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Iron gall inks have been among the most used writing materials after carbon black, thus representing an important element of the historical and artistic heritage of our society. Crucially, the preservation of manuscripts and drawings is influenced by the presence of these inks, leading to conservation issues related to paper degradation and text fading. Besides all the advances obtained in paper conservation, the study of iron gall ink's behaviour and ageing is still an important topic, which requires investigation through an accurate molecular characterisation to produce reliable models. In the present work a micro-destructive method based on liquid chromatography techniques (HPLC-DAD and HPLC-ESI-Q-ToF) has been optimised starting from a model gallic acid-based ink. An in-depth study of the behaviour of the ink in time was performed by natural and artificial ageing tests, monitored by colorimetry, showing the autoxidation of gallic acid to ellagic acid in the prepared mock-ups. The effect of relative humidity on ageing processes was also evaluated, allowing us to determine different intermediates depending on the environmental conditions. Finally, the analytical method developed was then successfully applied for investigating 19th-20th century historical ink samples, where one of the identified ageing markers was detected, besides the expected gallic and ellagic acids.

• We applied liquid chromatography-tandem mass spectrometry to the analysis of historical iron gall ink recipes. • We characterised ageing markers of iron gall inks by HPLC-ESI-Q-ToF. • We highlighted different degradation trends depending on historical recipes and ageing conditions. • We proved that it is possible to use 15 µg as minimum amount of sample for the HPLC-ESI-Q-ToF analysis of iron gall inks. Several manuscripts and drawings of our historical and artistic heritage have been produced with iron gall inks. To obtain an iron gall ink, ancient treatises cite the addition of ferrous sulphate and gum Arabic to a decoction of oak galls as a basic procedure. Owing to the development of synthetic chemistry, iron gall ink recipes were improved with new materials and procedures in the late 19th and early 20th century. Notably, many conservation issues arise from the interaction between iron gall inks and the paper support of manuscripts and drawings. To date, most of the research on the topic are focused on paper preservation by non-destructive analytical methods, which provide only limited information on degradation process trends and minor components, representative of iron gall ink's recipes. In the present work, three historical recipes of iron gall inks (alizarine ink, Reid ink, modern gall ink) , dated to 19th-20th century and differing for the preparation method and additives, were characterized. The molecular markers of iron gall inks and of gallic acid degradation were detected by an optimized protocol based on high performance liquid chromatography coupled to high resolution mass spectrometry (HPLC HRMS). Furthermore, by performing ageing tests on reference materials in different indoor conditions (natural light and stored in the dark), two degradation mechanisms were observed: hydrolysis of poly-galloyl glucose species and auto-oxidation of gallic acid. Thus, different chemical profiles and ageing trends were revealed depending on the starting recipe. The procedure Limit of Detection (LOD) was estimated, improving the approaches reported so far in the literature. Finally, the strategy was successfully applied for the characterisation of the ink employed in a very degraded 16th century manuscript, granting access to the complete molecular profile of an iron gall ink with just 15 µg of sample. [Display omitted] [ABSTRACT FROM AUTHOR]