Colour science and retouching

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A retouching palette

Van Gogh and his contemporaries were keen to apply modern theories of colour science to the art of painting. For me, an innovatory aspect of The bedroom treatment was the opportunity to work together with colour scientist Roy Berns in order to select an ‘ideal’ retouching palette that I could use to touch in old paint losses (so-called ‘inpainting’). Based on the known spectral properties of the Gamblin range of retouching colours, it was possible to determine a recipe to match different spots of colour that had previously been measured on the painting.

Importantly, this match is predicted to persist for different light sources, so that my retouches would not suddenly ‘change’ if the painting is hung in different lighting. I was curious as to how this process would compare to the usual trial-and-error approach of mixing and matching colours by eye.



Trial and error

Whilst waiting for the colour recipes and some missing pots of colour to arrive, I experimented with the Gamblin colours we had in stock. Intuitively, it turns out that I did reach the same choice of blue pigments that, based on calculation, would be recommended for use in the blue walls and outlines of the door panels (cobalt blue and ultramarine respectively) mixed with titanium white.


The recommended set of colours

However, I had problems with matching the elusive chrome yellow colour of the foot end of the bed with its aged greenish-brown translucent skin. Great was my delight when the recommended set of Gamblin colours arrived from America and indeed turned out to provide a very good basis for retouches in the bed frame (roughly equal quantities of  Naples yellow deep and Permanent green light with a little Dioxazine purple and Titanium white added).

Detail of the foot end of the bed after cleaning, before retouching.

Detail of the foot end of the bed after cleaning, before retouching.

The same area after retouching. Some old residues of brown overpaint that could not be safely removed have been touched out with new retouches.

The same area after retouching. Some old residues of brown overpaint that could not be safely removed have been touched out with new retouches.


Science and intuition

I am quite certain I would not have reached the same choice alone, since especially the rather lurid  Permanent green light seen in the pot was not an obvious candidate to me. Working with this basic set of colours, the old-fashioned skills of the conservator remained essential to make the subtle adjustments of colour, texture, opacity and layering required at each spot on the painting. As for Van Gogh, it seems that colour science and intuition can work well hand-in-hand.

Retouching materials and retouching palette

Retouching materials and palette


Finding the suspect of the discolouration of the floor

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What material is responsible for the discolouration of the floor?  

During the examination of The bedroom, Ella asked me to come by and look at the painting with her. She had noticed something that was probably a result of the materials used by Van Gogh. Specifically, she was curious about why the floor in The bedroom had changed colour. What material could be responsible? Could the paint contain eosin, an organic red pigment that we had found in the ground (the first paint layer) during our earlier examination of the painting Daubigny’s garden? 

Cross-section of a paint sample from Daubigny’s garden.

Cross-section of a paint sample from Daubigny’s garden. The ground (the pink paint layer at the bottom) consists of a mixture of lead white and eosin.


Geranium lake 

Eosin is a synthetic dye, first produced in 1873. It is a compound of carbon, hydrogen, and bromine. When this dye is precipitated on a substrate (usually alum), it produces a pigment that can be used in oil paint. This pigment was sold under the name of Geranium lake, because it is similar in colour to geraniums. Even though Van Gogh knew that Geranium lake faded when exposed to light, he used it frequently. In eight letters to his brother Theo (from Arles, Saint Rémy, and Auvers-sur-Oise), he asked for Geranium lake from the Paris firm of Tasset et l’Hôte. In total, he ordered no fewer than 44 tubes! 


Searching for evidence 

To take a closer look at the discolouration of the floor in The bedroom, we first asked our colleague Luc Megens to analyse the painting with X-ray fluorescence (XRF) spectrometry. This technique involved direct examination of the painting. In the floor in the foreground, he established the presence of zinc, lead, and mercury, but not of bromine. We were disappointed, because bromine is a telltale sign of eosin. This finding suggested that no eosin, or very little, was present in the pigment that Van Gogh had used for the floor. 

But I was not ready to give up. I obtained a sample from the bottom edge of the painting and took a cross-section. 

Cross-section of a sample from the bottom edge of The bedroom.

Cross-section of a sample from the bottom edge of The bedroom. On top of the light ground, we can see the pink paint layer used in the floor.

This made it possible to study the composition of the paint layers under a microscope, at a magnification of approximately 1,000 times. With the aid of a scanning electron microscope with energy-dispersive X-ray spectroscopy (SEM-EDS), which I use with Kees Mensch at Shell, I managed to identify most of the pigments in the pinkish paint layer. It proved to be a mixture of zinc white and lead white, coloured with small amounts of vermilion, Schweinfurt green, and chrome yellow. But this did not answer my question, because it did not seem possible that these pigments could be responsible for the discolouration in the floor. 

In a final attempt to discover the cause of the discolouration, another colleague of mine, Maarten van Bommel, took a sample of the pinkish paint used for the floor. He analysed this sample using a technique called high-performance liquid chromatography (HPLC), which is particularly well suited for analysing dyes. What he discovered was that, in fact, there was a small amount of eosin in the pinkish paint. And just as we’d suspected, the eosin was almost certainly the cause of the discolouration. Finally, the mystery was solved!

Restoration with a sewing machine

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The edges

Close scrutiny of the edges of paintings that are usually covered by the frame can provide valuable evidence for changes occurred over time, such as faded colour areas, or later adjustments in picture format (both topics to be covered in future blogs on The Bedroom).

Sewing and ironing

Another peculiar feature discovered on The Bedroom though, was the remnants of two tiny rows of even stitches in light-coloured thread through the tacking margins folded over the sides of the stretcher.

Tacking edge of The Bedroom

Detail of the tacking edge, with stitches












Evidently a sewing machine was run twice around the edges of the painting, something which, however carefully done, makes one shudder to think now. This feature was familiar from other Van Gogh paintings in the collection lined by the restorer Traas in the period 1926-1933, so that I could immediately recognize it as a characteristic of his early lining practice. The sewing was to attach fabric margins that extended the edges of the original canvas, so that it could be tensioned on a loom for lining by the so-called Dutch method. In this process, both the original and lining canvases were stretched on looms, brought into contact, and bonded by impregnation of wax-resin spread with a hand-held hot iron.

Vulnerable paint

In view of the vulnerable impasto (thickly applied paint), the painting would have been ironed face down on a cushioning surface, such as a felt mat or levelled sawdust bed, though this meant that the paint surface could not be monitored during the process.








In the case of The Bedroom it is striking  how the added lining with wax has significantly increased the stiffness and weight of the original canvas. 

I am sure that you can think of other drawbacks of this method, which has been practiced by restorers on such a wide scale in the past!

Completion of preliminary examination

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At the moment I’m still winding up the preliminary examination of The bedroom. This preliminary examination has taken about a year: understandably, since you don’t start on the restoration of a masterpiece without being fully prepared! I removed the work from the exhibition room a few times to study it at close quarters, to make X-ray photographs and infrared images, or to take minuscule samples of the paint.

Collaboration with scientists

These paint samples were then studied thoroughly by scientists at the laboratories of the Netherlands Institute for Cultural Heritage (ICN) and Shell. They have apparatus there that is even more advanced than the equipment in our own restoration studio. This research turned up answers to our questions, answers that we needed before we could embark on a new treatment of The bedroom.

Look at the video we made last year:


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The bedroom has been removed from its frame, and awaits me in the restoration studio. I am eager to get started on this restoration! Not long after my appointment as head of conservation at the Van Gogh Museum in 1999, I was given the file on this painting. I read that the painting has been on the list of works awaiting restoration since the 1980s. Still, it’s a good thing that we waited, since the research techniques available to us now have taught us far more than would have been possible twenty years ago.

A special trolley is used to move The bedroom safely from the museum’s exhibition room to the restoration studio.


This painting has been through a great deal since the moment it left Van Gogh’s easel. Some of the damage took place while Van Gogh was still alive, as we know from letters that Van Gogh wrote to his brother Theo. For instance, the artist rolled the painting up, so that he could send it through the post.

What is more, the flooding very close to Van Gogh’s house explains a great deal of the craquelure (tiny cracks in the paint) and paint loss. This photograph of the paint surface, made using a light microscope, clearly shows the marks left by the passage of time.

Through the light microscope, you can see the tiny cracks in the paint that have formed over the years.

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