What is iron gall ink?

The dark colour of iron gall inks derives from the complex formation between the ferric ion and tannin.

The tannins used for ink preparation have been extracted from plants and trees and have polyphenolic structure bearing vicinal hydroxyl groups which are responsible for the complex formation with the metal cations.

The metal was added as a sulphate salt. Most often, the green vitriol (FeSO4 x 7H2O) was used. In addition to iron, the natural raw material used for manufacture of iron gall ink usually contained numerous other transition metals, which may contribute to the decay of paper known as ink-decay. In particular copper have been shown to be a more powerful catalysts of oxidative degradation than iron ions. Although the corrosive effect of e.g. copper ions on the cellulose is well demonstrated, no systematic study of the historical ink recipes, compositions of historical inks, nor their role in ink corrosion has been performed so far. Such a study is of extreme importance, given that research on stabilisation of iron gall ink corrosion extensively relies on the use of model inks. In addition, study on composition of various vitriols and inks may lead to identification of the type of vitriol used in certain inks, an important information for art historians. Given the importance of the subject on subsequent stabilisation studies, the first objective of InkCor project is to gain a deeper insight into the various ink components and evaluate their relative corrosive properties. The problem of ink analysis will be approached by art historians, analytical chemists and physists.

Art technological source research

Analysis of inks in historical documents