The 17th century Dutch master Rembrandt van Rijn is justly considered one of the greatest artists of all time. He's particularly praised for his masterful depiction of light and shadow in his oil paintings, an almost three-dimensional effect achieved with his signature "impasto" technique. The recipes he used to mix his paints were believed to be lost to history. But now a team of Dutch and French scientists has used high-energy X-rays to unlock Rembrandt's secret recipe, according to a new paper in the journal Angewandte Chemie.

Impasto (translated as "dough" or "mixture") involves applying paint to the canvas in very thick layers. It's usually done with oil paint because of the thick consistency and slow drying time, although it's possible to add acrylic gels as a thickening agent to get a similar effect with acrylics. Rembrandt used it to represent folds in clothing or jewels, among other objects, in his paintings. As David Bressan notes at Forbes, "The layer causes light to reflect in certain ways, giving the painting interesting dark and light contrasts and a three-dimensional effect."

Like most artists of his era, Rembrandt mixed his own paints, experimenting with different recipes to get different desired effects. Common materials then included lead white pigment (produced via the corrosion of metallic lead) and organic substances like linseed oil. But nobody knew the precise recipe the master used to create the impasto effect.

One of the best, nondestructive ways to analyze precious historical artifacts is with high-energy X-rays from synchrotrons, a type of particle accelerator where the accelerating particle beam travels around a fixed-loop path. (It's a direct descendent of the cyclotron, invented by physicist E.O. Lawrence in 1930.) Synchrotron radiation is a bit different from conventional X-rays; it's a thin beam of very high-intensity X-rays generated within a particle accelerator. It's created by firing electrons into a linear accelerator (linac), thereby boosting their speeds before injecting them into a storage ring, where they zoom through at near-light speed. A series of magnets bend and focus the electrons, and in the process, they give off X-rays, which can then be focused down beamlines.

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Steph Cande

Steph Cande

The shorter the wavelength used (and the higher the energy of the light), the finer the details one can image and/or analyze. Back in 2008, European scientists used synchrotron radiation to reconstruct the hidden portrait of a peasant woman painted by Vincent van Gogh. The artist (known for re-using his canvases) had painted over it when he created 1887's Patch of Grass. The synchrotron radiation excites the atoms on the canvas, which then emit X-rays of their own that can be picked up by a fluorescence detector. Each element in the painting has its own X-ray signature, so scientists can identify the distribution of each in the many layers of paint.

Alternatively, researchers can zap tiny samples (less than 0.1mm in size) of paint to see what chemicals are present, like the French and Dutch scientists did in this latest study. The team took samples from three Rembrandt masterpieces: Portrait of Marten Soolmans, Bathsheba at her Bath, and Susanna and the Elders. Next they zapped them with X-rays produced by the European Synchrotron (ESRF), discovering the presence of a mineral called plumbonacrite in the impasto layer. It is not a common element in paints. Until now, the earliest painting known to contain plumbonacrite was Van Gogh's Haystack in Rainy Day (1890).

Apparently, the use of this mineral was intentional. "The presence of plumbonacrite is indicative of an alkaline medium," said Marine Cotte, a scientist at ESRF specializing in art conservation. "Based on historical texts, we believe that Rembrandt added lead oxide (litharge) to the oil in this purpose, turning the mixture into a paste-like paint.”

This kind of knowledge will help conservationists continue to preserve the Dutch master's works for future generations, but more research is needed. “We are working with the hypothesis that Rembrandt might have used other recipes, and that is the reason why we will be studying samples from other paintings by Rembrandt and other 17th [century] Dutch Masters, including Vermeer, Hals, and painters belonging to Rembrandt’s circle,” said co-author Annelies van Loon, a scientist at the Rijksmuseum in Amsterdam.

DOI: Angewandte Chemie, 2019. 10.1002/anie.201813105 (About DOIs).