The Ins & Outs of Iron Gall Ink

Anisha Gupta was the Assistant Conservator for Archival Materials. She holds an M.S. from the Winterthur/University of Delaware Program in...

While iron gall ink was an incredibly popular ink from the 4th to the 20th centuries, it is now known to have damaging effects on paper and parchment. We find this ink throughout the APS collection, meaning we conservators regularly have to decide when and how to treat it. In part one of this blog series, let’s dive into the basics of this ubiquitous material. 

What is iron gall ink?

Iron gall ink was developed in the 4th century. The Codex Sinaiticus (pictured above) is one of the earliest surviving manuscripts to be written on parchment in iron gall ink rather than carbon black ink. When properly made, iron gall ink is blue-black and permanent. Unlike carbon ink, it etches the writing surface and can’t be easily washed off, quickly making it the preferred ink for writing on parchment. It remained in use well into the 20th century whenever permanent ink was necessary. 

What is iron gall ink made of?

Iron gall ink is comprised of four main ingredients:

1. Oak gall nuts. Oak gall nuts are a tree’s protective reaction to wasps depositing eggs beneath its bark. They are not actually nuts! Once collected, these are then soaked in a solvent. 

oak gall nuts
Oak gall nuts. Aleppo galls with the insect still inside were particularly prized for their dark color. Image Credit: 

2. Solvents. Acidic solvents, such as beer or wine, or allowing mold to grow on the gall nuts as they soak help produce gallic acid and increase the color of the ink.

Solvents such as water, beer, or wine. Image credit:

3. Ferrous sulfate. Ferrous sulfate reacts with gallic acid to produce a blue-black iron-tannin complex.

ferrous sulfate
Ferrous sulfate, also known as green vitriol. Image credit: 

4. Gum Arabic. Gum Arabic increases viscosity (improves ink flow), keeps pigment particles in suspension, binds ink to the writing surface, and gives the ink shine and depth.

gum arabic
Gum arabic is the binder. Image Credit: 

Why is iron gall ink corrosive?

The formation of the iron-tannin complex releases sulfuric acid, so iron gall ink is always extremely acidic, with a pH of 1-3. Acids attack the cellulose chains that paper is made of, shortening the chains and making the paper brown and brittle. If the iron gall ink contains an excess of iron (II) ions, it will also catalyze oxidation of the paper or parchment, which results in crosslinking and brittleness. Eventually the acids and the oxidation reactions can corrode parchment and paper, undermining the ink and allowing it to drop out or crumble away. 

What are the stages of iron gall ink corrosion?

As iron gall ink corrodes, it often passes through distinct stages of deterioration. In the first stage, haloes develop around the inked lines. At first they are only visible in ultraviolet light, but they eventually become visible to the naked eye. 

The corrosive nature of iron gall ink and its various stages of deterioration can be seen in this manuscript. Image credit:

In the next stage, the ink begins to strike through the paper, becoming visible on the opposite side from which it was written. In the third stage, the ink begins to crack whenever the paper is handled. In the last stage, the ink falls out of the paper entirely. 

How do we treat this type of damage?

Stay tuned for part two of the series to learn more about how conservators mitigate and treat the effects of iron gall ink. 



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