Pictures hardly subject to leaching during cleaning

May 29, 2001

If picture restorers are careful when using solvents, very few organic molecules are likely to be leached away from the paint layer. Serious leaching does however occur when paint samples are immersed in solvents. These are some of the results of a study carried out at the National Gallery of Art in Washington DC as part of NWO's Molart project.

The researchers wanted to determine the effect of various solvents on oil paintings, includiging examples by Dutch masters. They focused primarily on the phenomenon of 'leaching', in other words the loss of organic molecules from the paint layer.

The chemists used realistic tests to show that leaching only takes place to a limited extent when pictures are cleaned with solvents. The solvent is in contact with the paint for such a short length of time that little of the organic content of the oil paint is removed from the paint layer.

Molart researcher Ken Sutherland counsels caution, however. Repeatedly cleaning a painting can potentially have a cumulative effect, so that detrimental effects may eventually result. Previous studies have already shown that cleaning can cause a paint layer to swell so that it can become soft and vulnerable.

In carrying out their research, the chemists used samples taken from 17th-century Dutch masters such as Jacob van Campen and Cesar van Everdingen from the Orange Hall at the Dutch Queen's residence, Huis ten Bosch. They also tested a famous panel forming part of Westminster Abbey's 'Westminster Retable' by an unknown 13th-century painter and a large number of samples from the laboratory of the National Gallery of Art in Washington DC. The molecules which had leached out of the paint were analysed by means of gas chromatography. They turned out to include molecules of fatty acids, monoglycerides, diglycerides and triglycerides and sometimes relatively large oligomers. The molecules are formed by complex reactions that take place in the paint as it dries.

Paint samples which had actually been submerged in a solvent turned out to contain considerably fewer volatile molecules than before they were treated. Examination under an electron microscope showed that the structure of the paint had been affected. Besides immersion tests, the research team also carried out experiments to simulate actual cleaning. These showed that cleaning led to few or no volatile molecules being leached out.

Breakdown and yellowing of the varnish, years of dust and dirt and the effects of earlier restorations mean that paintings have to be periodically cleaned. A wide range of methods and materials are used for this, but they generally involve cleaning agents containing organic solvents.
Further information:
Ken Sutherland (National Gallery of Art, Washington DC)
T +1-202-842-6949
F +1-202-842-6886
E-mail: or

Netherlands Organization for Scientific Research

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