How the Variability of Iron-Polyphenolic Complexes Affects the Degradation of Iron-Gall Inks: A Multi-Analytical Study

Iron-gall inks (IGI) were among the most widely used writing materials in historical manuscripts. However, their presence is now recognized as a major cause of degradation in many of these documents. Common forms of deterioration include ink fading and discoloration, embrittlement of the writing support, crack formation, and material loss. Understanding the mechanisms underlying IGI-induced degradation is therefore crucial for developing effective strategies for the preservation of historically valuable manuscripts. In this study, a systematic and multi-analytical approach, involving the use of Raman, electron paramagnetic resonance, and infrared spectroscopy, was employed to investigate the degradation processes associated with IGI, with a specific focus on their intrinsic chemical variability. Three key parameters were considered: the structure of the polyphenolic ligand, the pH, and the iron-to-ligand ratio. These variables were selected to evaluate their respective contributions to the observed degradation phenomena. The findings provide a comprehensive overview of the main degradation pathways and the factors influencing them. Among the results, some indicate the occurrence of hydrolytic processes involving the complex ligands, which seem confined to the acidic conditions applied during sample preparation. In addition, the results enabled comparison of the differences in oxidation rates observed during accelerated aging, revealing how these rates vary according to the structure of the complexes. Overall, the study establishes a robust and reproducible framework that lays the groundwork for future research on IGI-related degradation.