Impact of Fe³⁺/Polyphenol Ratio in Iron-gall Ink on Superoxide Formation: Rationalizing Historic Recipes from a Kinetic Study

Iron-gall inks, a vital part of our written cultural heritage, are at risk of complete loss due to degradation, a potential loss that we must urgently address. These inks are based on Fe³⁺-complexes with phenolic compounds, which grow to form a complex network of iron oxyhydroxides. Over time, these black inks turn into brownish tones, with extensive degradation in paper support leading to extensive breaking. The kinetics of iron-gall ink preparation explains the use of iron sulfate, FeSO₄, in all ancient recipes to obtain a stable amorphous ink. The novelty of this work shows that a low ratio of Fe³⁺/polyphenol is a crucial factor in allowing the ink's growth without its degradation. This ratio also prevents the formation of superoxide. This was achieved through a comprehensive research methodology involving spectroscopic techniques in the visible and the near-infrared regions, stopped-flow spectrometry and electrochemical studies.