Salt Green; Viride Salsum

Mineralisation in Cornwall is associated with the intrusion of the granites which began 290 million years ago and finished almost 60 million years later. Earliest mineralisation was associated with hot fluids which formed the tin-tungsten deposits and with decreasing temperatures, copper-zinc ores were formed. The weathering of ore deposits is a partly natural and partly man-made process. When exposed to oxygen and water, ores transform in a process known as supergene enrichment. These process are facilitated by the opening up of mine adits and the pumping out of groundwaters. Where copper is present in ore systems, as in the Cornish tin mines, the most spectacular products of supergene enrichment are encountered, in the form of precipitates of striking blue and green encrusting and stalagmitic deposits, on the walls of adits and at the outflow of mine drains. These include the copper carbonate minerals azurite and malachite, but a number of complex copper salts incorporating sulphate, chloride and arsenates also occur[1]. The earliest description of a pigment as viride salsum occurs in the 12th century and treatises on art during the medieval period through the Renaissance gave recipes for manufacturing these synthetic salt greens from mixing copper filings and organic material, including honey[2]. However it is just as likely that natural copper salts from mines were also used by artists.

Ruth Siddall

[1] Bowell, R. J., 1992, Supergene copper mineral assemblages at Botaliack, St Just. Cornwall.,  4 (2), 45-54.

[2] Scott, D.A., 2002, Copper and Bronze in Art: Corrosion, Colorants, Conservation., Getty Conservation Institute, Los Angeles.