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Concept
Iron gall ink
Summary
Iron gall ink (also known as common ink, standard ink, oak gall ink or iron gall nut ink) is a purple-black or brown-black ink made from iron salts and tannic acids from vegetable sources. It was the standard ink formulation used in Europe for the 1400-year period between the 5th and 19th centuries, remained in widespread use well into the 20th century, and is still sold today.
The ink was traditionally prepared by adding some iron(II) sulfate (FeSO4) to a solution of tannic acid, but any iron ion donor 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 glucose and gallic acid, which yields a darker purple-black ink, due to the formation of iron gallate.
The fermented extract was combined with the iron(II) sulfate. 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 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 air, it converts to a ferric tannate, which is a darker pigment. This product is not water-soluble, contributing to its permanence as a writing ink.
The darkening process of the ink is due to the oxidation of the iron ions from ferrous (Fe2+) to ferric (Fe3+) state by atmospheric oxygen. For that reason, the liquid ink needs to be stored in a well-stoppered bottle, and often becomes unusable after a time. The ferric ions react with the tannic acid or some derived compound (possibly gallic acid or pyrogallol) to form a polymeric organometallic compound.
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