During the Middle Ages, Christianity attached great value to relics, i.e. physical remnants from the life of Christ or remains of a saint, such as a piece of bone, textile or paper. The Scripture refers to the healing power of objects that were touched by Christ or his Apostles. Similar to modern homeopathy, it was thought that such remains, even when present in small doses, could have a beneficial effect.

In some cases relics have been hidden inside an art object without any external physical evidence of the relic’s presence. This seems to have been the practice notably with sculpture, and in some rare instances also with altarpieces. A good example is the Norfolk triptych from the collection of the Museum Boymans-van Beuningen in Rotterdam, which is considered to be one of the oldest paintings in The Netherlands.

The main scene in the central panel represents the Deposition of Christ (Figure 142). The X-ray transmission radiography of this area shows that an inset was made into the panel, into which a thin plank of softwood was inlaid with its grain direction perpendicular to the grain direction of the altarpiece. The primer and paint layer have been painted on top and therefore cover the wooden inlay. This raises the question of whether some sort of relic could be hidden behind this inset (Figure 142). The object could be a piece of textile related to the Shroud of Christ, depicted in the painting itself, or a small piece of paper with an inscription, or maybe even a bone or piece of wood from the cross of Christ.

Regular X-ray transmission radiography has failed to properly image what is hidden behind the panel inlay. Conventional radiography provides no three-dimensional insight into the construction of the panel. However, a more significant limitation is the low sensitivity of X-ray absorption to low-Z materials. The paint layer on the front contains heavy metals like Pb and Hg in pigments such as lead white and cinnabar. The wooden panel has a thickness of about 4 cm. Any organic materials such as paper or textile sandwiched in between the paint and the wood are therefore virtually transparent at the relatively high energies needed to penetrate the entire object (> 25 keV).

The present study describes the first-time application of two synchrotron-based imaging techniques in the study of paintings: X-ray computed tomography (ID17) and X-ray computed laminography (ID19). Both setups have phase-contrast imaging capability, being analyser-based and propagation-based, respectively. The tomography setup had a 0.35 mm resolution with a 15 cm field of view, while laminography had a 7.5 micrometre resolution with a 15 mm field of view, which allowed a more detailed inspection. The aim of this experiment was to see whether three-dimensional phase-contrast imaging would be adequate for the visualisation of an organic, fibrous object imbedded inside a painting.

We prepared a test panel into which a small inset was cut, mimicking the suspected construction of the Norfolk panel. Behind the inset we placed a small folded piece of paper with a short inscription in organic ink. The panel was given an authentic chalk-based priming layer and several layers of various paints containing heavy metals like Hg and Pb.

Fig. 142: Christ as the man of Sorrows, also known as the ‘Norfolk tryptych’ ca. 1415–1420, Museum Boymans-van Beuningen, Rotterdam, The Netherlands. The white frame corresponds to the X-ray radiography of the central panel showing the wooden inlay and its perpendicular grain direction.

A computer tomography scan with 0.35 mm resolution provided the three-dimensional structure of the entire panel, visualising features such as wood grain, and accumulation of glue and voids inside the panel. Computed laminography with spatial resolution on the micrometre scale allowed us to image local volumes, i.e. three-dimensional regions of interest, within the panel. In combination with propagation-based phase-contrast imaging, we succeeded in three-dimensionally imaging the piece of paper sandwiched in the wooden inset, the heavy metal paint layers and the bulk panel (Figure 143). The phase-contrast capability proved very effective in enhancing image contrast (at all material interfaces and especially between paper and air) which otherwise would have been dominated by the attenuation of the highly absorbing paint layer.

Fig. 143: Three-dimensional rendering of a mockup panel under different viewing angles, obtained by synchrotron-radiation computed laminography. The paint layer at the top, the perpendicular grain directions in the inlay (below) and the panel (bottom), and the folded paper (centre) are clearly visible.

In addition to the propagation-based setup we also imaged our panel with the analyser-based phase-contrast imaging. This phase-contrast method is particularly well suited to the detection of steady variations of the projected refractive index distribution. It permitted the hidden paper to be imaged, and even the contours of the ink writing on the paper were visible. The contrast in these images strongly depends on the refraction angle, the orientation of the analyser crystal with respect to the wood grain and, to a lesser extent, the density of the grain being imaged. The future availability of a laminography setup using an analyser-based phase contrast could significantly improve the detectability of such written texts. Plans are underway to examine the actual artwork, the Norfolk triptych, with techniques described above.


Principal publication and authors

K. Krug (a), L. Porra (b, c), P. Coan (b), A. Wallert (d), J. Dik (a), A. Coerdt (d), A. Bravin (b), P. Reischig (a, e), M. Elyyan (e), L. Helfen (b, e) and T. Baumbach (e), J. Synchrotron Rad. 15, 55 (2008)
(a) Department of Materials Science, Delft University of Technology (the Netherlands)
(b) ESRF
(c) Department of Physical Sciences, University of Helsinki (Finland)
(d) Rijksmuseum, Amsterdam (The Netherlands)
(e) ANKA/ISS, Forschungszentrum Karlsruhe (Germany)