Verdigris

Verdigris was made deliberately, not simply gathered as a natural weathering product, by suspending strips or plates of copper over a pot of vinegar in a sealed container, allowing the acetic acid fumes to slowly corrode the copper's surface into a thick green crust, which was periodically scraped off, dried and ground into pigment, a process practiced across Europe from antiquity and described in detail by writers from Pliny the Elder onward. The resulting pigment gave painters one of their only strong, saturated green colors before modern synthetic pigments, though it had real drawbacks: verdigris tended to darken and discolor over time, especially when mixed with certain other pigments, a problem conservators studying old paintings still contend with today when identifying areas of verdigris that have browned or blackened from their original vivid green.

Beyond painting, verdigris served as a mordant in dyeing, as a component in various metalworking fluxes, and in a handful of medicinal preparations applied externally, always with the understanding, at least among careful practitioners, that copper compounds in any quantity were toxic if taken internally. Its ready formation, requiring nothing more exotic than copper and ordinary vinegar, made it one of the more accessible pigments and reagents in the whole alchemical and artisanal repertoire, needing no rare mineral and no difficult metallurgy.

Verdigris, as deliberately manufactured by the vinegar process, is identified chemically as a basic copper acetate, principally a compound close to copper(II) acetate combined with copper hydroxide in varying proportions depending on the exact conditions of its formation, a genuinely variable mixture rather than one single fixed compound, which is honestly reflected in how mineralogists and conservators today describe historical verdigris samples as falling within a family of closely related basic copper acetates rather than as one uniform substance. It is worth distinguishing this manufactured pigment clearly from the naturally occurring green patina that forms on copper and bronze objects exposed to ordinary weathering, chiefly basic copper carbonate and copper sulfate depending on local atmospheric conditions, a chemically different family of compounds that early modern writers, without the analytical tools to separate acetate from carbonate from sulfate, frequently lumped together under the same word verdigris. Modern conservation science treats this conflation as a real and acknowledged complication when trying to determine exactly what compound is present in a given green corrosion sample from a historical artifact.