Rembrandt van Rijn revolutionised painting with what is known as the impasto technique which uses thick paint to make features protrude from the surface.
The relief of impasto increases the perceptibility of the paint by increasing its light-reflecting textural properties.
Researchers from Delft University of Technology and the Rijksmuseum in the Netherlands unveiled the precise recipe that helped Rembrandt achieve this effect.
Scientists have known that Rembrandt, epitome of the Dutch Golden Age, achieved the impasto effect by using materials traditionally available on the 17th century Dutch colour market, namely lead white pigment, and organic mediums (mainly linseed oil).
However, the precise recipe was unknown.
Plumbonacrite, Pb5(CO3)3O(OH)2 is the mysterious, missing ingredient of the impasto effect, researchers discovered.
It is extremely rare in historic paint layers. It has been detected in some samples from 20th century paintings and in a degraded red lead pigment in a Van Gogh painting.
"We didn't expect to find this phase at all, as it is so unusual in Old Masters paintings," said Victor Gonzalez, scientist at the Rijksmuseum and Delft University of Technology.
"What's more, our research shows that its presence is not accidental or due to contamination, but that it is the result of an intended synthesis", he said.
The European Synchrotron (ESRF) played an essential role in these findings, researchers said.
The team sampled tiny fragments from the Portrait of Marten Soolmans, Bathsheba and Susanna, three of Rembrandt's masterpieces.
Using the ESRF, they quantified the crystalline phases in Rembrandt's impasto and in the adjacent paint layers, modelled the pigment crystallites morphology and size and obtained crystalline phase distribution maps at the microscale.
The samples were less than 0.1mm in size, requiring the small and intense beam delivered by the synchrotron.
The scientists analysed them on two ESRF beamlines, ID22 and ID13, where they combined High-angular Resolution X-Ray Diffraction (HR-XRD) and micro-X-Ray Diffraction (μ-XRD).
"In the past, we have already successfully used the combination of these two techniques to study lead-white based paints. We knew that the techniques can provide us with high quality diffraction patterns and therefore with subtle information about paint composition," said Marine Cotte, scientist at the ESRF.
The analysis of the data showed that Rembrandt modified his painting materials intentionally.
"The presence of plumbonacrite is indicative of an alkaline medium. Based on historical texts, we believe that Rembrandt added lead oxide to the oil in this purpose, turning the mixture into a paste-like paint," said Cotte.
The breakthrough yields the path for the long-term preservation and conservation of Rembrandt's masterpieces.
However, the number of samples studied is not extensive enough to assess if lead white impastos systematically contain plumbonacrite.
"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 Dutch Masters, including Vermeer, Hals, and painters belonging to Rembrandt's circle," said Annelies van Loon, scientist at the Rijksmuseum.