Tracing the Alteration of Verdigris Pigment through Combined Raman Spectroscopy and X-ray Diffraction, Part I

Abstract

Abstract This research investigates chemical alteration in the important historical pigment called verdigris, both in the form of neutral verdigris (Cu(II) (CH3COO)2 . H2O) and basic verdigris (Cu(II)x(CH3COO)y(OH)z . nH2O), using reference pigment powders and historically relevant “mock-up” samples exposed to artificial aging. Analytical study of model samples by combined Raman spectroscopy, X-ray diffraction and visible spectroscopy provides new evidence that clarifies and builds on the often conflicting body of literature, first in terms of analytical identification of different forms of verdigris pigment, and second by tracing the alteration of neutral verdigris in systems that link to its behavior in aqueous media on historical types of paper. Results further suggest that the historical importance of neutral verdigris as a pigment is underestimated, since commercially available verdigris throughout its heyday – from before the Renaissance through the eighteenth century – was likely to have been dominated by the more easily manufactured neutral salt. This misunderstanding may arise from pigment alteration, whereby the neutral verdigris converts to basic copper salts, or forms organo-copper complexes.