A very durable ink that consists of gallic acid (also known as tannin – 3,4,5-trihydroxybenzoic acid, chemical formula: C6H2(OH)3COOH), iron sulfate and possibly a binder (gum arabic). Ferro-gallic ink has been used since antiquity. When fresh, pure ferro-gallic ink has only a pale color. Only after being applied to paper and exposed to atmospheric oxygen does it develop a strong black pigment. It is insoluble in water and thus very difficult to remove. After extended periods of time, ferrogallic ink attacks paper and parchment, causing what is known as ink corrosion.
- Ferrogallic Ink