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Masterpieces under the microscope

Zinc oxide has been industrially produced as a white pigment for painting since the 1840s. Artists were attracted to zinc oxide based paints as they were less prone to discolouration than traditional whites. Manufacturers valued its lower toxicity and the beneficial properties it conferred to other paints. Zinc oxide peaked in popularity in the first half of the twentieth century and is found in many paintings from this period through to the present day. Unfortunately its presence has recently been linked to serious deterioration resulting from reaction of zinc oxide with fatty acids from the oil-based paints. In some paintings the resulting zinc carboxylate soaps aggregate to the point where they cause the paint to crack and flake or even erupt as lumps through the painting surface. Art conservators now want to know how these zinc stearate compounds form, why they accumulate, what conditions promote formation, and the all-important question, can the formation be prevented or reversed?

Gillian Osmond is an art conservator at the Queensland Art Gallery and is currently doing a PhD at the Australian Institute for Bioengineering and Nanotechnology at the University of Queensland with Prof. John Drennan and Prof. Michael Monteiro. Through an ARC Industry Linkage Project, The Twentieth Century in Paint, she is investigating the mechanism of zinc stearate formation and its distribution in affected paint samples. She is using a combination of characterisation techniques in the AMMRF at the University of Queensland and at the Australian Synchrotron, including electron microscopy, X-ray analysis and Fourier transform infrared microspectroscopy.

The challenge is significant given the complexity of paint formulations and the many variables influencing drying and aging processes. Gillian has had access to reference paints from the Smithsonian’s Museum Conservation Institute enabling her to consider the influence of specific pigment, oil and paint additive combinations on the formation and behavior of zinc stearate in naturally aged paint films. She found that zinc stearate forms rapidly in the paint samples. She also observed that the problem was significantly worse in paint where aluminium stearate is present as it significantly influences the amount of stearic acid available to react with the zinc. Once formed in sufficient concentrations, the zinc stearate moves through the paint film, gradually aggregating into clumps. This aggregation has very important consequences for the stability of art works. A particularly high profile case involves Vincent van Gogh’s, “Falling Leaves (Les Alyscamps)”.

Continuing to improve our understanding of the formation and behavior of zinc stearates will enable art conservators to identify susceptible paintings and prescribe protocols for minimising associated deterioration and will have significant impact on art conservation.

Gillian’s findings have recently been published in Applied Spectroscopy and are part of a collaboration with researchers from JAAP Enterprise for MOLART Advice in Amsterdam, the AMMRF at the University of Queensland and the Australian Synchrotron.