Spectroscopic Studies of Copper-Based Pigments
Date
2019
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Publisher
University of Delaware
Abstract
Copper-based pigments are common in both Western and Eastern artwork as green and blue colorants, even though they are commonly associated with degraded organic materials as well. Spectroscopic analyses of copper-based compounds provide information on both the origin and manufacturing of these blue and green pigments. Furthermore, they provide insight into the preservation of cultural heritage objects as these pigments interact and degrade with the surrounding organic media. For example, Chinese architectural paints are composed of several different “copper green” pigments. Understanding their application and usage prompted investigation into green pigments across China using samples from the 12th to 19th centuries. Hyperspectral Raman images of the paint layers of several Chinese architectural paint cross-sections, including several from the Forbidden City, Beijing, China, were analyzed with multivariate curve resolution-alternative least squares (MCR-ALS) to distinguish component spectra of copper-based species of interest and their relative ratios. The differences in the relative ratios of copper chloride trihydroxide polymorphs indicate the historical synthesis of these pigments. Through this study, the transition to synthesized pigments for Chinese architectural materials is beginning to be understood, as well as the pigments’ role in the stability of the paint. Furthermore, to complement this investigation into copper-based pigments, accelerated degradation studies were carried out to study the interaction of copper in many historical objects. Common copper-based pigments were prepared following adapted historical recipes and then extensively characterized to identify the starting materials of various artworks. Also, these methodologies were successfully adapted for educational purposes to connect the art and sciences in the classroom as well as the research laboratory. The accelerated degradation of the most common of these pigments, both neutral and basic verdigris (copper acetates), was studied in relation to organic substrates. Through X-ray diffraction, infrared, Raman, and X-ray photoelectron spectroscopies, the alteration products of verdigris on cellulose with and without gum Arabic as a binder, show an intermediate species in the degradation process prior to the final product of copper oxide. Additionally, the changes of copper oxidation state were observed to relate to the amount of gum Arabic added to the samples. With these complementary studies, we better understand the usage and effects of copper-based pigments in both Western and Eastern cultural heritage, and this information can be utilized by conservators preserving the objects in question.