What It Actually Is

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.