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research profiles The art of analytical chemistry ally associated with Naples yellow, a lead–antimony oxide. However, the pigment in the Grünewald painting was a different color, and historical examples E. Lambert
Artists throughout history have been ingenious in their use of color, but it was the metallic glint of one pigment in particular that caught the eyes of chemist Marine Cotte and her colleagues at the Centre for Research and Restoration of French Museums (C2RMF). The painting was the Isenheim Altarpiece, the magnum opus created by German Renaissance painter Matthias Grünewald during ~1512–1516. The work is a large carved shrine with two sets of folding panels depicting scenes from the life and death of Christ. In the Resurrection scene, an unobservant guard in chain mail sleeps at the feet of an ascending Christ, and the realistic silvery appearance of that armor piqued the researchers’ interest. “If you see the painting, you really have the impression of a metallic aspect,” says Cotte. The team wanted to know how Grünewald achieved such an authentic portrayal of metal with the relatively limited materials of his day, and in the September 15 issue of Analytical Chemistry (pp 6988–6994) they joined with researchers at the European Synchrotron Radiation Facility to report the results of a synchrotron-based X-ray spectromicroscopic analysis of the metallic pigment. Curators and conservators wish to preserve all works of art as much as possible, so the first steps in any scientific painting analysis involve nondestructive techniques such as taking UV or IR pictures or performing X-ray fluorescence spectroscopy with a portable instrument. Scientists are usually looking for clues to the composition of the pigments or of the other strata that make up an artwork, including the primers, glazes, varnishes, and any sketches that the artist made before painting. The team applied all of these common techniques to the Grünewald painting, and the preliminary analysis of the metallic pigment detected the presence of antimony, lead, and sulfur. Antimony is not commonly found in paintings, but when it appears, it’s usu-
The Resurrection, from Grünewald’s Isenheim Altarpiece. Pigment samples were taken from the chain mail of the sleeping guard in the lower left corner. (Copyright 2007 C2RMF.)
of the use of Naples yellow do not appear until more than a century after the Isenheim Altarpiece’s creation. To get a more detailed picture of the metallic pigment’s composition, the researchers had to take a sample from the painting for analysis at the synchrotron. “You must take a very, very small fragment without damaging the rest of the painting,” says Cotte. The task is difficult, but because the synchrotron X-ray beam is very small, it can focus more precisely on a minuscule grain of pigment (typically
