An insight into the metal coordination and spectroscopic properties of artistic Fe and Fe/Cu logwood inks

Fe- and Fe/Cu-based logwood inkswere synthesized following recipes in nineteenth and early twentieth century manuals andwere characterized by EPR, ESI-MS, FTIR, and Raman spectroscopies. Thismulti-technique approach allowed us to shed light on the structures of the complexes responsible for the inks’ colors and to obtain vibrational signatures that can be used to identify the different inks in works of art and in historic documents. Information on the nature and chemical properties of the complexes formed between a dye and a mordant is important as these determine, at least in part, their lightfastness. EPR permitted to determine the coordination environment of the metallic ions. The results of the ESI-MS analysis demonstrated, for the first time, the breakdown of the hematein molecule during the ink preparation, and that the colorants are formed by the complexation of themetallic ions by hematein breakdown products,mainly catechol and/or bicyclic compounds. The FTIR spectra obtainedwere found to be dominated by bands due to the bindingmediumand sulfates used as reagents. The Raman analysis showed that the characteristic features for the different inks studied depend on the historic recipe used, attesting to the challenges that their identification and characterization inworks of art present. In the Raman spectra of the inks applied on paper, broadening of bands in the 750–400 cm−1 range are observed when compared to the spectra of the inks’ powders, possibly due to the interaction of the compoundswith the cellulose in the substrate.