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

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

doi:10.1186/s40494-018-0228-8

Rafael Javier Díaz Hidalgo, Paula Nabais + Show 6 more

Open Access

https://doi.org/10.1186/s40494-018-0228-8

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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.

Highlights

Degradation of manuscripts catalysed by iron-gall inks is a major conservation issue in heritage collections, posing a serious threat to world written heritage
In Europe, iron-gall ink recipes are profusely described in medieval treatises that mention the use of plant extracts such as Quercus infectoria that were combined with iron salts [1,2,3], Fig. 1
Selected historical ink reproductions are compared with standards prepared complexing iron with gallic, ellagic and tannic acids as well as with digalloyl and pentagalloyl glucose, with and without gum arabic, Fig. 1 and Table 2

Summary

Introduction

Degradation of manuscripts catalysed by iron-gall inks is a major conservation issue in heritage collections, posing a serious threat to world written heritage. Iron‐gall inks in technical written sources and references for the colour center Medieval recipes typically contained the three basic ingredients described in Fig. 1: Fe2+ obtained from an iron sulphate salt, a phenolic extract (tannins), and gum arabic [2, 9].

Results

Conclusion