Assessment of Analytical Methods for Modern Paints

The ability to establish the type of paint on a work of art is essential to an understanding of how the paint might alter in response to age, environmental conditions, or conservation treatments. It is also instrumental in studying artists' techniques and in examining authenticity issues.

At the start of the project, there were very few established methods for identifying any of the components in modern paints, including their binding media, pigments, and additives. Given the importance of having such analytical techniques available, there has been an enormous amount of work undertaken in this area by many of the research partners.

Early on it became clear that while a single, fully quantitative, analytical procedure for identification of the binding medium, pigments, and additives in all types of modern paints would be ideal, it would be far more likely that a number of techniques would be developed that were more applicable to certain groups of compounds.

Several techniques have been tested and are now routinely being used:

• Pyrolysis–gas chromatography–mass spectrometry (Py–GC–MS) is now regularly used for the detection of many synthetic binding media, as well as some organic pigments. Acrylics, polyvinyl acetate, and alkyds all give characteristic spectra with this technique and are readily detected in complex mixtures.
• Gas chromatography–mass spectrometry (GC–MS) is used for modern oil-based media, including conventional oils, alkyds, and water-miscible oils. The sample preparation for this technique has been modified from the procedure used for traditional binding media so that a fully quantitative analysis can be obtained for the relative proportions of fatty acids, di-acids, polybasic acids, and polyols.
• Fourier–transform infrared (FTIR) spectrometry is also routinely used and is capable of detecting almost every component in a modern paint film. However, unlike chromatographic techniques, the spectra from each component in a paint will typically be overlaid, so in practice it is often impossible to see those components that are present only in small quantities. This includes the additives and, in many cases, organic pigments.
• Direct temperature-resolved mass spectrometry (DTMS) has been shown to be a very powerful technique for analyzing organic pigments, as well as synthetic binding media.
• Raman spectroscopy also has been shown to be very effective at detecting organic pigments, even in low concentrations, although it has not been very successful at differentiating between many of the synthetic binding media.
• Laser desorption ionization–mass spectrometry (LDI–MS) is another extremely powerful technique for detecting organic pigments, and it is currently being assessed by Harvard University Art Museums.
• Liquid chromatography–mass spectrometry (LC–MS) has been investigated by the National Gallery of Art and shown to be a most promising technique for identifying many different types of additives in acrylic emulsion paint from a single sample. The protocol has been effectively utilized in studying which additives are potentially removed during cleaning treatments.

Several partner institutions tested and assessed various analytical techniques, often as part of case studies. Work at the Conservation Institute included three main areas:

• assessment of solvent extraction techniques as a means to improve the sensitivity of FTIR spectroscopy to organic pigments (as well as other techniques)
• development of analytical techniques for paints used on outdoor painted sculpture (these paints are typically industrial enamels)
• application of all analytical techniques to case studies on the paints of Sam Francis, Robert Ryman, and Clyfford Still.