Iron gall ink
Iron gall ink (also known as iron gall nut ink, oak gall ink, and common ink) is a purple-black or brown-black ink made from iron salts and tannic acids from vegetable sources. It was the standard writing and drawing ink in Europe, from about the 5th century to the 19th century, and remained in use well into the 20th century.
The ink was generally prepared by adding some iron(II) sulfate (FeSO4) to a solution of tannic acid (C6H2(OH)3COOH), but any iron ion donor (e.g., nails, iron metal scraps, etc.) can be used. The gallotannic acid was usually extracted from oak galls or galls of other trees; hence the name. Fermentation or hydrolysis of the extract releases tannic acid, which yields a darker black ink.
The fermented extract was combined with the ferrous iron(II) sulfate. After filtering, the resulting pale-gray solution had a binder added to it, (most commonly gum arabic) and was used to write on paper or parchment. A well-prepared ink would gradually darken to an intense purplish black. The resulting marks would adhere firmly to the parchment or vellum, and (unlike india ink or other formulas) could not be erased by rubbing or washing. The marks could only be erased by actually scraping a thin layer off the writing surface.
By mixing tannin with iron sulfate, a water-soluble ferrous tannate complex is formed. Because of its solubility, the ink is able to penetrate the paper surface, making it difficult to erase. When exposed to oxygen a ferric tannate pigment is formed. This complex is not water-soluble, contributing to its permanence as a writing ink.
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