Metal Stearate Distributions in Modern Artists' Oil Paints: Surface and Cross-Sectional Investigation of Reference Paint Films Using Conventional and Synchrotron Infrared Microspectroscopy

Abstract

Zinc oxide is a prevalent industrial-age pigment that readily reacts with fatty acids in oil-based paints to form zinc carboxylates. Zinc stearate aggregates are associated with deterioration in late nineteenth and twentieth century paintings. The current study uses both conventional and synchrotron Fourier transform infrared spectroscopy (FT-IR) to investigate metal carboxylate composition in a range of naturally aged artists' oil paints and reference paint film draw-downs. The paints contain zinc oxide alone or in combination with lead white, titanium white, and aluminum stearate and are prepared with linseed and safflower oils. Attenuated total reflectance (ATR)-FT-IR using the conventional source identifies marked differences in carboxylate profiles between exposed and protected surfaces in a large number of samples. Synchrotron FT-IR microspectroscopy of thin paint cross-sections maps metal carboxylate distributions at high spatial resolution and resolves broad concentration gradients and micrometer-scale phase separation of carboxylate species. Aluminum stearate, a common paint additive, is found to influence the distribution of zinc carboxylates more strongly than pigment composition or oil type. The presence of aluminum stearate results in higher concentrations and more pronounced separation of saturated C16 and C18 chain zinc carboxylates in the margin of paint nearest the polyester substrate. The presence of aluminum stearate in association with zinc oxide has a clear influence on zinc carboxylate formation and distribution, with potential implications for long term stability of vulnerable paintings.

20th century artists’ oil paints: The case of the Olii by Lucio Fontana

“…The high content of oleic acid and the consequentially high degree of unsaturation is most likely to be a consequence of the presence of a slow-drying oil, but this phenomena could also be explained in another way -in fact, the significant amounts of oleic acid may be linked to the presence of zinc oxide, which have also been noted in other artworks. It would thus appear that in an oil-based medium, zinc oxide forms a packed structure which is able to trap oleic acid in the pictorial layer: this effect can also occur years after the oxidation process ends [22][23][24]. The unusual degradation phenomena observed in all of these paintings might be the result of the presence of a slow-drying oil (rapeseed oil) containing a large amount of unsaturated acids (oleic, erucic and gondoic were detected after almost 50 years) whose triglycerides reacted with zinc white from the painted layers to form metal soaps.…”

The artists’ materials of P. S. Krøyer: An analytical study of the artist’s paintings and tube colours by Raman, SEM–EDS and HPLC

“…This seems to suggest that zinc white was introduced on Krøyer's palette somewhere between 1886 where he finished the portrait of Christoffer, and 1903 where he begun painting the Drachmann portrait. Zinc white has previously been linked to a number of conservation issues due to the pigment's strong tendency to form zinc carboxylates [13][14][15]. We therefore investigated the formation of zinc soap.…”

An infrared spectroscopic study of the nature of zinc carboxylates in oil paintings

“…Specically, the n a COO À band is frequently signicantly broadened and shied to higher wavenumbers (between 1570 and 1590 cm À1 ) in ZnO containing paint layers. 7,9,[14][15][16][17] We discuss Attenuated Total Reection Fourier Transform infrared microscopy (m-ATR-FTIR) analysis of a sample from the painting De Houthakker (naar Millet) ('The Woodcutter (aer Millet)') by Vincent van Gogh as a clear example of this phenomenon.…”