Rembrandt the chemist: lead formate found in masterpiece

An international team has identified lead formate in various areas of one Rembrandt’s best-known paintings, The Night Watch. The discovery - which has been published in Science Advances - provides some clues about the practices used by some of the great artists of the seventeenth-century. Chemistry World’s Phillip Broadwith took a look at the painting and the research paper...

Phillip - This particular paper is about Rembrandt's 'The Night Watch.' It was painted in the mid 17th century and it's now at the Rijksmuseum in Amsterdam. The Rijksmuseum itself is a fantastic museum - they have a great big chemistry department doing lots of conservation and analysis work - and what the scientists at the Rijksmuseum were trying to do in this case was look at how Rembrandt had actually painted 'The Night Watch;' what's the condition of the painting? Is it degrading and how can we preserve it? They did all sorts of analysis using x-ray techniques, and they found a very unusual mineral, lead formate, in the painting, which was a bit of a surprise.

Chris - People of Rembrandt's era were using lead all the time, weren't they? Because lead makes colourful compounds. People even used it in makeup, I think, didn't they? To get nice white skin tones and things?

Phillip - Lead compounds have been used in makeup since the ancient Egyptians. Lead oxide was also used as a hardener. The oil, the linseed oil that was the base of the paint, if you mix that with lead oxide, there are reactions that help the paint to harden and set. So there's all sorts of different kinds of lead compounds, and some of those can react together so it looks like this lead formate, which is strictly a compound of lead and formic acid, which is the acid that ants use in their defence, wouldn't have started out as lead formate: it wouldn't have been added to the paint in that form, but it has later formed by some kind of chemical reaction between the various different lead compounds, maybe the oils, maybe other things that are there at the time.

Chris - Is the purpose of this that - in the same way, as we say in biology, in order to conserve a species, you've got to understand it - are they trying to chemically unpick what Rembrandt did so that they can then understand why the picture looks the way it does, but also how we can sympathetically keep it looking good?

Phillip - Yeah, absolutely. That's exactly it. You know, looking at the composition of the paints gives us some insight into the materials that artists were using at the time. Rembrandt is particularly famous for his use of light and shade and colours to make those kinds of images, and this one is an absolutely spectacular example of that. But also, understanding how those materials change over time gives us an insight into how we can conserve these paintings in the best way for future generations to enjoy.

Chris - I was quite surprised actually when I read a bit more about the history of it. I have seen it, and it is a stunning picture. It's huge - 14 feet by 12. And you think when someone can paint on that sort of scale with the detail that he achieved and bring in, as you mention, that element of shading, light and dark, to emphasise certain bits of the picture, it just shows an incredible talent. But one of the things that someone wrote about this is that although we call it 'The Night Watch,' it's not actually intended to be a nighttime scene and it's years and years of accumulated dirt and other people's not so careful conservation that have made it look dark. So does this understanding of the chemistry that these sorts of analyses are giving us, does that mean we might be able to chemically unpick some of what's been done to it? Or are we kind of stuck with it?

Phillip - Yeah, well that's always a difficult thing in art conservation. There's always a balance between trying to preserve what's there, but not necessarily trying to turn back the clock. That's part of its journey, if you like. But if it's something that conservators have done that can be undone, for example, dirt, or if someone has added a varnish over the top that's damaging the painting, then that's the kind of thing that they will look to try and remove, to bring more of the painting back to reality, if you like, without necessarily interfering with the art itself.

Chris - Apart from being extremely good with a paintbrush, does this suggest that people in this sort of era, like Rembrandt, were also pretty good chemists?

Phillip - It's not entirely clear. You're talking about the mid 17th century, this is almost the birth of modern chemistry, the emergence from an alchemical, artisanal kind of practice into a more scientific way of thinking. It's not clear how much the artists of the time were taking that on. You say this painting is massive, if somebody wanted to have consistent colours over a painting that size, they're going to have to be mixing batches and getting some consistency. So my feeling is that they would've been taking a relatively artisanal but quite systematic approach to mixing those pigments, not necessarily understanding the underlying chemistry, but having a really good knowledge of the materials that they were using and how they interact and mix together to create the effects that they want to do. Other painters over time have been very famous for this kind of thing as well. Van Gogh and the other impressionists and expressionists. Edward Munch used a lot of metal compounds to make really vibrant colours. But one of Van Gogh's problems is that, quite quickly, only 50 to a hundred years later, those colours have changed significantly, and that's partly because of the chemistry of the pigments he was using. Now, whether he was using those pigments because he didn't have very much money and those were what he could afford, or because they gave him the effects that he wanted, we don't really know. And modern artists start to blend some of that chemical knowledge into what they're doing and they can deliberately make colours that will change over time and use that as part of the effect of the art.