Scientists recapture Renoir's reds
Researchers in Chicago have produced a visualisation of how they think a Renoir could have looked before its colours faded.
The picture of Madame Valentine Clapisson was painted by the great French Impressionist more than 130 years ago.
The original's impact has been degraded and dulled by the action of light.
But by using the latest analytical tools, conservators have been able to recover a sense of Renoir's rich reds.
"When we first brought this picture into the conservation studio for examination and removed the frame, we noticed that at the top and at the left-hand side there was a sliver of very intense colour," recalls Dr Francesca Casadio from The Art Institute of Chicago.
"This tipped us off to the fact that the mood of this painting that is now pretty cool and restrained with light purples and blues was once far more vibrant," she told BBC News.
Dr Casadio was speaking here at the annual meeting of the American Association for the Advancement of Science (AAAS).
She and other specialists in her field have been discussing the technologies they now use to investigate and restore artworks.
The visualisation of Madame Clapisson was produced after subjecting tiny samples of paint from the 1883 canvas to a technique known as Surface-Enhanced Raman Spectroscopy (SERS).
Pigments fade over time as the molecular structures responsible for colour are broken down through exposure to light.
SERS enables researchers to pick out even the smallest fragment of intact structure and determine its molecular composition. This was done with tiny samples taken from the newly uncovered edges of the canvas.
Dr Casadio's and colleague's investigation showed that the picture we see today has lost the impact of Renoir's use of the carmine lake, or cochineal, pigment.
Extracted from the bodies of certain insects, this crimson colour should dominate the scene around Madame Clapisson.
"The manuals from the 19th Century were already warning artists that carmine lake was a 'fugitive pigment'; it wasn't a permanent colour. And yet, the artists clearly loved it because they continued to use it over and over again even though they had more stable options available to them," Dr Casadio explained.
The study has allowed the conservation scientist to make a compelling computer visualisation of what Renoir originally saw when he stood back after signing his name on the completed canvass. Is it exactly the same?
"We can't quite say that," said Dr Casadio, who acknowledges that the mixing of pigments on a palette does not have the certainty of scientific equations.
"We can get very close to the tip of the brush of Renoir because we know the exact chemical mixture he used, but then if you and I were to make a cake with the same ingredients it may not taste or look quite the same."
Northwestern University chemist Prof Richard Van Duyne pioneered SERS. He said the Renoir demonstrated why the Raman technique was fast becoming an invaluable tool for studying artworks: "You get tremendous information about the origins of the painting, the techniques of the artist, an understanding of the fading mechanism, and the ability to restore the painting."
The picture of Valentine Clapisson was actually Pierre-Auguste Renoir's second attempt at capturing a portrait of his subject.
Initially, Valentine, the wife of Paris stockbroker Leon Clapisson, was depicted taking tea in a rose garden. But neither the artist nor the client was happy with the outcome and so the subject was moved indoors.
An exhibition based on the research opened this week at The Art Institute of Chicago.
The original picture is known as "Madame Leon Clapisson", 1883. Oil on canvas, 81.2 × 65.3 cm, Mr and Mrs Martin A Ryerson Collection. Digital recolourisation by Kelly Keegan, Conservation department, The Art Institute of Chicago
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos