Model compounds of iron gall inks – a Mössbauer study

Ferrogallic inks were used for at least two millennia before they became obsolete in the 20^th century. The chemistry of such inks is, however, still largely unclear. Today it is of particular interest for the conservation of old manuscripts. ⁵⁷Fe Mössbauer spectra of the ink on historical documents showed the presence of Fe(II) oxalate and of Fe(III) sites presumably representing iron oxihydroxides. To obtain more information on the behaviour of ink on paper we have performed Mössbauer studies at 300 and 4.2 K on iron gall inks prepared from FeSO₄⋅7H₂O and tannin. These inks were either written on paper or isolated as a precipitate by centrifugation. In the dried precipitate there is still a strong contribution of the FeSO₄⋅7H₂O which is absent in the same ink written on paper, for which a broad ferrous component with a quadrupole splitting (QS) of about 2.5 mm/s was found. The dominant Fe(III) site present in all inks on paper with QS ≈ 0.82 mm/s is not Fe(III) gallate and different from the precipitates. We propose that nanoparticulate oxidic clusters or molecular composites covered by a shell of polymerized oxidation products of the phenols are formed on the paper.