‘When I was 8 I was allowed to fill the jars of acrylic paint at work with my father,’ says Katrien Keune in a room in the studio building of the Rijksmuseum. Ahead, two restorers bend over The Standard Bearer, the painting by Rembrandt that is being prepared for his tour through the Netherlands. Keune’s father worked as a chemist at the Rietveld Academy and the Jan van Eyck Academy. “That made a big impression. Those smells of all acrylics. The large spaces. I was introduced to the atmosphere at the academy at a young age and I found it inspiring.”
Four years later, she decided that she wanted to study art with chemistry knowledge. “As a twelve-year-old I was fascinated by materials. Art is a wonderful means of communication to let materials give a message, it appeals to your feeling. I wanted to unravel those materials. What is paint? How is color created?”
In 2019 she became head of natural science research at the Rijksmuseum. She leads part of Operation Night Watch; the extensive research on this painting. She also works as a researcher at the University of Amsterdam. On 11 May she will give her inaugural lecture there as professor by special appointment of molecular spectroscopy.
What is it like to work on The Night Watch?
“Awesome! It’s a dream come true, it’s such an iconic piece. We are working on it with so many different scientists, restorers and conservators. The whole museum has to do with it.”
I am very happy with these kinds of problems
What made it so much fun?
“The longer you work on a painting, the more you understand it. You really bond with it. By placing the painting in high resolution on the website, we hope that the public can develop that bond as well.”
You found out the cause of the white haze in the dog at the bottom right of the painting.
“We initially thought that the discoloration of the dog was the result of chemical reactions in the paint. We discovered by taking a minuscule paint sample, among other things, that this is not the only cause. We embed these paint samples in resin so that we can view the paint cross-sections using various microscopic techniques. The white haze appears to be caused by wear of the paint, among other things. The top layers have disappeared, so you are now looking at Rembrandt’s light sketch.
Read about the restoration: Rijksmuseum discovers unknown sketch under ‘The Night Watch’
“I am very happy with these kinds of problems because it can provide so much insight. A painting is a small chemical factory in which all kinds of reactions take place. That is a wonderful world! If you understand what reactions occur and why a painting looks the way it does now, you can also reason back and determine what a painting has been through.”
Rembrandt’s palette was much more nuanced and colorful than we thought
Do you have an example of that?
“In the whitish haze we found, among other things, a component of palmierite. That forms a kind of crust on the painting. It consists of lead, potassium and sulphur. Lead comes from pigments such as white lead or red lead, potassium is often found in pigments such as smalt and red lacquer. But that sulfur? That may come from the plaster, but we see it in many paintings. So I’m convinced this came from the environment and precipitated onto the surface. In the 18th and 19th centuries, galleries were heated with coal ovens. Sulfur was released during that combustion.”
What else did you find striking?
„Everyone always thinks that The Night Watch painted very dark, because that’s how it looks now. But Rembrandt’s palette was much more nuanced and colorful than we thought. A blue pigment like smalt gets browner over the years.
“You can still see an example of that richness of color on Van Ruytenburch’s jacket. The embroidery is painted with arsenic, we discovered. We only know this from still lifes and we had never seen that with Rembrandt. Now we want to know how he used it and in what form. We are investigating that in the lab.”
Will the painting get darker after 380 years?
“That is an important question. You hope it stagnates at some point. When you look at the cross-sections of the red lacquers, I am impressed by how well preserved it is. That gives me confidence that it has stabilized. To investigate this, we are speeding up these kinds of decay processes in laboratories.”
Keune and colleagues use techniques from various fields. For example, optical coherence tomography. Ophthalmologists use this to create 3D images of the retina to detect abnormalities. The Rijksmuseum uses it to visualize the height differences in the layers under the varnish surface and thus to find out how Rembrandt created depth. Another example is reflectance imaging spectroscopy to determine which pigments and binding agents Rembrandt used. NASA uses that technique to photograph and chemically identify minerals on Mars.
A certain reaction can take place at one micrometer and something completely different can happen ten micrometers away
Which techniques from the art world end up in other fields?
“Macro-X-ray fluorescence scanning, or macro-XRF, was developed for the art world to investigate which chemical elements are in used paint. This technique is now being used in forensic research, for example, to detect biological traces such as blood and semen and gunshot residue on clothing.”
After your research, the restoration of De Nachtwacht will start. What can be seen from previous restorations?
„In the 19th century they laid paintings like The Night Watch in a coffin, along with all kinds of cloths soaked in alcohol. They then let it steam overnight to allow the varnish layer to regenerate; a kind of rejuvenation cure. But that effect wears off, so they use it more and more. I think they extracted a lot from the painting with that, such as binder. It could be one of the reasons why that dog is so blurry now. I would like to investigate that further. We are now looking at how you can make only the varnish layer swell with a minimal amount of alcohol so that no fatty acids or metal ions migrate from the layers below. We have to understand what you do with a material.”
You will deliver your inaugural lecture on 11 May. What are your plans?
“Until now, we saw paint as a homogeneous layer, but it is a heterogeneous environment in which different reactions take place. On one micrometer a certain reaction can take place and ten micrometers further on something completely different can happen, resulting in two different effects on the painting. This heterogeneity is an unexplored area and that also brings me back to my childhood fascination. A material is not one thing. So much is happening beneath the surface.”