Fine art: painting tricks of 19th century revealed

PARIS, FRANCE: To paint quickly while creating brilliant texture and volume effects artists relied on the development of innovative gels. All the rage in the 19th century—and still in use today—these compounds modify the properties of the oil paints they are combined with.

Now the researchers from Centre national de la recherche scientifique (CNRS), University Pierre and Marie Curie (UPMC), and College de France together have finally learned the chemical secrets behind these mixtures. Lead in its acetate form—is essential for the formation of the gels.

The findings are published in the journal Angewandte Chemie International Edition.

Oil paints were traditionally made by grinding pigments together with linseed, walnut, or poppy seed oil. When prepared in this way, oil paints have very long drying times. Completing a painting takes months, or even years if several layers of colour are to be applied. In the 19th century, artists like J M W Turner introduced innovations in an oil painting that shortened drying time and allowed them to add further dimensions of light and movement through the creative use of novel properties.

Aided by chemists, they combined materials to obtain new formulations that met their requirements: oil-resin gels containing lead—most often lead acetate, in aqueous solution or solid form. These 'gumtions', as they were called by the English painters using them, made it possible to finish a painting faster—in just days—while incorporating unchallenged volume and texture effects.

Answering questions like-How exactly do these materials assume their form? How do they bind with paint? How do they age? and thus making appropriate recommendations for conservation required an understanding of the structural, chemical, and other properties of these gels.

After rebuilding the original paint formulas of the 19th century, the researchers were able to reproduce the gels and study their rheological properties—i.e. flow and deformation behaviour. The addition of even minute amounts of the gels to pigments is enough to modify paint characteristics, yielding elastic properties greater to those of traditional oil paint. They are bona fide paint mediums! (substances added to oil paint to modify particular properties, such as texture and brilliance). Moreover, oil isn't even necessary to create a gel. Mastic resin and lead acetate are all that is needed.

The researchers combined several spectroscopic techniques like nuclear magnetic resonance (NMR), electronic spin resonance (ESR), and Fourier transform infrared spectroscopy (FTIR) to explore the gels on multiple scales. They managed to define the molecular interactions of the hybrid organic-inorganic gel system and the mechanisms of gelling.

The team uncovered processes similar to those behind the drying and ageing of oils. Lead is known to accelerate these processes, which explains the formation of the gel. These findings show that lead not only catalyses the gelling process but contributes to the structure of the medium. The findings of thesis research begun in November are expected to reveal the chemistry of these hybrid gels. The challenge is to determine how the lead binds with the resin, the conditions for conservation of these mixtures, and those under which they break down.

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