To the casual observer, a painting is two dimensional. But below the visible surface are often multiple layers of paint, and that’s true for The Night Watch, painted by Rembrandt in 1642. Some 377 years later, art conservators and scientists at or affiliated with Amsterdam’s Rijksmuseum started a research project to answer, among other questions, how the famous artist created the iconic masterpiece.
Fréderique Broers of the Rijksmuseum and Florian Meirer of Utrecht University and their colleagues found evidence that Rembrandt first applied a lead-containing impregnation layer to the canvas. A better understanding of the painting’s condition and what it’s made of will help the museum’s staff determine the best preservation treatments for the room-sized painting. (It’s about 3.8 meters by 4.5 meters.)
To see below multiple layers of paint, Broers and her colleagues used x-ray fluorescence (XRF), similar to the imaging methods that Broers used with some of the coauthors and other researchers to study another mid-17th-century painting (see Physics Today, August 2022, page 19). For The Night Watch, a tiny paint fragment was removed at a spot where lower layers of paint were exposed. Then the sample was taken to the German Electron Synchrotron in Hamburg to collect XRF and other microscopic-imaging measurements.
The results—a large set of 2D data slices of the sample—were combined, and an algorithm created a 3D rendering of the paint chip. Some lead was expected in The Night Watch—an XRF map of the elemental distribution showed lead was everywhere on the canvas. Although the metal was a common ingredient in pigments at the time, Broers and her colleagues found that the lead signal came from below the surface layer. In the elemental map, the signal appears as rough brush strokes, which were made in the painting’s many dark-colored regions.
The lead could also have come from drying additives put in the ground layer of quartz and clay. But the signal was only on the bottom of that layer. From the evidence, Broers and her colleagues conclude that Rembrandt likely applied a lead-containing oil to the canvas before applying the ground layer. The treatment would have protected the painting against humidity fluctuations, given that it was hung on a long exterior wall at an Amsterdam shooting range. For decades, art conservators have minimized the amount of time cleaning solvents are in contact with a painting to prevent various chemical reactions. The new lead finding reaffirms that such practices should likely continue. (F. T. H. Broers et al., Sci. Adv. 9, eadj9394, 2023.)