Preserving master art works

Materials World magazine
,
27 Feb 2019

Old paintings can develop a white crust on the surface which if left can cause damage, but a conservation team may have a solution. Idha Valeur finds out more.

On the surface of Rembrandt’s Homer, a white lead-rich residue has formed. The combination of the painting’s history, the passage of time and environmental factors has caused the crust. Chemical reactions in the materials put old paintings at risk, if they are not kept in stable conditions, like those found in museums, but unfortunately, this is not always the case. A conservation team has now discovered what the surface deposits are, opening up the possibility for repair
work and prevention.

By examining a small paint example using X-ray diffraction computed tomography (XRD-CT), conservationists from the Mauritshuis, the Rijksmuseum and University of Amsterdam in the Netherlands, with Finden Ltd, University College London (UCL), and Diamond Light Source in the UK, showed how paint containing lead reacts with pollutants in the air, such as sulphur dioxide (SO2).

Finden Ltd Research Scientist Dr Stephen Price explained to Materials World, ‘XRD-CT works in the same manner as any other 3D imaging techniques, such as medical CT scan, in that you image the sample from as many projections as possible, and then use computer algorithms to reconstruct the information into a 3D volume. The key difference here is that instead of taking a 2D X-ray “photo” at each projection, which would only give absorption contrast, we scan the sample through a micro-focused X-ray beam at the synchrotron, collecting a series XRD patterns.

‘Each XRD pattern gives us the average chemical composition – in this case eight-10 crystalline phases, each with their own XRD pattern, or “fingerprint” – through the sample at the point it was taken, and we can then reconstruct this to show where each different phase is located within the paint layer.’ Price and UCL Professor Andy Beale said the technique they developed is a new approach that has already been adopted in materials characterisation across many fields such as cultural heritage, catalysis and energy storage. They said that XRD-CT could be applied to a wide range of materials. Applications considered include batteries, fuel cells, industrial catalysts for chemical synthesis and automotive applications and geological samples.

This technology has revealed that the crust is a mixture of lead sulphates. Price said, ‘From the S:Pb (sulphur:lead) ratios throughout the paint layer, we can conclude that S is from an external source in the form of SO2 (sulphur dioxide) and that the nature of Pb-SO4 (sulphate) product is dependent on the degree of diffusion/absorption of SO2 into the paint layers. The source of sulphur is historical, therefore, no new sulphurous deposits are expected to be formed if the crust is removed.

‘In this case study, however, a complete removal of those products would not be possible without damaging the original paint layer, since the surface deposits and the paint are so closely intertwined.’

Price explained that in 2005-2006 when the last treatment of the painting took place, the surface residue was reduced and is now barely visible to the naked eye. The study shows the complex Pb chemistry, which takes place in mature oil paintings over duration of time.

‘The findings also give us confidence that no further sulphate degradation will occur in this or similar paintings,’ he said.

Rijksmuseum Paintings Researcher Scientist, Dr Annelies van Loon, highlighted that the way the painting is now stored is also serving as preservation. This is due to the conditions in the museum, factors such as light, temperature and humidity which all play a role in preventing sulphates forming. ‘There is no need, at the moment, for conservation treatment of the painting or removal of the crust. The painting was last treated in 2005-2006, and conditions are stable now,’ she said.

The paper, Unravelling the spatial dependency of the complex solid-state chemistry of Pb in a paint micro-sample from Rembrandt’s Homer using XRD-CT, was published ChemComm.