Any regular museumgoer will recognize the darkened, muted color of red vermillion pigment that immediately signals that a painting is centuries old. But the reasons for this darkening are a mystery that dates back at least 1,200 years. Now scientists have used x-ray analysis of pigments in a medieval Spanish mural to study the degradation and have proposed a new explanation that had not been considered before.

Color is determined by which wavelengths of light bounce off an object. When light hits a surface, certain wavelengths can be absorbed by the material's electrons, which use the boost of energy to jump up to a higher energy level. Different chemicals will be able to absorb different wavelengths of light, and whichever wavelengths they cannot absorb bounce back to be seen as a particular color by an observer. The process is complicated by interactions between excited electrons and the empty energy levels they left behind when they jumped up.

"One of the biggest challenges in this work was to describe, correctly, the effects caused by these interactions," says Fabiana Da Pieve of the Free University of Brussels. Together with Conor Hogan of the National Research Council of Italy and their colleagues, Da Pieve analyzed mural samples from the 14th-century Monastery of Pedralbes in Barcelona, which made ample use of vermillion paint. The researchers performed x-ray diffraction on the samples to identify the chemical composition of various layers in the mural and combined these data with calculations based on quantum mechanics to predict which color each chemical present should give rise to.

The analysis, published November 15 in Physics Review Letters, showed that some previously proposed chemical pathways for the darkening cannot be correct based on quantum mechanics. Instead the researchers have proposed a new pathway for vermillion to degrade that ties together suggestions put forth by various other studies.

Vermillion is made from a mineral called cinnabar, which is composed of mercury sulfide. The researchers showed that when the surface of a painting is illuminated by light, and humid air allows chloride ions (such as, for example, sodium chloride, or salt) found in dirt to deposit on the paint, the mercury sulfide can absorb the chloride ions, transforming it into another mineral called corderoite.

Sure enough, x-ray analysis revealed corderoite was present on the mural in several forms. This mineral is unstable and can directly give rise to metallic mercury, which is black in color, as well as to a mineral called mercury chloride, which can also eventually produce metallic mercury.

While the scientists detected mercury chloride in layers of the painting they examined, they failed to find metallic mercury, which is invisible to x-ray diffraction because the substance is a liquid at room temperature, and diffraction can only spot solids.

Nevertheless, the researchers have argued that metallic mercury is the most likely culprit for vermillion's darkening, and they are the first to lay out these precise chemical pathways for creating the compound. "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," says Marika Spring, a conservator at the National Gallery in London who has also investigated the darkening of vermillion.

Previous studies had suggested that electrically charged atoms of mercury in mercury sulfide might have directly converted into neutral atoms of metallic mercury, but the new analysis has shown that light hitting the painting could not provide the energy necessary for this conversion, refuting that potential pathway as an explanation for the darkening. It is too soon, however, to know whether Da Pieve and her colleagues' explanation is correct, says David Saunders, keeper of conservation and scientific research at the British Museum in London. First, direct proof of metallic mercury on the surface of vermillion paintings will be needed.

Understanding how vermillion darkens should help conservators reduce future damage to aging paintings. "And 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," Saunders says. We may never know exactly how old masterpieces looked to their painters, but science is bringing us closer than ever to seeing them as they were meant to be seen.