Reading the Unreadable. Advanced Imaging to Recover Illegible Text in Historic Documents

International conference on analytical techniques in art and cultural heritage (Technart)

Lisbon. Portugal
Conference. 2023

From fires to floods, from invisible inks to redactions, information has been accidentally or intentionally obscured on countless documents, maps and photographs, in archives, libraries, and museums across the world. Removed from catalogues and reading rooms and therefore inaccessible to researchers and the general public, what new and fascinating stories would these obscured documents reveal, if we were able to read them?

This paper demonstrates that current imaging technologies can be used to unlock this 'lost' archive. In this study, we imaged a selection of medieval documents relevant to the 'Virtual Record Treasury of Ireland', a digital reconstruction of archives destroyed in 1922 at the Public Record Office of Ireland at the outset of the Irish Civil War [1]. We used Multiband Imaging, Hyperspectral Imaging and X-Ray Fluorescence (XRF) scanning to reveal illegible letter-forms and whole words written in iron gall ink on parchment.

While both UV luminescence and UV reflected imaging were very effective at improving the readability of faded iron gall ink and enhanced the contrast of the text and the parchment in areas with old damp stains and ingrained dirt, the “galled” documents required more advanced imaging techniques. The iron distribution maps obtained by XRF scanning helped to recover text from the galled areas, but it posed several challenges, such as minimising movement of the parchment during overnight scans and separating the data corresponding to iron on the front which was combined with the signal from the back. Through subtraction of other elemental maps during post-processing, we managed to tackle the latter issue. Hyperspectral imaging also proved helpful to read galled documents, particularly after applying PCA image processing. However, care has to be taken with instrument configurations where the light source is relatively close to the object, as surface heating can occur quickly and cause the parchment to distort.

These combined imaging techniques enabled records experts to identify individual characters and whole words, thereby recovering the meaning of texts that were previously indecipherable [2]. Further research will adapt the methodology to the broad range of causes of information loss, as well as the variety of document media and temporalities, which require an extensive and multi-faceted approach of advanced imaging and post-processing techniques, in balance with the preservation needs of these fragile and invaluable historic materials.

[1] https://virtualtreasury.ie (consulted on 19/01/2023)

[2] Biggs E, Dryburgh P (forthcoming, 2023), ‘Report into the State of the Exchequer c. 1284: TNA E 101/234/19’, Analecta Hibernica

Illegible text. Digital reconstruction. Multispectral imaging. Hyperspectral imaging. Macro X Ray Fluorescence. Principal Component Analysis. Medieval Documents. Parchment. Iron Gall Ink.