14 Aug 2014

How Red Pigments Fade

From Our Changing World, 9:34 pm on 14 August 2014
Girl with a Pearl Earring by Jan Vermeer

Girl with a Pearl Earring by Jan Vermeer (1632-1675) Photo: Wikimedia Commons

One of the striking features in paintings like the Girl with a Pearl Earring by Jan Vermeer (1632-1675) is the red pigments, in this case the madder lake used in her lips.

But these red pigments have a tendency to fade.

Sarah Thompson, a PhD student from the University of Auckland, is using a femtosecond laser at the Photon Factory to look at how the paints fade when exposed to light, right down at the molecular level.

“A femtosecond is to a second as five minutes is to the age of the universe. It’s very, very short,” says Sarah. “But it’s only short on our timescale. On the timescale of a molecule it’s actually quite normal.”

In particular, Sarah is looking at the anthraquinone molecules, many of which produce the vibrant red translucent glazes used in artworks. Using pulses of light from the laser, she is probing the pigments to see what their excited state lifetime is.

When these pigment molecules absorb light they are excited, a state which is unstable and of high energy. Then the molecules relax back to a ground state.

Sarah Thompson in front of a cupboard of pigments in vials

Sarah Thompson with a cupboard of pigments Photo: RNZ / R. Beran

“The longer that excited state lifetime is…there’s more risk that the molecule will break apart, react with another molecule around it, do something that changes its structure, such that it loses that red colour or as we would think of it, fades,” says Sarah.

The faster the molecule returns to the ground state, which is more stable, the less likely it is to fade.

“So we would expect the longer excited state lifetime would correlate with poorer photostability,” says Sarah.

This has been confirmed by her experiments. For example, although the two main components of the red pigment madder are chemically very similar, other studies have shown that the compound purpurin fades much faster than alizarin. Sarah has found that purpurin’s excited state lifetime is substantially longer than alizarin’s.

“It’s demonstrating that there is a connection between the ultrafast processes that are happening inside the molecule and the centuries’ long fading that we see,” says Sarah.

However, there’s no guarantee that just because a molecule is in the excited state that it will break down or fade.

“It does take time for you to see a visible change in a painting,” she says, “But the processes that are affecting this are on the molecule’s timescale, which is in this case picoseconds to nanoseconds.”

According to Sarah, knowing more information about the pigments used in artworks can help conservators to prioritise treatment, to better understand how vulnerable a particular work is, and to tailor display conditions depending on the molecules used in the artwork.