Hyperspectral imaging on two German medieval illuminations of the treasury of the cathedral of Bordeaux, part of the Marcadé Collection (XV century, France).

This paper is focused on the hyperspectral imaging study of two German medieval illuminations. These cuttings dated from the XV century had been taken from the same liturgical book and are part of the Marcadé Collection, the treasury of the cathedral of Bordeaux. This Collection includes forty one illuminations come from France, Germany, Italy, Netherlands and Spain, dated from the XIII to the XVI century. These illuminations have never been studied and we examined them about the iconographic, stylistic, materials and conservation points of views. The aim of this study is the application of a hyperspectral system to characterize the pigments and to compare the materials used in two German illuminations of the same period.
Our hyperspectral imaging system is provided by SPECIM (Finland) and works between 400 to 1000 nm. Spectro-imagery associates reflectance spectra with each pixel of the image and treats the signal received in various wavelengths. The characteristics of the spectral signal in VIS range or NIR are used to get rapidly an identification and localization (mapping) of the components (binders, pigments) of the cuttings without sampling and in respect to these fragile artworks. The potential of a compact and affordable NIR spectrometer, the ARCoptix FT-NIR Rocket (900-2600 nm) to confirm the identification of pigments on manuscripts paintings is also discussed.
HSI is used to give a preliminary approach of the materials used. HSI allows us to have RGB, IRFC and grayscale images and gives a control image of its conservation state at a given moment. Comparison of RGB and IRFC images of illuminations to those of models samples as a colors chart made by an illuminator according to medieval techniques allows us to hypothesize about the pigments used. Then, comparison of each reflectance spectrum obtained, with our database of 150 medieval pigments, enables mapping the repartition of the pigments used on the entire image. When doubts remain in certain zones, others techniques as Raman or XRF spectroscopies are used.
In the studied illuminations, HSI allows the identification of a palette of more than ten pigments, as azurite, cinnabar, lead tin yellow, lead white, malachite, salt green, ochres and an organic pigment, possibly brazil wood. The gilding technique is close to the zwischgold technique which consists to a thin gold leaf glued on tin leaf. The palette is varied and typical of the XV century in Germany. HSI has proven to be non-invasive and a rapid technique to give information on the entire image, and a good helped to identify and locate most pigments even if point analyzes are often needed to confirm the presence of some pigments or mixtures.