Copper Complexes in Verdigris Painting Mixtures: An Electron Paramagnetic Resonance Characterization
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Riccardo Punis
and
Alfonso Zoleo
Abstract
Copper complexes, also called “copper soaps” when referred to carboxylate compounds, have become a concern in the conservation of historical artefacts. These complexes are suspected to play a significant role in degradation processes such as darkening and cracking of painting layers. Here we propose the Continuous Wave Electron Paramagnetic Resonance spectroscopy (CW-EPR) as a tool for the identification of these species. EPR spectroscopy is a micro-invasive and non-destructive technique that is highly sensitive to paramagnetic species, including radicals, crystal defects, and transition metal ions, e.g., Cu(II) and Fe(III). We have selected the verdigris (copper acetate) pigment combined with many organic binders, e.g., egg tempera and gum Arabic, as mock-up systems because of the known degradation tendency of the copper-based pigment. We would like to demonstrate that a precise analysis of the experimental spectra assisted with the calculus of the main magnetic quantities could lead to a good characterization of the copper complexes (or “soaps”).
Zusammenfassung
Kupferkomplexe, die auch als “Kupferseifen” bezeichnet werden, wenn sie sich auf Carboxylatverbindungen beziehen, sind zu einem Problem bei der Erhaltung historischer Artefakte geworden. Es wird vermutet, dass diese Komplexe eine wichtige Rolle bei Abbauprozessen wie dem Nachdunkeln und der Rissbildung von Malschichten spielen. Hier schlagen wir die kontinuierliche elektromagnetische Wellenresonanzspektroskopie (Continuous Wave Electron Paramagnetic Resonance, CW-EPR) als Instrument für die Identifizierung dieser Spezies vor. Die EPR-Spektroskopie ist eine mikroinvasive Technik, die sehr empfindlich auf paramagnetische Spezies reagiert, darunter Radikale, Kristalldefekte und Übergangsmetallionen wie z. B. Cu(II) und Fe(III). Wir haben das Grünspanpigment (Kupferacetat) in Kombination mit zahlreichen organischen Bindemitteln, z. B. Eitempera und Gummi Arabicum als Modellsysteme ausgewählt, da die Tendenz zur Zersetzung des Kupferpigments bekannt ist. Wir möchten zeigen, dass eine genaue Analyse der experimentellen Spektren, unterstützt durch die Berechnung der wichtigsten magnetischen Größen, zu einer guten Charakterisierung der Kupferkomplexe führen kann.
Acknowledgments
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. R.P.: performed the EPR spectroscopies experiments and the data processing. R.P. and A.Z. discussed the results. The paper was written by R.P. and reread by A.Z. We are grateful to Emanuela Zangirolami and Sabrina Mattiolo for her technical support.
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Research funding: This work was funded under the National Recovery and Resilience Plan (NRRP) and the NextGeneration EU Project (CUP. C96E22000460007; Grant code: 38-413-19-DOT1319897-8969). The authors acknowledge the Fondazione Zegna for its economical contribution under the Ermenegildo Zegna Founder’s Scholarship.
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Articles in the same Issue
- Frontmatter
- Editorial
- Editorial
- Preface
- Editorial
- Original Works
- Treating Brown Inks: Investigation into the Effects of Aqueous Treatment Methods on the Colour Stability of Bister, Sepia, Iron Gall and Logwood Inks
- Characterization of Historical and Modern Leathers Using FTIR, XRD, SEM-EDX, and Thermal Techniques
- Material Characterisation of 19–20th Century Manuscripts from Northern Thailand
- Review
- Proteomics to Study Parchment Degradation – From Bulk to Spatial Analysis
- Original Works
- The Dagulf Psalter (Austrian National Library Cod. 1861): A Multi-Analytical Approach to Study Inks, Dyes, and Pigments of this Early Carolingian Manuscript
- Copper Complexes in Verdigris Painting Mixtures: An Electron Paramagnetic Resonance Characterization
- Restoration of the Information on De-colourized Architectural Cyanotypes
- Study on the Material Properties and Deterioration Mechanism of Palm Leaves
- Analysis of Two Different Inks and Application Techniques on Palm Leaf Manuscripts Through Non-Invasive Analysis
