Examination Methods
Technical photography
To get the scientific examination underway, art and research photographer René Gerritsen photographed the Girl with the Pearl Earring with a digital camera on Sunday 25 February.
Gerritsen used several light sources when. To record the painting’s current condition, he photographed it in daylight. He then photographed it under ultraviolet (UV) light. In UV, the varnish layer fluoresces (emits light) and later restorations (from 1994) can be distinguished from the original paint as they show up as dark areas. To capture the texture of the paint surface, Gerritsen took an image using raking light. By shining a bright light across the painting’s surface at a sharp angle, the 3-dimensional texture of the paint is revealed. Finally, he used infrared reflectography to record images in which, among other things, the black pigments beneath the painting surface are visible.
These images will serve as reference material throughout the examination.
X-ray technology
René Gerritsen, who specialises in art and research photography, took x-ray images of the Girl with a Pearl Earring. An x-ray beam is aimed at the painting and an x-ray film behind the painting captures any radiation that passes through the canvas. The thickness and density of the paint determine how much radiation reaches the film.
X-ray images can provide information about the use of the pigment lead white. Lead is a relatively heavy element and absorbs a high level of radiation. The image below shows that there is a lot of lead white in the collar, pearl earring and white of the Girl’s eye. The structure of the woven canvas is also visible. So is the stretcher over which the canvas is pulled taut, and the nails that were used to attach it to the stretcher.
The black-and-white x-ray images are examined on a light box and digitised for further research.
MA–XRF
The focus of the examination with macro X-ray fluorescence (Macro-XRF) imaging is the chemical composition of the pigments, such as iron in earth pigments and mercury in vermilion. MA-XRF analyses how X-rays are emitted by the paint. This reveals, for example, how the iron or mercury are distributed across the painting. Annelies van Loon, who specialises in the scientific examination of paintings and works at the Rijksmuseum and Mauritshuis, will be operating the MA-XRF scanner.
The entire surface of the painting is scanned: from left to right, and top to bottom. As well the surface of the paint, the layer beneath it is also revealed.
Earlier research has shown that the paint layers of the clothes and headscarf contain pigments that are prone to degradation and may discolour over time, for example: potassium and calcium associated with ultramarine, and yellow and red organic lake pigments. The question is: how are these and other elements distributed across the painting?
This will tell us more about the layer structure and original appearance of the Girl’s jacket and headscarf.
The MA-XRF scanner being used to carry out the examination has been provided by the Rijksmuseum, Amsterdam. It was developed specifically for art historical research by the University of Antwerp and Delft University of Technology. This research technique is currently also being used in forensic science.
NIR (Near-infrared imaging)
NIR makes it possible to see pigments that are usually invisible to the naked eye. This gives us more information about the layer structure of the painting. The focus here is on charcoal and other black pigments that contain carbon. Researchers John Delaney and Kate Dooley from the National Gallery of Art in Washington will be examining the painting using various infrared cameras to look through the paint layers.
There are three kinds of infrared radiation: near, mid and far, depending on the wavelength. Pigments in the paint can either absorb near-infrared (NIR), reflect it or allow it to pass through. In particular, black pigments containing carbon become visible with near-infrared imaging. These pigments absorb the infrared light, which might reveal an underdrawing or later restoration.
The researchers first map the pigments individually, before identifying the material. This information is then brought together in an ‘image cube’, an accumulation of all the material and technical data gathered about the pigments. This provides more information about the paint layers than the individual techniques in isolation.
FORS (Fibre optic reflectance spectroscopy)
Which pigments did Vermeer use to paint the blue headscarf? FORS (Fibre optic reflectance spectroscopy) makes it possible to determine the properties of the blue pigments.
This is another examination technique that uses a light source. Depending on a pigment’s properties, the light will either be reflected or absorbed. This information is recorded in a diagram that displays the wavelengths of the different blue pigments. From this, researcher John Delaney and Kate Dooley from the National Gallery of Art (Washington) can analyse the raw materials used by Vermeer. FORS can also provide information about the binding medium: the oil with which the pigments were mixed to make the paint.
OCT (Optical coherence tomography scanner)
On the surface of the Girl are translucent layers such as glazes (semi-transparent paint) and varnish. Tom Callewaert and Joris Dik from the Delft University of Technology will be examining these translucent layers with an OCT scanner to learn more about their structure.
Using OCT (optical coherence tomography), the researchers can take a ‘virtual bite’ of the glaze and varnish layers. They can then display this as a cross-section so that the build-up of the translucent layers is visible.
Each of the translucent layers has a different brightness in the scan, which reveals the structure of the translucent layers. The entire painting will be scanned, but the researchers will pay special attention to the blue glazes in the Girl’s headscarf. Vermeer suggested volume in this fabric by alternating areas of light and shade. How did he apply the translucent blue glazes over the light blue base layer?
Ultra-high resolution colour / topography scanner
Fine cracks appear in paintings as they age. This craquelure is particularly visible in the Girl’s face. By taking ultra-high resolution images at a microscopic level with a colour/topography scanner, we can record the network of cracks.
Under the leadership of Joris Dik, a team of colleagues and PhD candidates from the Delft University of Technology will be applying this technique to various areas of the painting. The information gathered is important as it ensures that any future development or spread of the craquelure can be closely monitored.
A grid of thin horizontal and vertical lines is projected onto the painting. Two cameras positioned to either side of the painting measure where and how the paint deviates from the digital lines. A computer translates this information into a ‘topographical map’ of the craquelure.
HIROX digital microscope
The HIROX hybrid 3D digital microscope makes a three-dimensional profile of the surface of the painting, revealing the differences in height of the paint layers.
This microscope can magnify details up to 7000 times. Researcher Emilien Leonhardt will first zoom in on a few small areas: the eyes, lips and pearl earring. He will then lay the painting on the table so that larger areas can be scanned. Because the microscope head can rotate through 360 degrees, every element can be recorded in great detail – from individual brushstrokes to pigment particles.
The white paint that Vermeer used to suggest the reflection on the pearl is applied more thickly than the surrounding paint. Peaks and valleys are visible in this white reflection.
The digital microscope has been loaned to us for the purposes of this research by Hirox Europe: Jyfel.
MA-XRPD (Macro x-ray powder diffraction)
Colours can change over the course of time. Light, for example, can alter chemical compounds in the paint. Vermeer used the pigment ultramarine (ground lapis lazuli) in the Girl’s blue headscarf. This pigment’s chemical compound has a crystal structure. Over time this structure can change, affecting the colour that we see.
The University of Antwerp’s AXES research group will be using a MA-XRPD scanner to examine the crystal structure of the pigments and how they have changed over time. The painting is scanned from left to right and top to bottom. Every test point is exposed to a small amount of x-ray radiation. The (diffraction) pattern reflected back by the crystal structures in the pigments is like a unique fingerprint. These patterns can then be identified by cross-referencing them with a database of known crystal structures.
This fingerprint may also provide information about the origin of the pigments, such as where the lapis lazuli was mined.
Colour/Gloss/Topography scanner
Before the Girl with a Pearl Earring is returned to Room 15, researchers from the Delft University of Technology will carry out one last examination.
In total darkness, they will project a grid of horizontal and vertical lines onto the painting. Two cameras positioned to either side record any deviations from these lines. This provides topographical information about the upper layers of the painting. The colour/gloss part of the examination focuses on the background of the Girl. This background was originally a dark greenish-black, but the colour and gloss have changed over time.
This information will also be useful for the creation of high-quality 3D prints of the painting in the future.
