New Insights into Synthetic Copper Greens: The Search for Specific Signatures by Raman and Infrared Spectroscopy for Their Characterization in Medieval Artworks

Abstract

A systematic investigation of medieval copper green pigments was carried out based on written sources: 21 manuscripts, dating from 50-70 to 1755 AD, were sourced and 77 recipes were selected, translating into 44 experiments. Reconstructions from medieval recipes were prepared and characterized through a multianalytical approach to disclose the original pigment formulation that is often described as verdigris. Based on the results obtained, we propose three main groups of copper green pigments, group 1, in which only Cu(CH 3 COO) 2 ·H 2 O is formed; group 2, where this acetate is found together with copper oxalates; group 3, in which atacamite is present as the major green component or as a signature compound. The products formed are in perfect agreement with that predicted by the state-of-the-art research on the mechanisms of atmospheric corrosion of copper. This knowledge, together with our experience on craft recipes to prepare medieval paint materials, allowed us to recover a lost medieval recipe to produce a copper green pigment based mainly on atacamite, a basic copper chloride, which has been recently detected, by Raman and infrared spectroscopy, in artworks ranging from Catalonia and the Crown of Aragon panel painting to Islamic manuscripts.

Degradation of emerald green: scientific studies on multi-polychrome Vairocana Statue in Dazu Rock Carvings, Chongqing, China

“…The positions and the vibrational assignments of the main bands for atacamite are listed in Table 3. The presence of atacamite can be concluded from Raman characteristic bands in the range of 145-972 cm −1 and 3328-3433 cm −1 which coincide with the published literature [33], three intense bands at the identical positions are observed at 972, 902 and 818 cm −1 , the bands between 800 and 1000 cm −1 attributed to Cu-OH and OH bends; the intense band occurs at 511 cm −1 , the bands below 600 cm −1 assigned to O-Cu-O and Cl-Cu-Cl modes; three bands at 3328, 3349 and 3433 cm −1 assigned to OH stretches. Figure 7a, b and c mainly show that numerous atacamite particles appear as aggregates or masses, while malachite particles sporadically occur in the layer.…”

Degradation of Emerald Green: Scientific Studies on Multi-polychrome Vairocana Statue in Dazu Rock Carvings, Chongqing, China

“…The positions and the vibrational assignments of the main bands for atacamite are listed in Table 3. The presence of atacamite can be concluded from characteristic Raman bands in the range of 145 ∼972 cm −1 which coincide with the published literature [ 33 ], three intense bands at the identical positions are observed at 972, 902 and 818 cm -1 , the bands between 800 and 1000 cm −1 attributed to Cu-OH and OH bends; the intense band occurs at 511 cm -1 , the bands below 600 cm −1 assigned to O-Cu-O and Cl-Cu-Cl modes. Fig.7a, b and c mainly show that numerous atacamite particles appear as aggregates or masses, while malachite particals sporadically occur in the layer.…”

Application of Multispectral Imaging and Portable Spectroscopic Instruments to the Analysis of an Ancient Persian Illuminated Manuscript

“…Often, FORS is the only technique to identify in a non-invasive and portable way the type of copper pigment. Unfortunately, copper pigments are also prone to chemical transformation, resulting in a variety of different copper compounds, especially the synthetic green ones such as verdigris [ 38 ]. Through XRF and FORS, we identified the verdigris present in several zones of the manuscript.…”