Iron-gall inks: a review of their degradation mechanisms and conservation treatments

Abstract

Iron-gall inks are an essential element of our written cultural heritage that is at risk of a total loss due to degradation. This degradation leads to the loss of the support, particularly the cellulose-based support. Intending to stabilize it, we have come a long way from the nineteenth-century cellulose nitrate laminations to the relatively recent phytate treatments; nevertheless, less invasive treatments are needed. To pave the way for developing safer and more sustainable treatments, tailored as much as possible to the object, this paper reviews the conservation treatments and the advances that have taken place over the last decade in our understanding of the degradation mechanisms of iron-gall inks, based on a careful selection of references to support a concise microreview. This discussion is based on the currently accepted models based on the Fe3+-gallate and the identification of degradation products for iron-gall inks observed in heritage objects, including manuscripts dating from the fourteenth to seventeenth centuries and drawings from the fifteenth to nineteenth centuries. The degradation promoted by iron-gall inks induces scission of cellulose through acid catalysis and/or redox reactions. The causes of these acid-base and redox reactions are also assessed. Finally, we detail the state-of-the-art conservation treatments used to mitigate iron gall ink deterioration, covering treatments from the late nineteenth century to the beginning of the twentieth century, followed by the presentation of current phytate treatments and new postphytate treatments.

Post‐secretory synthesis of a natural analog of iron‐gall ink in the black nectar of Melianthus spp.

“…The high levels of gallic acid in the nectar suggested its coloration might be derived from a natural analog of iron‐gall ink. This ink was used extensively from medieval times up through the middle of the 20 th century, with recipes often involving the addition of vitriol (ferrous sulfate (FeSO 4 )) to extracts of ground oak galls, which are high in gallic acid and hydrolysable tannins (Melo et al ., 2022). More modern recipes for iron‐gall ink even explicitly call for gallic acid and tannic acid (decagalloyl glucose) as ingredients (Wailes, 1935).…”

Post-secretory synthesis of a natural analog of iron-gall ink in the black nectar ofMelianthusspp.

“…The high levels of gallic acid in the nectar suggested its coloration might be derived from a natural analog of iron-gall ink. This ink was used extensively from medieval times up through the middle of the 20 th century, with recipes often involving the addition of vitriol [ferrous sulfate (FeSO4)] to extracts of ground oak galls, which are high in gallic acid and hydrolysable tannins (Melo et al ., 2022). More modern recipes for iron-gall ink even explicitly call for gallic acid and tannic acid (decagalloyl glucose) as ingredients (Wailes, 1935).…”

“…3C). The apparent discrepancy in the composition of iron-gall ink (rich in gallic and tannic acid) and that of black nectar (rich in ellagic acid and tannins) could be due to pH effects, as iron-gall ink is acidic [pH ~1-3 (Melo et al ., 2022)] and the black anthesis nectar is alkaline (Fig. 1D).…”

A closer look at the Azzolino collection

“…The most basic ingredients in iron gall inks are iron (II) sulphate (ferrous sulphate), historically known as green vitriol or copperas, and tannin (often from oak galls) in water with a binder such as gum Arabic [ 6 ]. The actual mechanisms of the colour-forming process of iron gall ink still represent a field of ongoing research and the exact form of the compounds that we know under the name “iron gall” have not been determined [ 7 ]. The ink is, however, generally understood as containing a water-soluble ferrous tannate complex that oxidises to form insoluble ferric tannate as the ink dries.…”