Crystalline materials in art and conservation: verdigris pigments – what we know and what we still don’t know

Abstract Verdigris is a collective term for synthetic copper based green and blue pigments that have been used by mankind since the antiquity. As the term is only loosely defined it covers a great variety of chemical compounds. All of them contain copper but also acetate, formate, hydroxide, water and sometimes also chloride, carbonate and sulphate. This article focusses on the verdigris phases related to the ternary system Cu(CH3COO)2–Cu(OH)2–H2O, which are commonly denoted according to their chemical composition as x–y–z phases. Besides neutral verdigris (1–0–0 and 1–0–1 phase), several basic verdigris phases (2–1–5, 1–1–5, 1–2–0, 1–3–0, 1–3–2, 1–4–3, 7–1–4) have been reported. These compounds can be obtained either by intentional corrosion of copper metal by direct contact with acetic acid and its vapours, which represents the historic pigment manufacturing or by incomplete precipitation from copper(II) acetate solutions. Due to their slow crystallisation behaviour the synthesis of verdigris pigments usually leads to multiphase and polycrystalline samples, which impedes the phase characterisation, in particular of the basic verdigris samples. Hence, most crystal structures that have been published were solved ab-initio from X-ray powder diffraction data by applying global optimisation methods like simulated annealing or charge flipping. Up to now the crystal structures of the 1–0–0, 1–0–1, 1–3–2, 1–2–0 and 2–1–5 phases were determined, the spectral and physical properties of these compounds completely characterise and therefore their existence and their chemical composition unambiguously proven. Investigations on historic verdigris samples and systematic synthesis approaches, however, show that there are at least four additional, hitherto unknown pigment phases. Information on the thermodynamical stability and the solubility of the verdigris phases are also lacking.