From Ink to Mockup: An Integrated Study of Historical Gall Ink Formulations and Their Impact on Paper

Iron gall inks (IGIs) played a central role as a writing medium in Western countries, leaving behind a vast legacy and significant conservation challenges. This study presents a twofold methodological approach to investigate the physicochemical behaviour of IGI-based formulations found in historical Portuguese sources. Fresh and 6-month naturally aged precipitates and supernatant solutions (dried inks) were characterised, and the impact of IGIs on Whatman paper over ageing was studied using attenuated total reflectance Fourier-transform infrared (ATR-FT-IR) spectroscopy, Raman and Mössbauer spectroscopies and X-ray diffraction (XRD). Iron(II) sulphates comprised the primary crystalline phase in the precipitates, while the dried inks consisted of distinct Fe(III)-polyphenol (PPh) complexes. Over time, Fe(III) Mössbauer parameters supported complex structural alterations. IGI-induced degradation on aged mockups was attested by Fe(II) oxalate formation until total depletion of the Fe(III)-PPh fraction. pH, ATR-FT-IR and degree of polymerisation analyses suggested that cellulose oxidation is the primary degradation mechanism, and the physical properties and XRD corroborated that the cellulose structure became disordered over ageing. The ink with the higher weight ratio of Fe(II) sulphate:gallnuts (unbalanced) exhibited the most aggressive action on the support. The results confirm that the more unbalanced the ink composition, the more severe its impact, with ink concentration per surface area also being a critical factor in paper decay.