Iron-Gall Ink
Definition
A historical ink formulation of ferrous sulphate, tannic acid (from oak galls), and gum arabic. The ferrous-tannate complex oxidises to brown ferric-tannate over decades and is acidic enough to degrade underlying cellulose. FTIR-ATR and XRF identify it.
- Historical use period
- Roughly fifth century to early twentieth century
- Main components
- Ferrous sulphate, oak-gall tannic acid, gum arabic
- Color progression
- Blue-green when fresh, darkens to blue-black, then browns with age
- Analytical methods
- FTIR-ATR and XRF
Common questions
What is iron-gall ink made of?+
Iron-gall ink combines ferrous sulphate, tannic acid extracted from oak galls, gum arabic, and water. When first made, it has a blue-green hue that shifts to blue-black over time.
How can you identify iron-gall ink on a document?+
Forensic analysts use FTIR-ATR and XRF to detect the presence of iron-gall ink. You can also spot it by age: the ink browns over decades as the ferrous-tannate complex oxidises into ferric-tannate.
Does iron-gall ink damage paper?+
Yes. Iron-gall ink is acidic enough to degrade the underlying cellulose fibers. Over time, sulfuric acid byproducts from the ink chemically corrode the paper, which is why documents written with this ink often show browning and brittleness.
Related terms
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