Verdigris pigment: a mixture of compounds. Input from Raman spectroscopy

Abstract

The reproduction of an old recipe to obtain the pigment known as verdigris leads to a complex mixture of hydrated copper(II) acetates containing neutral verdigris and two varieties of basic verdigris. Since the characterization of such mixture is complicated by the lack of reference standards, we complemented our Raman measurements with Fourier transform infrared (FTIR) and X-ray diffraction (XRD) experiments to perform our analysis, which has been compared with available studies. The spectral analysis allowed us to distinguish the different verdigris varieties, with stretching and bending wavenumbers assigned to the following chemical groups: OH groups (OH hydroxy and OH H2O ) (3600-3050 cm −1 ), Cu-O bonds (Cu-O and Cu-OH) (1000-450 cm −1 ) and those corresponding to the acetate group (CH 3 COO − ), i.e. (CH 3 : 3000-2850 cm −1 ) and (COO − : 1600-1400 cm −1 and 700-600 cm −1 ). Both wavenumbers and Raman intensities were used to quantify the relative compositions of the mixture components.

On verdigris, part II: synthesis of the 2-1-5 phase, Cu3(CH3COO)4(OH)2·5H2O, by long-term crystallisation from aqueous solution at room temperature

“…The indexing of the pattern was carried out by an iterative use of singular value decomposition (LSI) 29 leading to a primitive monoclinic unit cell with lattice parameters given in the ESI (Table S1 ‡). The observed systematic reflection extinction pointed to P2 1 /c (14) as the most probable space group. The peak profile and the precise lattice parameters were determined by LeBail 30 fits applying the fundamental parameter approach of TOPAS.…”

“…Considering the unit cell volume, the site multiplicities in space group P2 1 /c (14) and the packing densities of the related copper(II)-acetate-hydroxide hydrates, 7,8,15 35 the positions of the acetate and hydroxide ions and water molecules were determined. Rigid bodies for the acetate ions were defined in z-matrix notation and translated and rotated freely through the unit cell.…”

“…Only the 1-3-2 phase 1,4,5,7-14 and very recently the 1-2-0 phase 15 were characterised in detail, as the purity of the obtained basic copper acetates could not be proved unambiguously because the phase characterisation led to contradictory results. In addition, ancient recipes, in particular the intentional corrosion of copper, were proved to lead to complex multiphase mixtures 14,16,17 comprising components which could not be identified due to a lack of reliable reference data. Historic verdigris samples are even more complex as most of them are multiphase mixtures, as well as they are usually already affected by degradation processes.…”

Raman spectroscopy in art and archaeology

“…The reproduction of an old recipe to obtain the pigment, developed by San Andrés et al, [15] leads to a complex mixture of hydrated copper(II) acetates containing neutral verdigris and two varieties of basic verdigris. As the characterisation of such a mixture was complicated by the lack of reference standards, authors complemented the Raman measurements with Fourier transform infrared (FTIR) and X-ray diffraction experiments to perform a multianalytical approach.…”

Raman Spectroscopy of cultural heritage Materials: Overview of Applications and New Frontiers in Instrumentation, Sampling Modalities, and Data Processing

“…Research focused on reconstructing the chemistries of production of traditional pigments following ancient recipes also includes the characterisation of deterioration products [70,79,101,270]. Occasionally, spectra of pigments that have not been previously characterised are still presented in the literature and studied in depth, significantly enriching available libraries [28,55,290].…”