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Synchrotron deep-UV photoluminescence imaging for the submicron analysis of chemically altered zinc white oil paints.

Zinc oxide (ZnO) is a II-VI semiconductor that has been used for the last 150 years as an artists' pigment under the name of zinc white. Oil paints containing zinc white are known to be prone to the formation of zinc carboxylates which can cause protrusions and mechanical failure. In this paper, it is demonstrated how a multispectral synchrotron-based deep-UV photoluminescence micro-imaging technique can be used to show the distribution of zinc soaps at the submicron scale, and how this information is used to further the understanding of zinc white degradation processes in oil paint. The technique is based on the luminescence of zinc soaps in the near-UV (~3.65 eV) upon excitation in the deep-UV (4.51 eV); transitions that are argued to subsequently involve ligand-to-metal and metal-to-ligand charge transfer with intermediate structural reconfiguration. Since the primary emission peak lies at a higher energy than the bandgap of ZnO (3.3 eV), the signal can easily be isolated from the pigment's very intense band gap and trap state emission, by employing a multispectral acquisition approach. Moreover, analysis at such short wavelengths, in combination with a UV-transparent optical set-up, allows for a lateral resolution on the order of 200 nm to be obtained. The unprecedented capabilities of the micro-imaging technique are illustrated by showing its application to the study of a historical cross-section from an early 20th century painting by Piet Mondrian. Revealing the submicron distribution of crystalline zinc soaps in this cross-section provides new insights that suggest that micro-fissures, the starting points of paint delamination, are the result of an overall expansion of a heavily saponified zinc white layer.