The verisimilitude of verdigris: a review of the copper carboxylates

Abstract

Copper formates and copper acetates occur both as corrosion products on works of art and as pigments. Some of the characteristics of these compounds are reviewed, particularly in relation to their occurrence in works of art, the deterioration of associated materials, and historical pigment recipes for the preparation of verdigris [copper (II) hydroxyacetate hydrates] and related compounds. X-ray diffraction analysis is discussed at length and new sets of powder diffraction measurements are provided, for both the basic copper formates and the basic copper acetates, which represent reference data for the verdigris salts [copper (II) hydroxyacetate hydrates], not previously available as a comprehensive set of data, as no entries for them at present exist in the International Centre for Diffraction (Joint Committee on Powder Diffraction Standards) (ICDD (JCPDS)) files. The utility of this new data is reviewed and the optical characteristics of the different verdigris preparations examined. Many of the historical recipes were replicated in the laboratory, providing further insight into the complexity of these verdigris salts and their chemical constitution, some of which include the basic copper chlorides, citrates, carbonates and other copper compounds which are often difficult to identify. The chemical literature relating to the copper acetates, and some of the classical literature describing pigment preparations based on verdigris, is reviewed.

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