Non-destructive analyses of iron-polyphenolic complexes for reconstructing iron-gall inks historical recipes

Iron-gall inks (IGI) show considerable variability due to the wide range of historical recipes used in their preparation. Previous research has explored how factors such as pH and iron concentration affect IGI structure and how to detect these changes. This study focuses on variability induced by different tannins interacting with iron. Iron-polyphenolic complexes were systematically synthesized using both commercial tannins and oak gall extracts prepared following historical recipes. All starting materials were characterized via spectrophotometric assays and, for commercial tannins, infrared spectroscopy. The resulting complexes were analysed using Raman, IR, and Electron Paramagnetic Resonance spectroscopies to build a reference dataset. This enabled the identification of spectral markers offering insight into the materials used in IGI production through a non-destructive, multi-analytical approach. Finally, the method was applied to the "Black Hours" manuscript (property of the Austrian National Library) to investigate its ink composition, highlighting both the strengths and limits of the applied techniques.