Numerous chemical transformations over time alter the color of pigments. Analyses combined with the study of artists' and critics' commentaries help us to better understand the nature of these mechanisms, and sometimes to envisage remedies.
These phenomena can be observed in a wide variety of cases, involving both mineral pigments and organic coloring molecules. Alterations in cadmium yellow, for example, were observed on the surface of a painting by James Ensor: white globular formations developed as a result of the pigment's reaction with moisture or varnish in the presence of light. This pigment, available since the 1840s, has been widely used by artists for its brilliant yellow color, excellent covering power and ease of manufacture. In other cases, the discoloration of organic molecules is due to exposure to light, humidity, oxygen, industrial air pollutants or the action of micro-organisms (bacteria, mold).
These phenomena were sometimes visible during the artist's lifetime, and Jean Renoir explained in Pierre-Auguste Renoir, mon père : "Having realized the danger of painting for the immediate future on his early canvases, some of which turned black, my father gradually developed a long-term method. The result was a paint capable of withstanding the test of time; better still, calculated to turn the action of time into a benefit [2]."
Recent research into these phenomena shows that numerous degradation mechanisms can occur, and underlines the complexity of the parameters to be controlled if we wish to limit or halt the phenomenon. Understanding these transformations is important for assessing the differences between the painter's artistic project and our perception of the works today.
