Illuminating the problem of blue verditer synthesis in the early modern English period: chemical characterisation and mechanistic understanding

Abstract

We present a study into early modern English production of blue verditer, an early synthetic copper-based blue pigment chemically analogous to azurite. Verditers have been identified in numerous wall and easel paintings. While initial documentation occurs in the mid 1500s and production recipes were documented by the 17th c., the synthesis was known to be unreliable. This study replicates historical and recent scientific work on blue verditer and represents a significant advance in our understanding of verditer production and its challenges. Procedures for verditer synthesis are drawn from both 17th c. documentation and 20th c. replication work. The effects of temperature, copper and carbonate sources, solution stirring, copper ion concentration, and atmospheric composition are studied in order to elucidate the mechanism of synthesis and explain its unreliability in early modern refineries. Products are characterised by polarised light microscopy, scanning electron microscopy, Raman spectroscopy, and powder X-Ray diffraction. Rouaite, a green basic copper nitrate, is for the first time confirmed as a product of the refiners’ synthesis and a precursor to blue verditer in laboratory syntheses. This result problematises the blanket identification of green verditer as basic copper carbonate and provides important clues to the mechanism for blue verditer synthesis. Solution chemistry and ion equilibria allow us to explain the route by which rouaite is first formed and then converted to blue verditer. Conditions favouring blue verditer production are also clarified further. Although it is commonly stated that low temperatures are required for blue verditer production, blue verditer is produced here at a range of ambient temperatures. The reaction is found instead to be controlled by solution equilibria and heavily favoured by high partial pressures of carbon dioxide. Alongside archival materials about refining and verditer production, these results are contextualised and explanations for the unreliability of historical synthesis are proposed.