Visible (420 – 720 nm) Hyperspectral Imaging Techniques to Assess Inks in Historical Documents

Visible (420 – 720 nm) hyperspectral imaging was used for the examination of different inks in portions of two historical documents. One document was the 'James Bay Treaty' (circa 1905 – 1906) which was written on parchment. The other document was an 'Adhesion to Treaty No. 6' (circa 1876) which was written on paper and parchment. Most of the text in these documents was written with iron gall ink. Some signatures and decorative underlining of text were written with other inks. The objective of this work was to use hyperspectral imaging for assessing and quantifying aspects of the inks such the extent of ink migration and ink loss in portions of these documents. Ink migration can occur for a number of reasons including diffusion of wet ink, smearing or if the ink has fugitive properties. In some cases the extent of ink migration is difficult to assess if the migrated ink is very faint in appearance. Single wavelength images (420 nm) provided images with higher contrast between ink and paper which made them useful for examining the extent of migration of ink components. A contour plot from a single wavelength image provided detailed information regarding the spatial properties of iron gall ink migration. In one portion of text where red ink was smeared over black ink, pixel unmixing was used to determine the extent of smearing of the red ink. This red ink was also used for underlining text in one treaty. An ultraviolet light source was useful to visually determine the extent of its migration as well as for differentiating the migrated portion from the original ink. Ink losses could also be assessed by constructing a density slice of a single wavelength image and relating the amount of ink lost to a decrease in absorbance.