New insights into iron-gall inks through the use of historically accurate reconstructions

Rafael Javier Díaz Hidalgo, Ricardo Córdoba, Paula Nabais, Valéria Silva, Maria J. Melo, Fernando Pina, Natércia Teixeira, Victor Freitas

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Abstract

Iron-gall inks have been described as complexes of iron ions with gallic or tannic acids, available in gall extracts. To assess this working hypothesis, we have prepared medieval inks using ingredients and methods appropriate to the fifteenth to seventeenth centuries. The five historical inks studied were selected based upon research into Iberian written sources of medieval techniques. Results are supported by comparison with iron complexes with a well-characterized phenol counterpart: gallic, ellagic, and tannic acids as well as digalloyl and pentagalloyl glucose; as either precipitates or prepared as inks by adding gum arabic. Raman and infrared spectroscopies show that medieval writing inks could not have been represented solely by iron complexes with gallic acid. Overall, writing inks display the infrared signature of gallotannins, indicating that complexes of Fe3+-polygalloyl esters of glucose are also formed. Our results also show that the commercial tannic acid solution is far more complex than the gall extracts, and cannot be used to represent a gall extract (as described in historic written sources). High-performance liquid chromatography–electrospray ionisation, HPLC–ESI–MS, reveals that the concentration of gallic acid varies in the gall extracts, depending on the extraction method and ink recipe. Importantly, in certain recipes, gallic acid is found as a minor compound, when compared with the galloyl esters of glucose.

Original languageEnglish
Article number63
JournalHeritage Science
Volume6
Issue number1
DOIs
Publication statusPublished - 1 Dec 2018

Keywords

  • Gallotannins
  • Iberian written sources
  • Iron-gall inks
  • Polygalloyl esters of glucose
  • Reconstructions

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