Iron gall ink, a purple-black or brown-black fluid made from iron salts and plant-derived acids, was the standard ink in Europe for over 1,400 years. Its use began in the Roman Empire and lasted well into the 20th century. Because of its long history, a vast number of historical manuscripts, official records, and personal documents were written with it.
The Two Key Ingredients
The name “iron gall” directly refers to the ink’s two primary components. The first ingredient, the “gall,” is an oak gall, also known as a gallnut. These growths are not fruits but are formed by oak trees in response to the larvae of insects like the gall wasp. The tree produces these bulbous growths, which contain a high concentration of gallotannic and gallic acids, as a protective measure.
The second component is the “iron,” which comes from iron(II) sulfate. This metallic salt has been known by several historical names, including green vitriol. In early recipes, this substance was sometimes mined from sources where it occurred alongside other minerals. The iron(II) sulfate provides the iron ions necessary to react with the acids from the galls.
The Ink-Making Process
The creation of iron gall ink is a straightforward chemical process. The first step involves crushing dried galls to expose the material inside. These crushed pieces are then soaked in a liquid—often water, but sometimes wine—to extract the tannic acids. This soaking can last for several days, and some historical recipes called for fermentation to produce a darker ink.
After the acid has been extracted and the solution filtered, iron(II) sulfate is added. This initiates a chemical reaction between the tannic acid and the iron salt, forming ferrous tannate. The final ingredient is a binder, most commonly gum arabic, a resin from the acacia tree. The gum arabic helps keep the pigment suspended and improves the ink’s flow and adhesion.
A Legacy Written in Ink
The use of iron gall ink spans from antiquity to the modern era. First developed around the 4th century, it quickly became the preferred ink for writing on parchment because, unlike carbon-based inks, it chemically bonds with and etches the surface, making it permanent and difficult to erase. It was the standard for scribes, artists, and officials across Europe and eventually the Americas.
This ink was used to create some of the world’s most significant cultural artifacts. Many famous manuscripts from the Middle Ages and the Renaissance were written with it. The notebooks of Leonardo da Vinci, the musical scores of Johann Sebastian Bach, and early drafts of the U.S. Constitution were all penned in iron gall ink. Its permanence made it the official choice for legal and royal records in countries like Great Britain.
The widespread use of iron gall ink was also due to the accessibility of its ingredients and the relative ease of its production. Ink was often made in-house, with recipes passed down through generations. This resulted in significant variation in the ink’s quality and composition.
Corrosion and Conservation
Despite its permanence, iron gall ink possesses a destructive quality known as ink corrosion. This degradation is a result of two chemical processes driven by the ink’s ingredients. The creation of the ink produces sulfuric acid as a byproduct, which causes acid hydrolysis of cellulose fibers in paper, making it brittle. An excess of iron(II) ions in the ink recipe also acts as a catalyst for oxidation, which breaks down cellulose.
This corrosive process can cause the ink to change color from black to brown, create a brownish halo around the written lines, and eventually make the paper so fragile that the inked areas crack and fall away. The severity of the damage depends on the specific ink recipe, the type of paper, and the environmental storage conditions, with high humidity and temperature accelerating the decay. This presents a significant challenge for archivists.
Modern conservation efforts focus on neutralizing the ink’s destructive effects. One common treatment involves immersing the document in a solution containing calcium phytate, a chemical that binds to the damaging iron ions and prevents them from catalyzing further oxidation. This is often followed by a deacidification bath to neutralize existing acids and provide an alkaline buffer against future acid attacks. These interventions aim to stabilize the documents and preserve our written heritage.