Wax paintings analysed

The care and restoration of wax-based paintings could be easier following a new gas chromatography method for characterising the wax used.

Waxes are used to create paintings via the encaustic technique in which pigments and resin are mixed with molten wax and applied to the surface. Once the paint has cooled, it provides a permanent, lustrous finish that can be heated and reworked.

When historical artworks created by this technique are in need of restoration, it is essential to know the type of wax employed. Spanish researchers have developed a method for their characterisation based on analysis of their fatty acid and hydrocarbon components.

Jose Gimeno Adelantado and colleagues from the University of Valencia and the Polytechnic University of Valencia selected beeswax, carnauba wax and ceresin, representing the most frequently used animal, vegetable and mineral waxes in paintings. The waxes were hydrolysed to release the fatty acids, which were extracted along with the hydrocarbons. The fatty acids in the mixture were converted to their ethyl esters with ethyl chloroformate before gas chromatographic analysis.

Peak area ratios for the C₂₀-C₃₅ hydrocarbons relative to C₂₇ and for seven fatty acids relative to palmitic acid gave unambiguous differentiation between the three wax types, clearly illustrated in a principal components analysis plot.

The technique was applied to wax scrapings from paintings by the Spaniard Rafael Sanchis Yago and the Mexican Carmen Lopez. It was clear, despite natural ageing of the waxes, that both paintings were prepared with beeswax.

But the work is of limited use, suggests a leading expert in the field who preferred not to be named. ’I would be unlikely to choose this method as the three waxes described can be readily distinguished by qualitative analysis of unhydrolysed sample extracts,’ she told Chemistry World. ’Moreover, working with unhydrolysed samples, it is much easier to spot the presence of other sample components such as fats or oils - which the authors acknowledge would interfere with interpretation their fatty acid data.’ Steve Down