Geochemical and mineralogical characterisation of vitrified waste material discovered in large quantities on Roman salt production sites along the southern North Sea coast

Along the southern North Sea coast, Roman salt production sites are characterised by extensive refuse zones containing large quantities of what has been described as ‘salt slags’. These ‘salt slags’ are in fact amorphous, heavily vitrified waste materials. This is rather surprising since large-scale vitrification has never been associated with the salt production process. In this paper, these materials are for the first time systematically studied macroscopically, mineralogically and geochemically to determine their composition, formation and relation to the salt production process. To achieve these objectives, 30 samples from 7 Roman salt production sites were analysed by combining several analytical methods (thin-section petrography using Polarised Light Microscopy, Scanning Electron Microscopy coupled with Energy Dispersive X-Ray Spectroscopy and X-Ray diffraction). This approach enabled a detailed characterisation of the vitrification process in the waste materials, as well as the identification of high temperature mineral transformations formed in specific (archaeological) conditions. Based on these results, the amorphous waste materials should be interpreted as vitrified hearth base fragments. This study shows that the current interpretations regarding the firing conditions on Roman salt production sites needs major adjustments. In addition, this paper demonstrates the value of geochemical and mineralogical research on discarded waste materials to study poorly understood aspects of not only the salt production process, but artisanal activities in general.