Protrusions in oil paintings are a worldwide problem

June 24, 2003

Dutch paintings conservator Petria Noble has established that tiny craters and lumps observed on the surface of hundreds of oil paintings throughout the world are caused by aggregates of metal soaps that protrude through the surface of the paint, hence the name protrusions. Noble presented her team's findings during the "Highlights of Molart 1995-2002" symposium that was held on 19 June 2003.

Noble first detected this deterioration phenomenon in paint layers containing the pigment lead white. Small craters on the surface of the paintings appeared to be caused by firm, light-coloured protruding lumps. An international survey revealed that hundreds of paintings from the fifteenth to the twentieth century exhibit these symptoms. The phenomenon occurs on both panels and canvasses. Up until now the phenomenon has only been found in paints containing lead and zinc.

It seems that the aggregates are often formed in an underlying ground or paint layer of a painting. However, aggregate formation in a surface paint layer also occurs. Sometimes the aggregates form just under the surface of the painting causing a bumpy surface texture.

Microscopic analysis of cross-sections of the aggregates shows that the protrusions are composed of metal soaps. These are formed during the drying process from organic fragments from the binding medium of the paint and metal ions from the pigments or drier. Metal soaps are normally dispersed throughout the paint layer. They aggregate under specific circumstances, such as high relative humidity, and probably in combination with a weak chemical network in the paint. A consequence is that the soaps swell and protrude through the surface of the painting. In some cases this process still seems to be active.

The research formed part of the Molart Programme from the Netherlands Organisation for Scientific Research. The programme had a budget of 3.4 million euros and started in 1995 as a Priority Programme from the Netherlands Organisation for Scientific Research. In the Molart programme, art historians, painting conservators, analytical chemists and technical physicists worked together to produce a scientific framework for the conservation of painted art at the molecular level.

In 2002 the successor of Molart was started, the 'De Mayerne programme'. The programme has a budget of about 2 million euros and a duration of about 5 years. One aspect of the De Mayerne programme is a follow-up study of the circumstances that can lead to protrusion formation in paint layers.
For further information about the research project please contact Petria Noble (Senior Paintings Conservator Mauritshuis), tel.31-70-302-3463, e-mail:

For further information about the policy surrounding MOLART and De Mayerne please contact Marloes Telle (Exact Sciences, Netherlands Organisation for Scientific Research), tel. 31-70-344-0738, e-mail: The richly illustrated end report from the MOLART programme can be obtained in PDF format from

For further information about the symposium please contact the coordinator of the MOLART project Prof. Jaap Boon (FOM Institute for Atomic and Molecular Physics), tel. 020-608-1234, e-mail:

The research was funded by the Netherlands Organisation for Scientific Research.

Netherlands Organization for Scientific Research

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