Part 2: Making iron gall ink

Iron gall ink is made by mixing plant material containing gallotannic acid (a plant polyphenol), primarily obtained from oak gall nuts (or apples), with hydrated iron (II) sulfate (FeSO₄∙7H₂O), a compound also known as (green) vitriol.

According to Kremer pigmente’s information sheet about the oak gall nuts we used for this experiment (last accessed online 09.01.2023), the main constituents of the best quality Aleppo galls, i.e., those collected before the insect escapes the gall, are 50% to 70% gallotannic acid, 2 to 4% gallic acid, mucilage, sugar, resin, and lignin (insoluble matter).

The colourant that gives the dark tones of the produced solution is ferrous gallotannate, a dark brown compound that becomes black after exposure to air with the formation of ferric tannate (oxidation from Fe²⁺ to Fe³⁺).

Before their wide use for the manufacture of ink, these tannin-rich sources found numerous applications in antiquity. Pliny the Elder mentions their medicinal properties and their use as a black hair dye (Plin. HN 24.9), dyeing wool and tanning (Plin. HN 16.26-29).

Regarding ink manufacture ancient recipes suggest that, besides galls, a range of tannin-rich plants, such as for example tree bark or fruits and alternative iron sources deriving from everyday-life materials, such as iron nails, could be used for the production (Colini et al., 2021).

For this pilot phase of our study we wanted to experience the workability of the most commonly used materials for the production of iron gall inks (galls nuts, gum arabic, green vitriol).

• How easy is it to grind the gall nuts?
• Is gum arabic easily dissolved in water?
• Can the process be carried out by one single person?
• How does the colour of the tannin extract change over time and what is the impact of time on the final product?

There are several recipes out there and we chose to use a well documented and already reproduced recipe, following closely the quantities and process as described by Sara Charles (last accessed online 19.12.2022).

* It was difficult to find distilled water at a regular store in Oslo, so we decided we could afford to use tap water since no chemical analysis is planned at the moment and contamination from the water source was not considered important.

The galls were placed in a transparent plastic bag and wrapped in paper and a cloth, to avoid losing the material through the process of crushing.

And then the pieces were further processed in a marble mortar.

The colour of the crushed galls was pale yellow, pointing to the so-called ‘white galls’, i.e., galls collected after the insects have escaped. These galls contain less gallotannic acid than the best quality ones, which are of darker colour and are collected while the insects are still inside.

The extraction of the tannins without heating takes a few days, so the process had started already at home before we all met at the laboratory of the Department of Archaeology, Conservation and History of the University of Oslo.

At first, the mixture is yellowish:

According to the recipe we followed, the container should be placed by the window for three days. Oslo in December is rather dark, and the window could not help with the maintenance of a warm temperature. The jar therefore changed several locations during these three days, according to which place was the warmest and not the sunniest.

After a couple of hours, the mixture was already darker.

Three days later, and we were already in the lab setting up the hyperspectral imaging study that aimed to explore the impact of the glass frames that house papyri on the imaging process (part 3).

The mixture had a dark brown colour, indicating the successful extraction of the tannins in the water medium. We strained the solution through a coffee filter and since we wanted a small quantity of ink, we used 70 mL of the dark brown liquid to produce our iron gall ink, mixing it with 40 g of green vitriol and 70 g of gum arabic.

Ingredients for iron gall ink:

With the addition of vitriol, the solution became intensely black et voilà!

Our ink was ready to use.

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Many people contributed to this pilot study, whom we would like to thank:

• Hilda Debora and Riestiya (Tia) Zain Fadillah from the Colourlab (NTNU), who set up the hyperspectral imaging experiment.
• Anastasia Maravela (IFIKK, UiO) and Federico Aurora (UB, UiO), who kindly lend us spare glass frames.
• David Grimaldi (who is currently completing his MA thesis at IFIKK) and Despina Wilson (artist) were extremely helpful throughout the experimental session.