Iron-gall Ink
Olivier Bonnerot, Grzegorz Nehring
Iron-gall inks are obtained by the reaction between iron II ions (Fe2+) with tannins (phenolic and polyphenolic compounds) in a water-based solvent (water, wine, beer, vinegar, etc.), often with the addition of a binder, normally gum arabic, to avoid bleeding. Due to their characteristics, they are a borderline case between the soluble inks and dispersion inks groups. In fact, by mixing iron ions and tannins, a water-soluble ferrous gallate complex is formed; this product belongs to the type ‘soluble inks’. Exposure to oxygen leads to the formation of insoluble black ferric gallate pigment, thus acquiring characteristics typical of a ‘dispersion ink’ (Krekel 1999; Ponce et al. 2016; Teixeira et al. 2021).
Though the arguably best source of tannins, such as e.g. gallic acid, are gall nuts (pathological growths on the leaves and branches of oaks), they could be substituted by a variety of tannin-rich plant matter (e.g. tree bark, fruits and fruit rinds, leaves, flowers, etc.) that was cooked or macerated. The most common source of iron ions is vitriol (a mixture of hydrated metallic sulphates), but the recipes also attest to non-vitriolic iron-gall inks, which were prepared using filings, slags, nails or other pieces of iron. Natural sources of vitriol are found as efflorescences next to iron and copper mines, and consist of a varying mixture of several metal sulphates. These metals do not contribute to colour formation in the ink solution, but possibly change the chemical properties of the inks.
Due to its solubility, the ink penetrates the writing substrate's surface, making it difficult to erase. Their colour varies between dark brown and black, and they often show a brown halo around the writing. Especially due to unbalanced reactions that cause an abundance of acid solution or oxidising compounds, they can catalyse several types of reactions in the writing surfaces that often result in heavy damages to the latter (i.e. ink corrosion characterised by the burning and piercing of the support, particularly of paper).
The first mention of the use of vitriol together with gall nuts can be traced to Philo of Byzantium in the 3rd century BCE (Zerdoun 1983, 91−92). It describes the recipe of an invisible ink made of gall nuts, used to write on the skin or hat of a message-bearer in the context of a siege. When vitriol is later added to the written surface, the message turns black and can be read. However, the first recipes appeared in the Middle East only in the 9th century CE (Schopen 2006), while European recipes are attested only from the 12th century CE (Zerdoun 1983, pp. 153-155).
The main analytical techniques to investigate iron-gall inks have been described in detail in several references (Hahn et al. 2004; Colini et al. 2021) and grouped in a standard protocol for the analysis of black inks. It includes:
- 3-colour microscopy generally used for a quick identification of the ink type.
- XRF, which allows the differentiation of iron-gall inks, based on the ratios of various elements relative to iron.
- Complementary techniques such as Raman spectroscopy, FTIR, ASAP-MS can then be used, depending on the objectives of the analysis.