Due to the degradative effect of iron gall inks, paper documents written with them change their appearance over time. Visual examination of a wide range of originals showed that the appearance of the ink, as well as of the paper support, can vary significantly: there are differences in the colour of the ink, halos have formed around some ink lines, the paper on the verso is often brown discoloured beneath the inked areas on the recto and some inks have faded or support crystalline deposits on their surfaces. Research has shown that the visible progress of ink corrosion is similar for all examined originals, as well as for test papers which I have been artificially aged. In order to illustrate this process, a model has been developed showing the cross-section of an iron gall inked paper during its ageing process. In order to facilitate decision-making concerning preservation, a condition rating system has been developed for objects written or drawn in iron gall ink. A classification system incorporating four condition ratings allows a fast and easy evaluation of the condition of an object. Advice for storage, handling and the treatment of artefacts is provided alongside the condition rating.