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Reflecting on modern painting

When analysing paintings, non-invasive analytical techniques such as reflectance infrared (IR) spectroscopy are clearly the preferred option, because they don’t require the removal of a physical sample. Such analysis is often conducted to determine the kind of binder used in the paint, whether a natural binder such as oil or, in more modern paintings, synthetic binders such as acrylic, because the nature of the binder can help determine the best way to preserve or restore a painting.

The problem with using reflectance IR spectroscopy for this purpose is that other components of the paint such as pigments and filler can interfere with the reflection, as can the physical shape made by the paint. This can make it difficult to pick out those parts of the reflected spectra that are specific to the binder. To help resolve this problem, a team of Italian scientists, led by Costanza Miliani at the Institute of Molecular Science and Technologies in Milan, set out to identify the most reliable IR wavelengths for identifying five commonly-used paint binders, both natural and synthetic.

This involved first creating standard paint samples by combining the five binders, comprising acrylic emulsion, polyvinyl acetate resin and oil-modified alkyd resin as examples of synthetic binders and linseed oil and proteinaceous tempera as examples of natural binders, with various different pigments. The scientists then applied these paint samples to a cotton canvas and analysed them with a portable Fourier transform IR (FTIR) spectrometer operating at near- and mid-IR wavelengths.

By comparing the spectra reflected from the various paint samples containing each binder, Miliani and her team were able to identify those wavelengths associated most closely with the specific binders, as they report in the Microchemical Journal. Using this information, they then analysed 11 modern paintings with the same portable FTIR spectrometer, finding that most of them used paint with synthetic binders but that a few used more traditional oil- and protein-based binders.

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