Charcoal Fire Assaying

During the Early Modern period, the assaying of gold and silver by cupellation (fire assay) was undertaken in charcoal-fired furnaces. Though the process of fire assay is still practiced on occasion, during the twentieth-century, assayers came to rely first on gas, and then electricity, to power their furnaces. This technological shift gave easier control of the furnaces’ temperature and atmosphere. However, as a consequence, the skills needed to manage a charcoal-fuelled fire assay furnace, as well as a close understanding of the demands and direct experience of undertaking cupellation in such furnaces, have been lost. In addition, it is no longer clear why the furnaces described and illustrated in specialist texts from the sixteenth to twentieth century took the forms they did, or why the changes in design that occurred over these four centuries were adopted.

The project is inherently interdisciplinary, combining established library research as practiced by historians with an exploratory practical programme of iterative furnace building and testing, following the experimental reconstructions that are becoming increasingly typical in science and technology studies. In addition, the project has borrowed the method of cardboard prototyping from design practice, due to the practical benefits and unique insights it offers on artefact construction and use. The results from the prototyping work, the practical experiments and library research will all inform the assembly of the final historic furnace reconstructions.

The project has involved the close examination of imagery and historic texts from the sixteenth century onwards, including early printed publications held in the Rijksmuseum in Amsterdam, twentieth-century academic translations, and online digitised versions uploaded by libraries across Europe. The textual research also included reading technical manuals published during the nineteenth century that described the equipment needed for the fire assay process as well as how to conduct cupellations, and the practicalities and economics of running an Assay Office.

The knowledge gained from this work informed a practical programme of iterative furnace building and testing. This started with the aim of constructing a viable functional charcoal-fuelled furnace similar to a nineteenth-century model using industrially manufactured components. The creation of a functioning furnace was followed by a sequence of alterations to this model, which introduced features found in older designs of furnace. The consequence of each of these changes and their implications for the practice of fire assaying were then considered.

Considerations of the style and possible intended audiences for the examined books, as well as their author’s espoused claims to authority and the validity of the material presented, have led to a deeper understanding of the development of the precious metal assayer as a distinctive technical professional, both during the Early Modern era and later historical periods. The complex relationship between the assayer and the alchemist in Early Modern Europe has also become a topic of interest, in part due to the role played by assayer’s aristocratic patrons who also had alchemical connections, but also the evident geographic proximity of specific individuals.

Presentations

‘Reconstructing a Sixteenth-Century European Goldsmithing Furnace: Technical principles, Structural Requirements, and Practical Issues’, Fire Arts: Pasts and Futures workshop, Texas A&M College Station Campus, College Station, Texas, Nov 2024.

‘The Magician, the Alchemist, and the Assayer: Elizabethan Characters on the Stage and in the Streets’. Presentation given to the Early Modern Studies Working Group at Texas A&M RELLIS Campus, College Station, Texas, 13 May 2024.

‘How to Regulate the Fire’: Early Modern Cupellation and Furnace Management’. SHAC Spring Meeting, held at the Maison Française d'Oxford, Oxford, 28/29 May 2024.