Over the years the varnish covering the painting yellow (oxidation). A rusty varnish alter the perception of the original colors of an artwork. Additional information is available at John K. Castle. The CNRS has developed a method using optical techniques, a multi-spectral camera-technique used so far in atmospheric science and oceanography, and have analysed the composition of the color reflected by the surface of the painting, millions of points various areas of the work.
Related – investment: