Lead 'soaps' behind iconic artwork damage uncovered

Lead stannate, Pb₂SnO₄, has been identified as the culprit responsible for disfiguring masterpieces by Rembrandt, John Singer Sargent and Johannes Vermeer. Over time, the reaction between lead stannate and palmitic acid forms lead carboxylates, or ‘lead soaps’, which appear as tiny bumps that distort the surface of historic paintings. ‘Almost every painting that’s old enough will have some problem like this if it has a lead based pigment in it,’ says Cecil Dybowski of the University of Delaware, US, who led the investigation.

Lead soaps are a big problem for conservationists. To create a masterpiece, medieval artists used to mix their own paint, combining a pigment with an oil-based binder. A long period of time may pass without soap formation, but once the reaction begins, observable damage can be seen in a relatively short, 10–15 year, timescale. As they grow, lead soap aggregates become less dense and expand, pushing up the paint layers above. The aggregates, which can reach up to 200µm in diameter, cause protrusions on the painting’s surface and appear as small transparent spots.

In a collaboration with researchers from the Metropolitan Museum of Art in New York, Dybowski and his team used a combination of nuclear magnetic resonance (NMR) spectroscopy and computational analysis to investigate the reactivity of a modern sample of lead–tin yellow type I (LTY-I), a pigment used in European paintings between the 15th and 18th century. Their aim is to understand the chemistry of lead soap formation, so that treatments may be developed to prevent further damage.

Currently, a large paint sample is needed for analysis, making the technique too destructive for use on a historic painting. But as NMR technology advances, it may one day be possible to apply the technique to heritage objects.

Results show that it is lead stannate, as opposed to other lead oxides, which acts as the lead ion source. It is too soon to draw conclusions about the chemistry involved, for example whether lead stannate precipitates directly, or if it forms an intermediate; however, early indications suggest that water plays an important role in the process.

‘Exploring pigment-binder interactions within paints has remained one of the more complex and elusive tasks within heritage science,’ says Bronwyn Ormsby, senior conservation scientist at the Tate, UK. ‘This paper represents a welcome development towards understanding the complex pathways behind the changes we see in works of art over time.’

Credit for reproduction of The Birth of Saint John the Baptist: Purchase, Gwynne Andrews, Harris Brisbane Dick, Dodge, Fletcher, and Rogers Funds, funds from various donors, Ella Morris de Peyster Gift, Mrs. Donald Oenslager Gift, and Gifts in memory of Robert Lehman, 1970s

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