Black inks (II): Iron gall ink

II Ink colours

Another type of black ink, the earliest evidence of which dates to the Hellenistic period, is iron gall ink. This ink is the result of a chemical reaction between a liquid rich in tannins, obtained from vegetable sources such as oak (from the bark and especially from oak-galls, boiled or macerated in water), and iron salts (generally iron sulphate or vitriol, but there are also recipes that use iron filings or nails). When the two components are mixed, a reaction takes place that produces an intense black colour.

The earliest reference to this ink is by the engineer, inventor and writer Philo of Byzantium (3rd century B.C.E.). In his treatise on military engineering, Belopoeica (4, 77), he states that, during wars and sieges, this reaction was used to exchange secret messages that could be written on unusual writing surfaces and remained invisible until the second reagent was applied. Its regular use in writing is attested somewhat later, in Late Antiquity. However, it was during the Middle Ages that it became widespread, due to its excellent penetrability on parchment.

In comparison to carbon inks, iron gall ink possesses several advantages, such as its ease of production and low cost, its intense black colour and its excellent adhesion to various writing surfaces. It does not rub off in water, making it an ideal choice for documents that should not be altered, such as legal documents. However, its use has also been associated with certain conservation problems, as its reagents have frequently contributed to the deterioration of writing media.