Shooting A Painting Reveals Its Pigments

Nicolas Wyplosz investigated laser desorption mass spectrometry (LDMS) as an analytical technique for studying organic pigments. The technique can determine the composition of the paint using a sample just ten micrometres thick. A knowledge of this composition is important for a better understanding of the ageing process.

In this technique, laser light is fired at a small cross-sectional area of a paint sample. The laser beam vaporises molecules and atoms from the paint sample surface. These particles are identified by a mass spectrometer. Finally, the composition of the paint layers can be determined from the particles detected. For example, a certain pigment can be obtained from two different plants and Wyplosz managed to determine which plant the pigment came from.

The preparation of the samples was found to be crucial for the success of the analysis technique. The paint samples must be smoothly polished with as little roughness as possible, and Wyplosz developed a new technique for this.

At present the best techniques for studying pigments in paintings are mass spectroscopy and chromatography. The new technique is a valuable addition to these, as the present techniques cannot be used for cross-sectional samples. A typical old master's painting consists of a canvas, a layer of animal glue, a primer, various layers of paint, and finally a layer of varnish. Studying cross-sectional samples is easier because the different layers do not need to be separated prior to analysis. The paint layer is just tens to hundreds of micrometres thick.

This study was part of NWO's MOLART programme. During the programme, fundamental research into the varnish, paint and pigments was carried out in order to understand the molecular aspects of the ageing processes. Wyplosz's doctoral thesis is the eighth in a series of reports which summarise the programme's results.