Modern Art Meets Modern Analysis

My background is in organic and analytical chemistry but I have been involved in art research since 1995. In the last ten years, my research has focused on the chemistry and optical properties of paint surfaces and the impact of conservation measures on 20th century oil paintings. These paintings are distinctly different from the paintings of previous centuries and present a range of challenging conservation problems – in particular, the presence of new materials such as synthetic organic pigments and metal soaps. Plus, the paintings are often unvarnished, making the surface vulnerable to degradation as a result of the interaction with light, noxious gases and particulate dirt. Challenges for conservators of the artworks include the formation of vulnerable surface “skins” of medium and exudates on paint surfaces, efflorescence, unpredictable water and solvent sensitivity, and even paint dripping, which can occur for several years after paintings are completed.

The big picture

To understand these phenomena, my group collaborates with colleagues from museum laboratories and collections such as Tate, the Courtauld Institute of Art, The Getty Conservation Institute, Stedelijk Museum Amsterdam and the Universities of Pisa and Amsterdam. One of our most fruitful areas of study has been exploring the archives of art materials supplier Royal Talens – a rich source of paint compositions over the years, information on historical materials suppliers, production philosophy and development. We also carry out studies on the degradation of oil paints and conservation research into alternative surface cleaning methodology. To analyze oil paints, a range of analytical techniques are used, including light microscopy, XRF, SEM-EDX, Raman, Fourier-transform infrared spectroscopy (FTIR), direct temperature-resolved MS, GC-MS and flow injection analysis, and LC- electrospray ionization MS (LC-ESIMS).

All white?

An interesting recent project studied the degradation of oil paints containing titanium white pigments. These pigments were introduced in the first half of the 20th century as an alternative for zinc white and (especially) the toxic lead white. The early titanium white pigments used by artists such as Pablo Picasso and Piet Mondrian were mostly produced from the mineral anatase. This pigment may absorb UV radiation, producing radicals that break down the paint binder, leading to chalking of the paint surface, compromising the appearance of the painting and leading to loss of original material. Nowadays, artists pigments are derived from another titanium oxide mineral, rutile, and are coated with thin layers of alumina and/or silica to block the detrimental effect of radicals on paint. In addition, museums often routinely block out the most damaging UV radiation. Nevertheless, especially for early titanium white-containing paintings, monitoring the paintings for degradation may prove useful.

PhD student Birgit van Driel investigated numerous aspects of titanium dioxide pigment and presented a number of analytical approaches and techniques that detect the degradation of titanium white paint before the damage becomes visible. This approach may be used as an early warning system by museums to see if additional lighting measures should be taken (1).

Scrub up

Soiling of paint surfaces with particulate dirt is inevitable and most paintings will need to be cleaned once in a while. For this, conservators generally use water and aqueous solvents with cotton swabs. While this works well for older paintings, modern oil paints are often sensitive to water and other solvents, creating a real challenge for conservators. We wanted to find the root cause of the problem, so we carried out a series of studies investigating the chemical reactions involved. We found that, in some cases, sensitivity is caused by the formation of water soluble salts following a reaction with atmospheric pollutants (2,3) and more recently we discovered that degradation of the binding medium itself may also play a crucial role. As oil paint dries, the binder, which often consists of linseed oil, will polymerize thanks to its double or triple unsaturated fatty acids; however, there are competing oxidation reactions that form diacid moieties. Hydrolysis of these triglyceride molecules may also undermine the stability of the paint but this effect may be counteracted with the formation of metal soaps (4,5). In collaboration with our colleagues from the Tate Gallery and the University of Pisa, we have been able to find, through analysis with direct infusion and LC-ESIMS, a firm analytical correlation between high concentrations of free oxidized degradation products and water sensitivity, whereas the more stable paints show high degrees of polymerization and/or metal soap formation (6).

It’s satisfying to know that we are helping to optimize the storage, presentation and guardianship of works of art that are of immense cultural value in our society. By understanding what materials have been used for paintings, and the changes that have occurred as a result of aging and the conditions of storage and display, we can predict (and avoid) changes in the future. In addition to the cultural value, paintings also represent economic value, reflected in the extravagant prices paid for paintings at auction. As a result, forgeries are ubiquitous, even in renowned museum collections, and this is another area where our knowledge of historical painting practice can be put to practical use!