Insights into mixed carbon–iron gall ink and its degradation in eighteenth-century manuscripts

The study of mixed ink recipes has a broad interest since along the millennial historical transition from carbonbased inks to iron-gallate inks, mixed carbon‑iron gallate might represent a long transitional phase. The knowledge of the impact and interaction of chemical and microbiological agents with the tangible archivistic heritage strengthens our ability to preserve historical documents over time. Herein, we propose a multianalytical approach including optical and electron microscopies, vibrational spectroscopies, liquid chromatography with mass spectrometric detection. Moreover, microorganism isolation and characterization were carried out on selected samples. Our case study (Vanvitellian letters) demonstrates that, in the presence of carbon black, integration of spectroscopic technique with HPLC-MS/MS analysis is essential for the proper characterization of iron-gallate complex and its degradation products. Therefore, we propose and apply a protocol in which a Raman-based pre-screening easily detects carbon black in ink, and, in case of carbon black presence, HPLC-MS/ MS is required to verify gallic acid/oxidation sub-products, to correctly assign the ink to the mixed recipe. Both ink formulation details and warnings on the diagnostics of mixed carbon‑iron gall inks arise from the present study, calling for further investigation into their degradation mechanisms.