HYPERSPEC project or the contribution made by spectro-imagery to the study of the paintings and the taphonomy of decorated caves

Mots clés : imagerie hyperspectrale, pigments, liants, roches, altérations, conservation.

Projet HYPERSPEC – Imagerie hyperspectrale et spectrofluorimétrie appliquée aux matériaux archéologiques

(caractérisation, altérations, taphonomie)

The HYPERSPEC project (Imagerie hyperspectrale et spectrofluométrie appliquée aux matériaux archéologiques – Hyperspectral imagery and spectrofluorimetry applied to archaeological materials) proposes a new approach, based on methods that are usually used, of analysis and the characterisation of archaeomaterials (wall paintings, taphonomy of decorated caves), via the acquisition of hyperspectral analysis equipment.

Hyperspectral imagery appeared in the 1980s. It is defined as: “the acquisition of images in hundreds of contiguous spectral bands, so that a reflectance spectrum can be obtained from each pixel of the image” (Goetz, 1985). It was developed for remote sensing in astronomy, geography etc, and it was first used in the field of cultural heritage in the 1990s.

In 2013, thanks to LabEx financing, IRAMAT-CRP2A acquired a hyperspectral camera for its research project needs.The LaScArBx HYPERSPEC project (Sept. 2013 – Dec. 2014) consists of exploring the possibilities and adaptation of the hyperspectral system for studying archaeological or heritage objects where the it is essential to use non-invasive methods of analysis in situ (a critical issue for heritage) such as for paintings (on walls, manuscripts etc) or decorated caves. 

Classic cameras called multispectral that are usually used for analysing paintings produce an image by analysing three components of the visible spectrum: blue, red and green. Hyperspectral imagery enables the acquisition of images from numerous spectral bands simultaneously – up to more than 500 – separated only by a few nanometres. They therefore offer much more information and make a much finer analysis possible (identification of pigments, for example). Also, the information obtained means that mathematical processing (statistics etc) from spectroscopy can be applied as well as the processing specific to imaging. The system is polyvalent, and adapts equally well to studies in a laboratory (small objects, pictures, illumination, reference materials etc), but it has also been designed to be mobile, with a view to applications on external sites (wall paintings, caves etc). 

The applications anticipated within the framework of the HYPERSPEC project essentially concern the study of paintings (wall paintings, parietal art etc) and the taphonomy of decorated caves.

Regarding the study of paintings, imagery under UV light enables the observation of fluorescence due to later additions, repainting, organic binders, varnish etc. Retouches appear darker, and the original remains brighter. UV rays therefore reveal the state of conservation of paintings and show if they have undergone repainting. Infrared light reveals the preparatory drawing under a layer of paint, adding information on the degradation of inks, revealing hidden signatures, etc. 

Mediaeval illumination

The hyperspectral camera was first used in the Région “Enluminures” (Region Illumination) project (2012-2015), concerning the study of the Mediaeval illuminations in the Marcadé Collection (the Bordeaux Cathedral Treasure).

• The study of the manuscript paintings is currently relatively undeveloped, partly because of the nature of these heritage objects from which samples cannot be taken, and partly because of the limits in existing analysis techniques. At the same time, there is a great demand for information about materials and techniques in the field of art, which justifies the use of hyperspectral imagery for this type of heritage object. In fact, this method (infrared or UV imagery), along with others, will reveal areas that have been affected by an addition (gilding, re-painting, restoration etc), and enable preparatory drawings to be studied and the vision of the object to be completed, identifying the nature of the pigments and binders. The preliminary trials for finalisation were carried out on a facsimile, and the first analyses on illumination from the collection will take place at the beginning of March, with the collaboration of Coralie Barbe, the restorer

Wall paintings

Within the framework of the project, apart from the study of Mediaeval illumination from the Marcadé collection, we have carried out analyses in situ on wall paintings in Belvès (Dordogne) and in Biañez (Biscay, Spain) (in collaboration with the University of the Basque Country, K. Castro, S. Fernandez).

• The Belvès paintings (15^th century) are located on the top floor of a Mediaeval house, today transformed into an attic. The theme is that of the “Neuf Preux” (the Nine Worthies), a group of legendary historical heroes, bearers of the knightly values of the Nobles of the Sword in the late Middle Ages. The hyperspectral system has enabled the acquisition of RGB images and infra-red false colours, revealing the different types of pigments used as well as the areas that have been restored and re-painted.
• In the chevet of the church of San Andrés de Biañez. a 16^th-century painting is preserved on the front wall, discovered behind a wooden 18^th-century altarpiece. These paintings were the subject of a restoration campaign in 1993. The objective of the study is to scan part of lower areas of the paintings, representing the Last Supper and a female Saint holding a book, with the aid of a hyperspectral camera, to identify the pigments and compare the results obtained with those obtained by the University of Bilbao team with a portable Raman (785nm laser) and a portable fluorescence X-ray instrument. Due to the height of the paintings relative to the Mediaeval floor level, a platform was installed and the camera was used in its highest position (around 2 metres). The results corresponded to those obtained with a Raman and EDXRF by the team from the University of the Basque Country, partner of the HYPERSPEC project. Their analyses have notably shown the presence of lead-tin yellow for the figures’ haloes and cinnabar and red lead for certain reds.

Détail des peintures de l’église San Andres de Biañez. Image RGB et IR fausses couleurs.

Decorated caves

In the study of the alterations and the taphonomy of the decorated caves, hyperspectral imagery enables the characterisation of a specific type of alteration, due to natural environmental conditions or the effect of heating on the walls, The HYPERSPEC project also anticipatesbeing involved within the framework of several LaScArBx research projects concerned with this subject: the IThEM[1], ESPACE GROTTE[2], ArTaPoC[3] projects.

• As far as the particular constraints linked to the environment of the caves are concerned, a trial has been carried out (with the collaboration of PACEA, C Ferrier), at Lugasson on an urgonian limestone wall subjected to a fire (ITHEM project) and the reflectance spectra obtained were compared to references of limestone of the same type heated in a laboratory to different temperatures between 200 and 800°C. The limestone appears pink from 300°C up to 400°C, then grey up to 700°C. At 800°C, the limestone becomes white, because it is transformed into lime. The results are similar to those obtained by spectroradiometry for characterising the reflectance spectra of the walls according to the temperatures attained in a fire in this confined space. 

Références de calcaires chauffés (image RGB) et spectres de réflectance correspondants.

• Une mission est programmée en avril (avec la collaboration de PACEA (C. Ferrier, S. Konik), TRACES (C. Bourdier), l’Ecole des mines d’Alès (D. Lafon),…) à l’abri Cap Blanc pour traiter des traces colorées sur des sculptures préhistoriques.

>>>En savoir plus sur le projet Hyperspec (cf présentation du projet lors de l'AG du LaScArBx, novembre 2013) et la caméra hyperspectrale

[1] IThEM : Les traces de feu de la grotte Chauvet : Des Impacts Thermiques sur les parois à la caractérisation des feux, Expérimentation et Modélisation (AAP1-LaScArBx, P. Guibert)

[2] ESPACE Grotte : Etats de Surface et Phénomènes d’Altération des Calcaires dus à l’Environnement en Grotte (AAP2-LaScArBx, C. Ferrier)

[3] ArTaPoC : ARchéologie et TAphonomie de la Paroi Ornée à Cussac (AAP2-LaScArBx, J. Jaubert)