Methods and techniques 

HOW DO WE APPROACH OUR WORK?

Van Gogh’s Studio Practice is a multidisciplinary research project, investigating wide-ranging aspects from the perspectives of many different fields. Conservators and natural scientists conduct materials research on the works of Van Gogh and his contemporaries. Art historians (both curators and researchers) are responsible for the art history component. This collaborative approach, in which the two groups share information, yields new answers and new questions.

The research question at hand guides the researchers in their choice of methods and techniques. Pure art-historical research, for instance, may involve tracking down and reading the manuals mentioned in Van Gogh’s letters. The researchers investigate how widely read these books were in Van Gogh’s day and whether he put their lessons into practice. This last question is explored in collaboration with a conservator. The first step is always to study the work of art carefully with the naked eye. The conservator then decides what methods and techniques to use for further examination. In any case, the paint surface is always studied with the aid of a microscope. Infrared reflectography, X-radiography and analysis of paint samples – tiny cross-sections of the paint layer – can be used to reconstruct the creation of a painting. Layer by layer, researchers determine the physical structure of the work of art. This often leads to surprising discoveries about the materials used.

For examples of these methods and techniques in practice, see the blogposts.

See below – or choose one of the letters of the alphabet above – for detailed explanations of the individual methods and techniques.

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Eye: studying the paint surface with the eye

Vincent van Gogh, Avenue of poplars in autumn, 1884.Details from the photograph in raking light, clearly showing the texture of the paint.Vincent van Gogh, Head of a woman, 1885.The same painting in ultraviolet light. The resin varnish produces a greenish-blue fluorescence. The dark patches are the result of removing varnish for closer examination. They show the fluorescence of the oil paint under the varnish.

Normal light

Paintings are always examined first with the naked eye and under normal light. This direct study of the painting forms the basis for the entire investigation, the standard to which all subsequent research findings are compared. With a magnifying glass or microscope, a researcher can magnify details and features such as brushstrokes, cracks and fingerprints in the paint layer.

Raking light

The next step in examining a painting is to look at it under raking light. A light source is placed to one side of the painting, so that the light ‘rakes’ across the paint surface. This method can be helpful when studying the paint texture.

Transmitted light

It is also possible to place a light source behind the painting, so that it shines through the canvas, revealing any breaks, cracks, loss of paint particles or thin areas.

Ultraviolet light

When exposed to ultraviolet light, the chemical elements of pigments, binders and varnishes glow fluorescent colours. Each colour indicates the presence of a specific element. This makes it possible to identify a range of pigments and varnishes. Ultraviolet light can also make later restorations clearly visible, because they fluoresce very differently from the original paint layer.

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Infrared reflectography

Vincent van Gogh, View of Paris from Theo’s apartment in Rue Lepic, 1887.Infrared reflectogram of View of Paris from Theo’s apartment in Rue Lepic, with lines that indicate the use of a drawing aid in creating perspective.Schematic illustration of the technique of infrared reflectography

Examination of paintings often involves infrared reflectography. The researcher uses invisible, warm infrared light to look beneath the surface and detect features such as an underdrawing, a preliminary sketch made by the artist on the canvas before painting. Some of the infrared light penetrates the top paint layers and is reflected or absorbed in varying degrees by the carbon in the underdrawing. An image captured by an infrared camera, called an infrared reflectogram, shows this drawing in shades of grey. Infrared reflectography can also provide information about the pigments used and about later restorations.

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Microscope

Vincent van Gogh, Autumn landscape with four trees, 1885 (Kröller-Müller Museum).Details illustrating the different types of brushstrokes in Autumn landscape with four trees.

A microscope is an optical instrument with lenses that reveal details of an object which are invisible to the naked eye. The degree of useful magnification provided by a microscope depends on the wavelength of the light source and the quality of the lenses. Even with perfect lenses and lighting, an optical microscope is only helpful for studying objects larger than half the wavelength of white light. In practice, the maximum magnification is about 1,000 times. To see even smaller particles, we need greater magnification. This requires a shorter wavelength than visible light. For this purpose, we use electron microscopes with electromagnetic lenses and specialised detectors. Instead of light, these microscopes use high-energy electrons.

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Nanomachining with a focused ion beam (FIB)

A FIB section was prepared from a cross-section of the original sample from the ground layer of Bridge in the rain. In the first photo (an SEM micrograph), the place from which the cross-section was taken is circled. The second image, made with TEM, shows the entire FIB section. The third photo is a TEM image of the area circled in the second photo, with a high concentration of the pigment animal black.

Nanomachining is a fairly new method of preparing paint samples for Transmission Electron Microscopy (TEM). This technique involves a focused ion beam (FIB). An ion is an atom or molecule which has an electrical charge because of the electrons in it. A beam of ions can be used to carve out a paint sample with great precision. This enables much greater control of the exact location from which a paint sample is taken, as well as of the thickness of the section, which can be controlled to within a couple of nanometres (a nanometre is one-millionth of a millimetre). Nanomachining avoids problems that can accompany traditional methods of preparation, such as crumbling of the paint sample. This technique reveals details of the morphology (form), chemical structure and composition of pigments and binders in the paint at a nearly atomic level.

The accompanying photo is a FIB section of the original paint sample from Van Gogh’s Bridge in the rain (1887). It was prepared from a cross-section of the original sample from the ground layer of Bridge in the rain. In the first photo (an SEM micrograph), the place from which the cross-section was taken is circled. The second photo, made with TEM, shows the entire section. The third photo is a TEM image of the area circled in the second photo, with a high concentration of the pigment animal black.

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Optical microscopy

To study the uppermost layer of a painting, a researcher can use an optical microscope. There are various kinds of microscopes, varying from small sizes that magnify 3 to 10 times to large ones that magnify up to 50 times. Magnification with optical lenses helps us to learn more about the condition of the painting and certain material features. It reveals details of brushstrokes, the nature of the pigments, the texture of the paint layer, restorations and damages. A camera can be attached to the microscope to take highly magnified photographs of the work of art, known as micrographs.

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Polarised light microscopy (PLM)

Polarised light microscopy is a research technique for identifying pigments and fibres. The particles to be examined, generally between 1 and 20 microns in size – smaller than one-thousandth of a millimetre – are placed under a microscope and a polarised light source is shone through them from below. Because each type of pigment and fibre reacts differently to these light rays, each individual particle can be identified.

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Sampling

Dutch-language video

Sampling and sample analysis

A paint sample is removed from the painting with a small knife. The sample is lifted from the canvas with a very fine brush. The sample is placed in a vial.After the sample is embedded in resin, ground down and polished, it provides a cross-section of the painting, which can be examined under a microscope.Here we see one cross-section from a paint sample. From bottom to top, it shows three different layers of the painting: the ground, the paint and the varnish.

After studying the paint surface, we can remove tiny paint samples, no larger than the head of a pin. These samples cut across the layers of the painting: the varnish, the paint and the ground. Because taking a sample involves permanently removing part of the paint layer, the technique is used sparingly. This type of cross-section can help us to investigate the composition and structure of the paint layers. The paint sample is prepared for examination by pouring it into a small block of synthetic resin. After the resin cures (that is, hardens), it is ground down and polished until only an ultra-thin layer of resin remains. Through the resin, the paint layers are visible on the surface. An optical microscope is used to magnify the paint sample 100 to 1,000 times. The researcher examines the sample under normal and ultraviolet light. This makes it possible to study not only the composition of the paint layers, but also the individual particles of pigment.

Scanning electron microscopy – energy-dispersive analysis of X-radiation (SEM-EDX)

Sketch illustrating the SEM-EDX analysis technique.SEM image of the original paint sample from Vincent van Gogh's Banks of the Seine (1887), indicating a pure white lead ground layer. This ground was probably applied to the canvas by Van Gogh himself. It differs from the ground usually found on commercially prepared canvases, in which the white lead is often mixed with less expensive fillers and applied more thickly. The composition of the ground was determined with EDX (energy-dispersive X-ray analysis). The EDX spectrum of the area circled in the black-and-white image is characteristic of lead (Pb).

In this technique, a scanning electron microscope directs a narrowly focused beam of high-energy electrons at the paint sample. Some of these electrons are reflected back by the sample. This makes it possible to create an image of the surface of the sample on a screen, often in the form of a backscattered electron image (BEI). Other electrons lose energy through interaction with the atoms in the paint sample. Some of this energy is released in the form of X-rays. When bombarded with electrons, each element produces X-rays with characteristic energy levels. By recording the X-ray energy peaks in the form of a spectrum, we can determine what elements are present in the sample.
This technique is known as SEM-EDX, which stands for scanning electron microscopy and energy-dispersive analysis of X-radiation.

Learn more in the Dutch-language video below.

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Transmission electron microscopy (TEM)

Electron microscopes have electromagnetic lenses and special detectors that enable study of very small particles. A transmission electron microscope accelerates electrons in a vacuum until their wavelength is extremely short. A beam of these accelerated electrons is then aimed at a very thin paint sample (see the image to the right). Because the sample absorbs some electrons and transmits others (that is, allows them to pass through), the detector on the other side can create a detailed image of it, magnified up to about one million times. The technique makes it possible to study objects as small as a single atom.

This sample is approximately twelve microns thick, or twelve-thousandths of a millimetre. The electron microscope reveals that the large particles have a complex porous structure. The tiny, evenly distributed particles of white pigment come from the binding medium.

Transmitted light

A light source can be placed behind the painting so that it shines through the canvas, revealing any breaks, cracks, loss of paint particles and thin areas.

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X-radiography

Vincent van Gogh, Baskets of potatoes, March-April 1885.This X-ray photograph of Baskets of potatoes has been rotated 90 degrees. The photograph shows the underpainting – apparently the same scene as Woman sewing, which Van Gogh had painted half a year earlier.Vincent van Gogh, Woman sewing, September 1885.

X-radiography, like infrared reflectography, is used to examine material under the top layer of paint. This is comparable to the medical use of X-rays to examine the inside of the body. X-rays are a form of electromagnetic radiation with a shorter wavelength than visible light. They therefore penetrate low-density materials relatively easily and are blocked (that is, absorbed) by higher-density materials. For instance, pigments containing heavy metals absorb more X-radiation than other pigments. These differences are visible in X-ray images: thick dots or strokes of paint containing heavy metals appear lighter in colour, or even pure white. X-radiography can be used to detect changes made during the painting process, such as overpaintings.