Saturday, February 15, 2014

Surface enhanced Raman spectroscopy to reveal Renoir's original palette

The lady in reds. Credit:
Art Institute of Chicago
The Art Institute of Chicago is presenting the results of a recent conservator's analysis of the portrait of Madame Leon Clapisson by Renoir. This portrait was undergoing routine conservation when it was discovered that edge areas of the painting, which had been protected from light by the frame, showed much stronger scarlets and purples. The conservators immediately suspected that Renoir had used carmine,  a brilliant red pigment which is very susceptible to fading, in his original color palette, meaning that the painting would originally have been much more lively.
Carmine itself is produced from the cochineal beetle, a native of the Americas which infests cactus leaves. (A neat overview of carmine production is here. ) Carmine was introduced to European artists by the Spanish, and the pigment became especially popular during the Renaissance, when it was prized for creating translucent, glowing washes. In addition to the brilliant red-pink pigment, the cochineal beetle extract can be precipitated with copper sulfate to create Indian purple.
The Art Institute has several works in its collection in which carmine-based pigments used during painting have since faded.

To confirm that carmine pigment, now faded, was used in the main body of the Mme. Clapisson portrait, Richard van Duyne of Northwestern University used surface-enhanced Raman spectroscopy, an enhancement of of Raman scattering detection optimized for anticipated low target abundances or for weak scatterers. (A YouTube overview of the method, which is widely used in forensic applications, is here).
You can see can see a reconstruction of how the original might have looked at the BBC web page here. The most apparent difference is in the background, which has  much more raspberry than the present-day painting.

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