Distinct strategy using an integrated methodology of innovative and traditional techniques for discriminating superimposed historical inks in revised manuscripts

This study concerns a non-invasive, multi-analytical methodology proposed to overcome legibility limitations in compromised handwritten parts of historical manuscripts. In such official documents, literary works, or personal letters, ink traces may be revised after the original draft through overwriting, crossing out, or erasure. The inks used for these later modifications often closely resemble those of the original text in both appearance and chemical composition, making them particularly challenging to distinguish. Over the centuries, a variety of black inks have been produced using similar ingredients, such as polyphenols derived from galls or logwood and metal salts involved in ink formation, resulting in comparable chemical and physical properties when analysed. The main aim was to characterise and discriminate overlapping inks to achieve effective separation and visualisation of superimposed writing layers through a systematic methodological approach. The methodology combines a multispectral imaging system, Hypercolorimetric Multispectral Imaging (HMI), with complementary spectroscopic techniques, including single-point and mapping X-ray fluorescence (XRF) and micro-Raman spectroscopies. The approach was first tested on laboratory specimens, simulating overwritten handwriting, prepared using inks produced according to historical recipes (carbon-based, iron-gall, and logwood inks), dating from the 15th to the 18th centuries. It was subsequently applied to a specific case selected (e.g., text with a stroke of later corrections) from the original manuscript Ms. Tic. 67 (late 18th–early 19th century), an important testimony to Pavia’s erudition and historiography, preserved at the University Library of Pavia (Pavia, Italy). HMI was applied to inks for the first time, providing high-resolution images with spectral reflectance and colourimetric coordinates for each pixel. Multispectral data processed with image processing tools enabled enhanced visualisation and contrast between underwritten text and cross-out marks, particularly when inks belonged to different classes, and in some cases even when compositions were similar. XRF and Raman spectroscopies provided complementary elemental and molecular characterisation. XRF proved effective in distinguishing metal-tannin inks with different metal contents, while Raman spectroscopy enabled identification of the characteristic bands of carbon-based inks and the metal–polyphenol complexes associated with iron-gall and logwood inks, as well as the detection of layered and mixed ink systems. Overall, the results demonstrate both the advantages and disadvantages of the proposed methodological approach, highlighting that each case represents a complex and unique paper–overlapping ink system in which imaging and analytical outcomes are influenced by ink composition, application technique and deposition ink amount.