Why Red Paint Fades Over Time


SC Assistant Science Editor

Walking through a museum, one might realize that older pieces of artwork that contain red paint have darkened, the red instead appearing black in color.

Recently, researchers from the National Research Council of Italy and the Free University of Brussels worked to analyze samples from the Monastery of Pedralbes in Barcelona in order to uncover the science behind this phenomenon.

The Monastery of Pedralbes is a museum that houses collections from Barcelona City’s History Museum, including art from the fourteenth to the twentieth century.

In order to analyze the samples of artwork, the researchers used x-ray diffraction to study the degradation of the murals and to identify the chemical composition of the paints utilized.

Based on accepted quantum mechanics, however, the degradation of vermillion paint observed by the researchers could not be explained.

Vermillon is a bright red pigment that was used widely in Ancient Rome, during the Middle Ages and Renaissance, and in the lacquerware of China.

Vermillion is composed of a mineral known as cinnabar, which contains mercury sulfide, a chemical compound that has been accredited for the red color of vermillion.

It is now believed that when the murals are exposed to humid air and light, chloride ions found in dirt particles may be absorbed by the mercury sulfide to form a mineral called corderoite.

Corderoite is highly unstable and can transform into metallic mercury and mercury chloride. While the scientists were able to detect mercury chloride, they were not able to find traces of metallic mercury, which is black in color.

Though they were not able to detect any metallic mercury within the paintings, the researchers still have hope that it is responsible for the darkening of vermillion paints.

Marika Spring, a conservator at the National Gallery in London who aided in the study, said, “It’s a pretty convincing argument for what this black product is that’s produced, and it definitely takes us forward in our understanding of exactly what’s happening.”

Before the scientists can further attempt to explain the chemical pathway that causes the darkening of vermillion, one of the first steps that must be taken is to discover the presence of metallic mercury within older pieces of art.

According to David Saunders, keeper of conservation and scientific research at the British Museum in London, “Knowing how color may have changed allows us to imagine how works might once have appeared and to interpret them accordingly, avoiding erroneous interpretations of color that arise not from original intent but from the changes wrought by time.”

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