Chemical restoration: X-rays get beneath the paintwork

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  • Published: Apr 15, 2017
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Chemical restoration: X-rays get beneath the paintwork

Brotherly artwork

The Ghent Altarpiece was a family affair painted around 1432 by Flemish/Netherlandish artist Jan van Eyck and according to art historians probably his brother Hubert. Now, chemical imaging based on element-specific X-ray analysis of a large area of the work has revealed the original paint layers and assisted in cleanup and conservation. (Credit: Angewandte/Wiley/Van der Snickt  et al)

The Ghent Altarpiece was a family affair painted around 1432 by Flemish/Early Netherlandish artist Jan van Eyck and according to art historians probably his brother Hubert. Now, chemical imaging based on element-specific X-ray analysis of a large area of the work has revealed the original paint layers. The results point to new conservation imperatives.

Geert Van der Snickt, Stijn Legrand, Piet Van Espen, and Koen Janssens of the University of Antwerp, in Belgium and colleagues Helene Dubois, Jana Sanyova, Alexia Coudray, Cecile Glaude, Marie Postec, of the Royal Institute for Cultural Heritage (KIK-IRPA) in Brussels, and the University of Ghent emphasise once again that Van Eyck's Ghent Altarpiece is perhaps the pinnacle of mediaeval painting. Unfortunately, the years have not been kind to the Altarpiece and it is currently undergoing its most extensive conservation treatment for more than a century. The 4-metre-wide by more than 3-metre high winged altar stands in the Cathedral of St. Bavo in Ghent, where it is viewed by about 200,000 visitors every year. The artwork has had an eventful history, to say the least.

Imagine imaging

While The Ghent Altarpiece has had repeated attempts at restoration and conservation in the past, a new, stepwise restoration was initiated in 2012. The decision was made to remove all overpaint based on sound scientific arguments: Now, Belgian researchers writing in the journal Angewandte Chemie explain how they have created chemical maps to help conservators visualize the original paint layers under the overpainted surface. They analysed the scenes on the reverse of the eight wing panels, which are visible when the altar is closed using element-specific X-ray analysis. The data then informs the conservation strategy allowing it to be adjusted and optimized according to the particular elements present in different regions and to facilitate monitoring of the process of removal of the overpaint during the cleaning phase of the restoration.

Until now, conservators and chemists were only able to examine such delicate and invaluable works of art point by point, rather than over large areas, which is not particularly helpful when it is those large area that must be cleaned and preserved. Van der Snickt and his colleagues had the assistance of a mobile scanning system to allow them to image the entire surface by X-ray fluorescence spectroscopy.  Specifically, the team carried out non-invasive macroscopic X-ray fluorescence (MA-XRF), supplemented with secondary electron microscopy/energy dispersive X-ray analysis (SEM-EDX) and synchrotron radiation-based micro-XRF (SR m-XRF) imaging on a number of paint samples.

Gigabyte spectra

"More than 16 million spectra were collected and swiftly processed with our in-house software to yield over 1 gigabyte of spectral data for each panel," explains Van der Snickt. "By using computer calculations, we were able to translate these into chemical maps that depict the distribution of elements. X-rays penetrate the layers of paint without damaging them. In this way, we were able to visualize the original Van Eyck paint layers hidden under the [skilfully applied] overpaint." In order to gain more detailed insight into the layers of paint the team also analysed the structure of tiny cross-sectional samples of paint.

"In an analysis of the portrait of kneeling donor Joos Vyd, the iron, lead, and mercury maps revealed substantial damage to the original paint in an area of the vermillion red robe that appeared undamaged," Van der Snickt explains. "It also showed how the gaps were filled with a red iron-containing paste before being covered with a thin layer of red mercury sulfide paint."

Through their work, the team has revealed the overall good condition of the original scenes, the chemical maps thus supported the decision to remove all overpaints that were previously thought to be Van Eyck’s work and so bring the artwork closer to its original condition albeit almost seven centuries after it was painted.

Related Links

Angew Chem 2017, online: "Large-Area Elemental Imaging Reveals Van Eycks Original Paint Layers on the Ghent Altarpiece (1432), Rescoping Its Conservation Treatment "

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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