Oak 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 traditionally prepared by adding some iron(II) sulphate (FeSO4) to a solution of tannic acid (C76H52O46), but any iron ion donor can be used. The gallotannic acid (= tannic acid) was usually extracted from oak galls or galls of other trees; hence the name. Fermentation or hydrolysis of the extract releases glucose and gallic acid (C6H2(OH)3COOH), which yields a darker purple-black ink, due to the formation of iron gallate.
The fermented extract was combined with the ferrous iron(II) sulphate. After filtering, the resulting pale-grey 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.
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