Researchers in the UK and the Netherlands have succeeded in identifying for the first time all of the solid phases present within whiteish lead-rich deposits on the surface of an Old Master painting using a technique called X-ray diffraction computed tomography (XRD-CT). The work, carried out at the UK’s National Synchrotron, the Diamond Light Source, proves how important it is to house these precious works of art in a protected environment.
Old Master paintings often have a layer of whiteish crust on their surface. Although barely visible this indicates that potentially damaging chemical reactions have occurred. It is difficult to characterize these crusts, however, since only a very small amount of sample is usually available to test.
Synchrotrons are incredibly useful here, explains study lead author Stephen Price from the Diamond Light Source and Finden Ltd. “Our sample was less than 100 μm in size, so a lab-based source (with typically mm-sized beams) wouldn’t have the resolution to spatially resolve the lead compounds in it.
“The XRD-CT we employed works in the same way as any other 3D-imaging technique (such as medical CT scans, for example) in that you image the sample from as many projections as possible,” says Price. “Computer algorithms then reconstruct the information obtained into a 3D volume.”
Micro-focused X-ray beam
The key difference in the technique employed in this work is that the researchers scan the sample through a micro-focused X-ray beam at the synchrotron instead of taking a 2D X-ray “photo” of it at each projection (which would only yield absorption contrast), collecting multiple XRD patterns.
XRD-CT in fact uses diffraction to take a “fingerprint” of the different chemicals present and tomography to take a 3D picture of how different chemical species are distributed throughout the painting stratigraphy, adds team member Claire Murray, also of Diamond.
The technique revealed that the lead-containing paint in Rembrandt’s Homer (painted in 1663 during a period known as the Dutch Golden Age) had reacted with atmospheric pollutants such as sulphur dioxide (SO2) to form insoluble lead-rich SO4 salts. This SO2 was present in the harsh environments the painting was exposed to in its past.
Complex mixture of lead sulphate minerals
The researchers identified the surface crust as being a complex mixture of lead sulphate minerals – palmierite (K2Pb(SO4)2 and anglesite (PbSO4). The reaction products depend on how much SO2 had diffused/been absorbed into the paint layers, say Price and colleagues.
Deeper in the painting’s layers, the researchers found that lanarkite (Pb2(SO4)O) and leadhillite (Pb4SO4(CO3)2(OH)2), were the main components. This shows that SO2 permeated to these lower layers to a lesser extent than to the upper ones. Underneath these lower layers, they identified lead soaps of palmitate and azelate, which suggests that multiple chemical reactions had occurred.
No further degradation from SO2
“The change in the S:Pb ratio away from the surface we observed allows us to conclude that the S came from an external source in the past,” Price tells Physics World. “Since this SO2 is no longer present in the environment in which the painting finds itself today (the Mauritshuis collection at the Rijksmuseum in Amsterdam), further degradation from this pollutant should not continue. This result shows how important it is to keep valuable paintings in a stable climate.”
Diamond Light Source – when a tool becomes a gem
The team, which includes conservation scientists from University College London (UCL), the Mauritshuis in the Hague, the Rijksmuseum in Amsterdam and the University of Amsterdam, says that it has now begun to study other similar painting using its technique. “We already knew that the same phenomenon had affected many Old Master paintings and our work will help us better understand the complex Pb chemistry that takes place in these oil paintings over time.”
Full details of the research are reported in Chemical Communications 10.1039/C8CC09705D. The paper is particularly timely given that 2019 is the 350th anniversary of Rembrandt’s death.
