The most appealing results of the research done in the Research Agenda 2008-2012 set in the spotlights.
In 2004 Louise Wijnberg and Emile Froment of the Stedelijk Museum were cleaning the large painting Untitled 1964 - '65 by the American artist Jasper Johns. However, to their surprise, paint adhered to every moist cotton swab rolled over the surface.
All colours except black and white proved sensitive to moisture. Very unexpected for an admittedly unvarnished but glossy oil painting on which the pigments seemed well bound and the painter had used normal oil paint, not paint with water-soluble components. How ought the treatment continue?
The paintings conservation workshop of the Stedelijk Museum submitted a research request to the RCE to determination the composition of the paint. Thanks to this request, research on 20th-century oil paintings accelerated markedly. A survey of conservators nationally and internationally revealed that this phenomenon is much more common with paintings from the 20th century, especially from the 1950s and 60s. Just like Untitled 1964 - '65, most of the paintings at that time were deliberately not varnished, a tradition that dates back to the time of the Impressionists. Artists wanted the colour and gloss of the oil itself to be fully expressed. However a painting is much more sensitive to environmental influences and becomes dirty more rapidly without a protective coating. Quite a problem as subsequently the paint cannot be cleaned well.
Besides the cleaning problems the project also investigated other frequently asked questions about modern oil paint in the areas of deterioration, conservation and restoration. To find answers to these questions the RCE, alongside the Stedelijk Museum, worked in close collaboration with the Courtauld Institute of Art in London, the Getty Conservation Institute in Los Angeles, the Tate Modern in London and the University of Amsterdam. The study became an international incubator for conservation students who ultimately made vital contributions.
In the Netherlands there are two leading manufacturers of artists' oil paints: Talens and Old Holland. In Germany, it is Schmincke, in Italy Maimeri and in England, Michael Harding and Winsor & Newton. Due to the addition of new pigments, modern oil paints have different characteristics than the paints with which Rembrandt or Van Gogh painted. Technical resources, patent literature, and paint tubes from various collections provided information on the composition of oil paint from the first sixty years of the 20th century. Staff who had been working at these factories since the 60's were interviewed to form an overview of paint manufacture in recent decades. Test strips from Winsor & Newton made in the late 1940s to the present were very useful.
Jasper Johns once told Wynberg that in 1965 - at the time of painting Untitled - he used the Utrecht line of paints from Winsor & Newton. Examination of tubes of this line from 1969 showed that the Untitled paints have many similarities. The Winsor & Newton test strips proved to be very sensitive to water. The surface presented needle-shaped crystals too small to be seen by the naked eye. The crystals had the same shape as those on Untitled and many other water-sensitive paintings.
The foundation for the explanation was laid during a visit of the RCE to the Getty Conservation Institute. There an X-ray diffractometer (a device that determines crystal structures) identified the crystals as magnesium sulphate, a magnesium salt of sulphuric acid that is very easily dissolved in water. This finding gave rise to the following hypothesis. The discovery was related to two other issues. Many modern oil paints contain the filler magnesium carbonate. Due to air pollution between 1950 and 1990, a high concentration of sulphur dioxide had been present in the atmosphere. As a result, magnesium carbonate in the paints reacted with sulphur dioxide in the air to form water-soluble magnesium sulphate.
The suspicions were confirmed. In collaboration with the Courtauld Institute the RCE made paints with as many of the same ingredients as possible as those on Untitled and subjected them to accelerated-ageing tests and sulphur dioxide. These tests resulted in the same magnesium sulphate crystal formation and indeed rendered the paint very water sensitive. When in contact with a moist cotton swab the magnesium sulphate instantly dissolves and the paint develops a spongy surface that easily detaches.
To find an answer to the question of how to cleaning water-sensitive paintings, Maude Daudin at the RCE investigated various cleaning methods that excluded the use of water. What worked? Dry make-up sponges gave the best result on many paintings. They took up dirt well and with a minimal effect on the paint surface due to their softness. The disadvantage is that such sponges contain many additives (antioxidants, plasticizers, vulcanisation agents), which in the long term could affect the paint. Mostly these substances can be easily flushed out, however it would be better if sponges without these additives can be used.
The research led to the Dry cleaning of paintings workshop, which the Netherlands Cultural Heritage Agency has since given in several countries.
Although emissions of sulphur dioxide have fallen strongly in recent years (by capture in power plants and cleaner domestic fuels), it remains important for collection managers and conservators to monitor levels of such gases in the environment. In addition, it would be beneficial if the paint manufacturers would seek alternatives to magnesium carbonate so that unvarnished paintings will present fewer problems with cleaning for conservators in the future.
Jasper Johns (1930) followed painting classes at a commercial art school in New York. After his military service, he returned to the city in 1952, where he worked as a window dresser amongst other jobs. In 1954 he began painting 'flags', in a style between abstract expressionism and Pop Art. He first exhibited his large paintings at the Leo Castelli Gallery in 1958. That same year his work was exhibited in Europe at the Venice Biennale. Johns often chose everyday subjects such as flags and numbers, which he rendered in thick layers of paint combined with other materials such as coloured wax and real objects. This produced a constant tension between reality and imagery in his work.
Klaas Jan van den Berg
Henk van Keulen, Ineke Joosten, Luc Megens, Suzan de Groot, Zeph Benders; Laura Mills, Hannah Tempest, Genevieve Silvester, Anna Cooper (studenten)
Aviva Burnstock (Courtauld Institute, Londen), Louise Wijnberg (Stedelijk Museum Amsterdam), Tom Learner en collega’s (Getty Conservation Institute), Bronwyn Ormsby (Tate Modern, Londen)