A very durable ink that consists of gallic acid, iron sulfate and in some cases a binder (gum arabic) and that has been known for several centuries. The ink was used for documents of all kinds until the advent of chemical dyes in the recent past.
When first applied, pure ferro-gallic ink has a pale color; only when exposed to atmospheric oxygen does it form a distinctly black pigment. It is insoluble in water and thus very difficult to remove.
After extended periods of time, ferro-gallic ink decomposes paper and parchment, causing what is known as ink corrosion.