When conservators at the Art Institute of Chicago wanted to find the missing pigment in a portrait by Pierre-Auguste Renoir, they used laser light to reveal the original hues.
Nothing lasts forever as proved by an advanced technique for painting color analysis. A laser technique called surface-enhanced Raman spectroscopy reveals that the red has faded in a Renoir painting circa 1883. This is due in part to the fact that the red pigment, carmine lake, was ‘made from crushed and ground cochineal insects that live on cactuses in Mexico and South America.’
See on www.nytimes.com