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Zinc oxide (ZnO) has been used for pharmaceutical applications since ancient times and was introduced as a pigment in the late 18th century, but it was not until the mid-19th century that its production and sale really took off. Its low toxicity compared to lead oxide was one reason for its rapid growth, despite its poor hiding power and the time required for oil to dry in its presence. It is used in graphic arts such as watercolours and oil painting, alone or in a mixture. It became popular at the height of the Impressionist period, and Van Gogh, for example, used this oxide a great deal.
When used as a minor ingredient, zinc oxide can sometimes be difficult to identify unambiguously, even with a scanning electron microscope. One of the challenges is to detect its presence when it is mixed, sometimes in tiny amounts, with lead oxide. The modularity of the radiation from the DISCO beamline at SOLEIL and its optics, optimized for work in the UV range, now make it possible to acquire a sub-micrometric image of the luminescence properties of heritage materials. This allows the direct study of complex and diverse samples, particularly stratigraphic sections taken from works of art. Because the luminescence of the zinc oxide in the ultraviolet domain is very characteristic, it is a reliable signature of the pigment, even when mixed with other materials. | 
Sample of zinc oxide
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The impressionistic painters were the first ones to use the zinc oxide. As an illustration: a view of a Pissarro painting.
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Synchrotron multispectral luminescence microscopy images of zinc oxide particles dispersed in linseed oil. The bands centred at 383 nm and 510 nm seen on the zinc oxide emission spectrum in the middle are imaged via interferential filters between 300 and 360 nm (left-hand image) and between 480 and 520 nm (right-hand image), respectively.
The method developed should provide important evidence to track the evolution of the pigment manufacturing method (nature of the mineral used, grinding, heating) for a finer characterisation and more accurate dating.
This technique has also been used to study other cultural heritage materials. For example, in the varnish on a cello manufactured in the 18th century by the Parisian instrument-maker Jacques Boquay, it was possible to identify the specific nature of the first coat applied to the wood by showing the similarity between the spectra obtained on this three-century-old sample and those of an original animal glue.
 | Multispectral imaging of a violin varnish stratigraphy ("Provigny", A. Stradivari, Cremona 1716, Museum de la Musique, E.1730). The images collected in several spectral bands made it possible to reconstruct an image in false colours, highlighting the distinct composition of the varnish layers. |
Reference :
Thoury, M., Echard, J.-P., Réfrégiers, M., Berrie, B. H., Nevin, A., Jamme, F., Bertrand, L., Synchrotron UV-visible multispectral luminescence micro-imaging of historical samples, Analytical Chemistry, 2011, pp. in press. dx.doi.org/10.1021/ac102986h
