Characterisation of paper containing iron gall ink using size exclusion chromatography

Abstract

Iron gall inks were the most often used inks from the Middle Ages until the 20th century; examples include the drawings by Leonardo and scores by Bach. These masterpieces are threatened by the corrosive properties of the ink. In order to increase the longevity of the materials, stabilisation treatments have been developed during the last two decades. Their effectiveness has often been determined by comparing rates of degradation during accelerated tests. However, such evaluation using historical materials is limited by the amount of material required by various analytical methodologies.The paper describes the use of size exclusion chromatography (SEC) of cellulose containing iron gall ink, derivatised using phenyl isocyanate. A linear correlation is observed between the weight average degree of polymerisation obtained with SEC and the one obtained using viscometry, with a slope of 1.4. The method was used to evaluate the extent of stabilisation of historic documents containing iron gall inks during accelerated degradation. Despite considerable uncertainty, also caused by the heterogeneity of historic materials, it is demonstrated that efficient stabilisation by factors of 10 and 20 times is achievable using a calcium phytate treatment.