XANES analysis of Fe valence in iron gall inks

In this study, we investigate the feasibility and reliability of Fe K‐edge XANES spectroscopy as a tool in determination of Fe 2+ /Fe 3+ ratio in historic inks. We have measured Fe K‐edge XANES spectra of several historic and model iron gall inks with different Fe 2+ /Fe 3+ ratios, together with some standard reference Fe 2+ and Fe 3+ compounds with known local atomic structure and symmetry around Fe atoms. We examine different approaches to determine relative amount of Fe 2+ in iron gall inks from the Fe K‐edge shifts, and demonstrate that a proper choice of the Fe reference compounds with similar symmetry, same type of neighbour atoms in nearest coordination shells, arranged in a similar local structure is crucial for the absolute calibration of the Fe K‐edge shift and consequently a reliable determination of Fe 2+ /Fe 3+ ratio in the sample. Best results, with the accuracy of ± 2%, are obtained by a linear combination fit with XANES spectrum of FeSO 4 · 7H 2 O as a reference for Fe 2+ and spectrum of an iron gall ink containing predominantly Fe 3+ .