeological and contemporary technical treatises allows new insight into regional variability in silver metallurgy, the transfer of technical knowledge in post-medieval Europe, and the development of quantitative analytical chemistry.