Researchers at the VU have studied white lead paint on 77 Rijksmuseum paintings by Dutch painters from the 17th century. They discovered that the composition of the paint has changed over time. This can help to link paintings to a specific time, place or painter, says chemist Paolo d’Imporzano, who obtained his PhD for the research.
Why did you choose white lead paint?
‘We needed one specific color that we could compare the chemical composition of between the paintings. It is not possible to compare different colors, because they differ too much. We chose white because it was simply the most common color in the paintings. Lead paint was also the most suitable because lead has four stable isotopes.
Isotopes are slightly lighter or heavier versions of the same atom. Normally, the heavier isotopes decay into the lighter variant after a certain time. This does not happen with these four lead isotopes. They are each formed by the decay of other, even heavier metals, such as mercury, polonium and thallium. Because the ratio of these even heavier metals differs per area, the isotopic composition of the lead also differs per area. This allows you to deduce from the isotope ratio where a piece of lead comes from.’
You discovered that the composition of the paint also depends on the time. How did that happen?
‘We saw that changes in the paint coincided with certain wars. Warfare in the 17th century required a lot of lead. All normal supply lines were used to make bullets and cannons. As a result, paint manufacturers had to resort to more creative methods of obtaining lead, such as by melting things down. As a result, there is much more variation in the isotope ratio of the lead during that period.
That variation only depended on the time, and on whether it was painted inside or outside the Netherlands. Within the Netherlands it did not matter who the painter was. As long as two works of art were made approximately simultaneously, we saw no difference.’
Did any new information about certain paintings come to light?
From the painting Cimon and Pero Willem Drost was thought to have painted this after traveling to Venice. However, it turned out to be painted with Dutch paint. At the time, paint was easy to obtain in Italy. It is therefore illogical that Drost had brought the paint from the Netherlands. The painting was therefore probably already made before he went to Venice.
We also found Dutch paint on two portrait paintings, while we know for sure that painter Martin Palin was staying in Jakarta at the time. This can have several causes. One option is that the paint ended up there via a Dutch trade route, but it is also possible that those paintings were not made by him at all. However, you need additional information to be able to make statements about this.’
How could you examine the paintings without damaging them?
Our measuring method was so accurate that a few micrograms of paint were enough. The paintings are therefore not damaged. Where possible, we also used material that had already been removed from the painting.’
Are you more of an art or chemistry enthusiast yourself?
‘I am a chemist myself, so the paintings were more research objects than works of art for me. But of course, just because I see myself as a scientist in the first place doesn’t mean I don’t like art. It’s not for nothing that I chose this topic for my PhD!’