Causes of water sensitivity in the oil paints of 20th century paintings.
Modern unvarnished paintings can be susceptible to water, which is a problem when removing surface dirt. This project investigated the causes.
Detail of Jasper Johns 'Untitled 1964-'65', collection Stedelijk Museum Amsterdam.
In the 20th century, paintings were often unvarnished to allow the surface of the painted areas and their individual optical qualities to be expressed as much as possible.
In addition to the natural changes that the surfaces of these paintings undergo, they are also susceptible to dirt accumulation, which is potentially problematic for their conservation. A major problem is the occurrence of water-sensitive paint surfaces.
The project began with the question that Louise Wijnberg and Emily Froment of the Stedelijk Museum posed to the RCE about a work of Jasper Johns, Untitled 1964 - '65 from 1965. At first glance there seemed to be nothing wrong with the paint: good cohesion of oil paint and beautifully saturated colours. However in this work all colours, except for black and white, proved to be sensitive to aqueous solutions. Thus the paint would be removed along with the dirt while cleaning with a cotton swab and saliva or water.
Further investigation showed that this problem is well known to conservators of modern paintings; many more examples were found in interviews with conservators both in the Netherlands and abroad.
Various paintings were investigated for the causes of this particular solvent sensitivity of the paint. Not only works by Jasper Johns were investigated, but also works by Willem de Kooning and Karel Appel in Dutch collections and works by other artists, including Paula Rego and Francis Bacon. This led to a more fundamental research into the relationship between the fabrication of oil paint, ageing and changes in the surface.
Ultimately, the water-sensitivity of Untitled 1964 - '65 and various other works were found to be caused by the formation of needle-shaped crystals of magnesium sulphate, which appeared as a white bloom on the surface. The discovery was related to two other issues. Firstly, many modern oil paints contain the filler magnesium carbonate. Secondly, due to air pollution between 1950 and 1990, a high concentration of sulphur dioxide arrived in the atmosphere. Magnesium carbonate paints react with sulphur dioxide to form magnesium sulphate.
The specific and common water-sensitivity of paints made with ultramarine could be partially explained by the fact that ultramarine is a hydrophilic pigment and has the tendency to degrade the binder. For other works, the water sensitivity is not yet fully explained: the research continues. The current theory is that the presence of the surface layer ('paint-medium skin'), also known as a patina, plays a role. This often fluorescent, greyish surface layer is found on many unvarnished paintings and is frequently susceptible to removal while cleaning.