‘A painting like The Night Watch is a small chemical factory’

‘When I was 8, I was allowed to fill the glasses with acrylic paint at work with my father,’ says Katrien Keune in a room in the Rijksmuseum’s studio building. In front, two restaurateurs bend over The flag bearer, the painting by Rembrandt, which is being prepared for his journey through Holland. Keune’s father worked as a chemist at Rietveld Academy and Jan van Eyck Academy. “It made a big impression. They smell of all the acrylic. The large spaces. I was introduced to the atmosphere of the academy at a young age and I found it inspiring.”

Four years later, she decided she wanted to study art with the knowledge of chemistry. “As a twelve-year-old, I was fascinated by materials. Art is a wonderful means of communication to let materials convey a message that appeals to your feeling. I wanted to unravel those materials. What is paint? How to create color? “

In 2019, she became head of scientific 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 May 11, she will give her inaugural lecture there as a specially appointed professor of molecular spectroscopy.

What is it like working on The Night Watch?

“Fantastic! It’s a dream come true, it’s such an iconic piece. We’re working on it with so many different scientists, restaurateurs and conservators. The whole museum has to do with it.”

I’m very happy with that kind of problem

What made it so fun?

“The longer you work on a painting, the more you understand it. You really commit to it. By placing the painting in high resolution on the website, we hope that the public can also develop that bond. ”

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. Among other things, we discovered by taking a small paint sample that this is not the only cause. We embed these paint samples in resin so we can see the paint cross sections using various microscopic techniques. The white haze appears to be caused, among other things, by wear and tear on the paint. The top layers have disappeared, so you now look at Rembrandt’s light sketch.

Read about the restoration: Rijksmuseum discovers unknown sketch under ‘Nattevagten’

“I’m very happy about that kind of problem because it can provide so much insight. A painting is a small chemical factory where all kinds of reactions take place. It’s 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. It forms a kind of crust on the painting. It consists of lead, potassium and sulfur. Lead comes from pigments such as white lead or red lead, potassium is often found in pigments such as narrow and red lacquer. But that sulfur? It may come from the plaster, but we see it in many paintings. So I am convinced that this came from the environment and fell to the surface. In the 18th and 19th centuries, galleries were heated with coal stoves. Sulfur was released during that combustion. “

What else did you find striking?

“Everyone always thinks so The night watchman painted very dark, for that is how it looks now. But Rembrandt’s palette was much more nuanced and colorful than we thought. A blue pigment that narrows becomes browner with age.

“One 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 life, and we had never seen that with Rembrandt. Now we want to know how he used it and in what form. We investigate this in the laboratory. ”

Does the painting get darker after 380 years?

“It’s an important question. You hope it stagnates at some point. When you look at the cross-sections of the red lacquers, I’m impressed with how well-preserved it is. It gives me confidence that it has stabilized. To to investigate this, we are speeding up that kind of decay process in laboratories. “

Keune and colleagues use techniques from different areas. For example, optical coherence tomography. Ophthalmologists use this to create 3D images of the retina to detect abnormalities. Rijksmuseum uses it to visualize the height differences in the layers below the lacquer surface and thus find out how Rembrandt created depth. Another example is reflectance imaging spectroscopy to determine which pigments and binders Rembrandt used. NASA uses this technique to photograph and chemically identify minerals on Mars.

A certain reaction can take place on one micrometer and something completely different can happen ten micrometers away

What 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 wounds on clothing. “

After your research, the restoration of De Nachtwacht starts. What can you see from previous restorations?

“In the 19th century, they laid paintings as The night watchman in a coffin, along with all sorts of rags soaked in alcohol. They then allowed it to steam overnight to allow the lacquer layer to regenerate; a kind of rejuvenation cure. But that effect diminishes so they use it more and more. I think they have extracted a lot from the painting with it, such as binder. That may be one of the reasons why that dog is so blurry now. I would like to investigate this further. We are now looking at how to make the lacquer layer only swell with a minimal amount of alcohol so that no fatty acids or metal ions migrate from the layers below. We need to understand what you do with a material. “

You will give your inaugural lecture on 11 May. What are your plans?

“Until now, we have seen paint as a homogeneous layer, but it is a heterogeneous environment where different reactions take place. At one micrometer a certain reaction can take place, and ten micrometers further ahead something completely different can happen, resulting in two different effects on the painting. This heterogeneity is an unexplored area and it also brings me back to the fascination of my childhood. A material is not one thing. There is so much going on beneath the surface. “

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