Enlarge /. The central adoration of the mystical lamb plate. The groups of figures are counter-clockwise from top left: the male martyrs, the pagan writers and Jewish prophets, the male saints and the female martyrs.
Over the past eight years, conservationists have carefully restored Ghent's famous altarpiece in St. Bavo's Cathedral in Belgium. Using several advanced imaging techniques, they were able to determine where overpainting from previous restorations hid the original work. Researchers from the University of Antwerp and the National Gallery of Art in Washington, DC have published a new article in Science Advances that shows how the combination of different techniques significantly improves their analysis and reveals previously unknown revisions of the figure of the Lamb of God inside Middle section.
The Ghent Altarpiece – also known as the Adoration of the Mystical Lamb – is a 15th century polyptych attributed to the brothers Hubert and Jan van Eyck. The altarpiece, which originally consisted of 12 panels, consists of two "wings" with four panels each, which are painted on both sides. These wings were opened on church holidays so that the parishioners could see the four central panels inside. The upper inner register shows Christ the King, the Virgin Mary and John the Baptist, flanked by the outer panels, which represent angels and the figures of Adam and Eve. The lower inner register shows John the Baptist and John the Evangelist. The adoration of the Lamb includes the middle panel, on which the Lamb of God stands on an altar in a meadow surrounded by angels and groups of martyrs, saints and prophets gather around the altar.
The first major restoration was carried out in 1550 to repair damage from an earlier cleaning. It was cleaned again in 1662 by the Flemish painter Antoon van den Heuvel. After the altarpiece was damaged during storage in Austrian mines during World War II, another restoration was carried out in the 1950s using X-ray radiography (XRR) to support this effort. In particular, the researchers mapped tiny color samples from the cross-section of the altarpiece and provided useful information about areas that had been painted over during the previous restoration, obscuring the original Eyckian work, including the lamb's head.
Artists of that time prepared an oak panel with a layer of chalk mixed with a binder such as glue. The artist then sketched the picture he wanted to paint on this layer in black before sealing the surface with a translucent primer (usually drying oil with lead white, chalk, soot, or earth pigments). Then the artist would apply a layer of different colored paints to define the signature. After all, the artist would paint the finer details and add the richest colors. About a year later, once the paint was completely dry, the artist applied a final coat of paint.
According to the authors of the new paper in Science Advances, the techniques available at that time were not sufficient due to the frequent use of lead-white paint to pinpoint the entire overpainting. It was simply not possible to achieve the required chemical contrast. In such cases, "conservators and curators are mistaken on the side of caution and leave areas with possible overpainting in place until further investigation provides definitive evidence of their origin," they wrote. The restoration of 1950 only removed the overpainting from the area around the lamb's head. This revealed the original gilded rays and ears of the original Eyck lamb, while the over-painted ears were preserved.
Most recently, the Museum of Fine Arts in Ghent began restoring the altarpiece in October 2012 so that the public could see the process behind a glass pane while restorers from the Royal Institute for Cultural Heritage worked on individual panels. The restoration of the eight outer panels was completed in 2016, and the five lower panels have been restored over the next three years.
This latest restoration also produced a series of high-resolution images of the various panels using different imaging techniques. As we reported last year, these images in turn formed the basis for an investigation into the application of AI analysis to the altarpiece images by researchers from Duke University, the National Gallery, and University College London. This team published a paper in 2019 showing that its techniques could make it easier to depict the two-sided painted panel of the altarpiece.
The fact that the wing panels are coated on both sides posed a unique challenge for conventional X-ray analysis. X-rays penetrate so deeply that it can be difficult to determine which content applies to which side of the panel, since "all images are visibly overlaid or" mixed "", the authors wrote. They developed a deep neural network algorithm to examine mixed X-ray images that included features from the front and back of the painting's double-sided panels. They successfully applied their technique to X-ray images of the Adam and Eve wing panels to break this data down into two clear images – which significantly improves the performance achieved with previous methods.
Left: color image after treatment in the 1950s. Eyck's lamb's ears were uncovered after removing the 16th-century overpainting that covered the background. Right: color image after the treatment in 2019, in which all overpainting from the 16th century was removed and the face of Eyck's lamb became visible. The dotted lines indicate the outline of the head before the overpainting was removed from the 16th century.
Lukasweb.be – Art in Flanders vzw
Left: colored composite elementary map showing the distribution of gold, mercury and lead. Right: The composite false color infrared reflection image shows signed lines that indicate facial features.
University of Antwerp / National Gallery of Art
Top left: color image before removing all overpainting from the 16th century. Top right: color image after removal of all overpaintings from the 16th century. Bottom left: The false color infrared reflection image shows signed lines. Bottom right: Map derived from processing the infrared reflection image cube and showing the original lamb with a slightly sagging back, rounded hindquarters and a smaller tail.
Lukasweb.be – Art in Flanders vzw / National Gallery of Art, Washington
The MA-XRF instrument during the experiments in the central panel.
The MA-XRF instrument during experiments on the middle plate.
Construction of near infrared reflection imaging spectroscopy.
National Gallery of Art
In the meantime, Geert Van der Snickt and colleagues at the University of Antwerp used macroscopic X-ray fluorescence images (MA-XRF), combined with other traditional methods (including XRR and infrared reflection reflection or IRR) on the four inner center plates to determine the full extent of the overpainting during the restoration in the middle of the 16th century.
The combination approach was so successful that the Antwerp researchers decided to collaborate with the National Gallery of Art in Washington, DC, and combine MA-XRF with infrared reflection image spectroscopy (RIS) for the second phase of the restoration, which involved the complete restoration of the lamb comprised of God. "Each method offers a degree of chemical contrast in cases where the other does not," the authors wrote. "In particular, every technology can reveal materials to which the counterpart is insensitive. For example, overlaid layers that strongly attenuate X-rays can prove to be transparent to infrared radiation and vice versa."
For example, MA-XRF imaging mapped the vermilion-colored mercury and revealed the nostrils of the original Eyck lamb. Meanwhile, the RIS revealed the original layer of the ungulate face that absorbs infrared light. At the end of the analysis, three different versions of the lamb were discovered: the original, painted by the Van Eyck brothers; a second version – either by the brothers or one of their contemporaries – with larger, angular hindquarters; and a third version from the 16th century restoration that changed the head of the lamb considerably.
The new analysis helped conservationists eliminate the 16th-century overpainting and revealed facial features that came very close to the researchers' predictions. The face of the lamb in particular is quite human in its expressiveness, with distinctive pursed lips, smaller V-shaped nostrils and a well-defined jaw, as well as eyes that are directed towards the viewer in a rather unsettling, direct look.
These eyes are still the subject of considerable debate among art historians. Some suggest making the lamb more human as a symbol of the embodiment of Jesus. "A simpler explanation could be that painting animals with forward-looking eyes is typical of an older style that existed in the 15th century but disappeared as artists mastered a more naturalistic representation of animals," the authors wrote. In the altarpiece of Ghent itself there are examples of animals depicted with both forward-looking and more naturalistic outward-looking eyes. "It is therefore not unexpected that the Van Eyck brothers painted the lamb with their eyes turned forward, which directly addressed the viewer," the authors concluded.
DOI: Science Advances, 2020. 10.1126 / sciadv.abb3379 (About DOIs).