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Imaging behind glass (2/3)

Part 2: Making iron gall ink

Gall nuts (Aleppo galls, Kremer Pigmente, n. 37400. Photo: AKM).

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 (FeSO47H2O), a compound also known as (green) vitriol.




Oak gall nuts are created by the female oak gall wasps (Cynips Gallae-tinctoria) as a cozy nest to lay their eggs and to provide a nutritious environment for the larvae.

The gall wasp
(Image by Judy Gallagher, CC BY 2.0)

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).



For the extraction of the gallotannic acid, we used:

  • 80 g gall nuts
    (Kremer pigmente, n. 37400)
  • 300 mL tap water*


Gall nuts posing with a hammer before being crushed (photo: AKM).
Gall nuts posing with a hammer before being crushed (photo: AKM).

* 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.

Partially crushed galls in a transparent plastic bag (photo: AKM).

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.

Gall nuts in a glass jar and 300 mL of water in a measuring container (photo: AKM).

At first, the mixture is yellowish:

The gall nuts/water mixture in a glass jar is of pale yellow colour (photo: AKM).

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.

The glass jar containing the mixture is placed next to the window. The sun is relatively low since the experiment was carried out in December in Oslo (Norway).
The glass jar changed several positions during the day trying to find the highest temperature.

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:

  • 70 mL extract from the galls
  • 40 g vitriol (ferrous sulfate FeSO4∙7H2O, from Kremer pigmente, n. 64200)
  • 70 g of powdered gum arabic (Kremer pigmente, n. 63330)
Weighing the ingredients for the synthesis of iron gall under the fume hood.
The pitch black of the final iron-gall ink vs. the brown colour of the extract in water before the addition of vitriol.

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

Our ink was ready to use.

The three different inks produced during the project. Fresh iron gall is intensely black with a bluish hue.

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.

By Ariadne_Marketou

Ariadne Kostomitsopoulou Marketou is a postdoctoral researcher in the project “EthiCodex.” Her research revolves around the materiality of the codex, viewing it as a medium to approach provenance and dating. Ariadne received her PhD in Archaeology and Conservation from the University of Oslo and holds a MSc in Protection, Conservation and Restoration of Works of Art and a BSc in Chemistry from the Aristotle University of Thessaloniki. Her broader research interests include materials analysis, the materiality of colourants, pigment manufacturing technology, and the organization of craft production in the ancient Mediterranean world.

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