CULTURAL HERITAGE Spectral imaging
Giorgia Marucci, from Pro-Lite Technology, visits the National Gallery in London to find out how hyperspectral imaging was used to reveal the history behind Leonardo da Vinci’s The Virgin of the Rocks
A
n image is the representation of an object or person. Te ability of Leonardo da Vinci to represent
reality was extraordinary. It resulted from his understanding of science, the study of light, and curiosity about the world. On the 500th anniversary of his death, the National Gallery in London celebrated the artist with its exhibition Leonardo: Experience a Masterpiece at the end of last year, where visitors could discover and rediscover one of the most famous of da Vinci’s pieces hung in the gallery, Te Virgin of the Rocks. Te show brought together findings from research, including an investigation using imaging techniques performed by the scientific and conservation departments of the National Gallery.
A problematic commission In 1483 the Confraternity of the Immaculate Conception of the Virgin of San Francesco Grande church in Milan commissioned da Vinci, and the de Predis brothers, to paint the central panel of the altarpiece of the Chapel of the Immaculate Conception. Da Vinci completed a first version of the painting in about 1485 but, thinking that the fee offered by the Confraternity was lower than the painting’s true worth, ended up selling it to another buyer. Today, this earlier version of Te Virgin of the Rocks hangs in the Louvre in Paris. In the early 1490s, da Vinci started work on another version of the same subject, thought to have been delivered by 1499. However, with his customer still unhappy with the finish, da Vinci kept working on this second painting – the one now owned by the National Gallery – between 1506 and 1508. It was only then that he received final payment, 25 years after starting the first version. At this point, da Vinci had a different perception of the world, a fuller understanding of the behaviour of
light and his artistic ability, which makes the two works of art distinct. (For further details on the historical context, see the exhibition catalogue, L. Kharibian, Leonardo: Experience a Masterpiece, London, 2019.)
Unexpected discoveries To better understand da Vinci’s modus operandi and his creative process, the National Gallery’s scientific and conservation departments made a number of studies of Te Virgin of the Rocks, using non-invasive and non-destructive imaging techniques. X-ray radiography – the same as that used in medicine – was first performed in 1948, and then later in 2005 with more advanced technology. In both cases, it showed little information concerning the painting itself, the image being dominated by the cradled panel.
‘Unexpectedly, the distribution map of the element zinc highlighted an angel and child’
New research was conducted in 2005 and 2006 using infrared reflectography, an imaging method that relies on the reflective and absorptive proprieties of materials when illuminated by an infrared light source. Paint layers are usually transparent in this wavelength region, while carbon-based materials, such as the charcoal used to make a preparatory sketch hidden underneath the paint, absorb in the infrared. Traces of the underdrawings for Te Virgin of the Rocks came to light in this study thanks to the infrared reflectogram. Surprisingly, however, these images also showed lines referring to a different composition. Tis unexpected drawing shows a different figure of the Virgin
4 IMAGING AND MACHINE VISION EUROPE FEBRUARY/MARCH 2020
sheds light on Leonardo masterpiece
Figure 1: Macro XRF map showing a distribution of zinc that reveals an angel and baby of the first composition
Mary, with her head in a 3/4 pose, possibly turned toward the child who is unfortunately not visible in the infrared image, and her left hand brought to the chest, her right arm outstretched to the side. Fortunately, technology evolves, and last
year a new study was performed to find out more about these underdrawings and a design that looks so different from the final composition known today. Te analysis included macro x-ray fluorescence (XRF) scanning and hyperspectral imaging. Te macro XRF outputs maps of the distribution of chemical elements at, and below, the surface of the painting. Unexpectedly, the distribution map of the element zinc highlighted figures of an angel and child on the right-hand side of the painting (figure one), which the infrared reflectogram in 2005 had not shown.
Hyperspectral imaging reveals more Hyperspectral imaging was used to try to obtain a clearer and more complete image of the underlying abandoned composition. Te hyperspectral acquisitions were performed with an imaging system built at the museum that operates in the shortwave infrared range (1,000 to 2,500nm). Tis is a push-broom
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The National Gallery, London
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