Research techniques - glossary
Focused Ion Beam (FIB)
See: Nano-machining with the Focused Ion Beam.
Widespread use is made of this technique, based on invisible, warm infrared light, in the examination of paintings. It enables the researcher to study the painting underneath the visible surface and, for instance, detect the underdrawing: the preliminary sketch that the artist draws on the canvas.
Infrared light partly penetrates the upper paint layers and is reflected and absorbed to a greater or lesser degree by the carbon present in the underdrawing. The image captured by an infrared camera, the infrared reflectograph, then shows this drawing in grey tones. Infrared reflectography can also yield information about the pigments used and later restorations.
Infrared reflectograph (right) of View of Paris from Theo’s apartment in Rue Lepic (left). The lines visible on the reflectograph indicate the use of a drawing device to help with perspective.
A microscope is an optical instrument which uses lenses to reveal details that are invisible to the naked eye. The useful (‘sharp’) magnification that a microscope can achieve depends on the wavelength of the light and the quality of its lenses. By means of an optical microscope - even one with perfect lenses and lighting - only objects larger than half the wavelength of white light can be viewed. In practice, this means that maximum magnification is about 1000x. In order to study even smaller objects, i.e. achieve greater magnification, lighting with a shorter wavelength has to be provided. This can be done by electron microscopes with electromagnetic lenses and special detectors. Such microscopes use energy-rich electrons to capture an image of the target.
Nano-machining with the Focused Ion Beam (FIB)
Focused ion beam milling offers a new way to produce samples for the Transmission Electron Microscope (TEM). An ion is an atom or group of atoms that has been electrically charged by electrons. It is now possible to select the region where the section is to be made with sub-micron precision and to control the dimensions of the section to within a few nanometres thickness (1 nanometre = one millionth of a millimetre). Nano-machining avoids the problems that can occur with conventional preparation methods, such as crumbling of the paint sample. The ability to prepare such samples means that it is now possible to examine the morphology (form), structure and composition of the various pigments and binders used in paints at virtually atomic level.
A FIB section was prepared from the cross-section of the original ground sample from Bridge in the rain. In the first image, the site of the section is circled (SEM micrograph). The second image shows an overview of the section imaged with the TEM. The third image shows a TEM image of the circled region of the overview micrograph where a cluster of the boneblack pigment is visible.
Research into a painting always begins with examining the surface with the naked eye under normal lighting conditions. This direct inspection of the painting remains the standard against which all research is compared. Details can be enlarged with a magnifying glass or a microscope. Microscopic examination of the paint surface allows the study of features such as the brushwork, crack patterns and fingerprints.
To study the uppermost layer of a painting, researchers use an optical microscope. There are various kinds of microscope available, ranging from small models giving 3x to 10x magnification to large instruments with magnification factors of up to 50x. The microscope lenses enable the condition and certain physical aspects of the painting to be assessed, revealing details of the brushwork, the kinds of pigments used, the texture of the paint layer, restorations and damage. By means of a camera attached to the microscope, photomicrographs can be taken of the highly magnified image.
Polarized Light Microscopy (PLM)
This investigative tool enables pigments and fibres to be identified. The particles to be examined, generally between 1 and 20 μm (microns) in size - smaller than one-thousandth of a millimetre - are placed under a microscope and a polarized light source is shone through them from underneath. Each type of pigment and fibre reacts differently to these polarized light rays so that every single particle can be identified.
Examination with the aid of raking light is the next step after examination examining the surface under normal lighting conditions: a light source placed at the side of the painting emits light at an angle across the canvas. This assists study of the paint texture.
SEM-EDX (Scanning Electron Microscopy - Energy Dispersive X-ray analysis)
In a scanning electron microscope, the sample under investigation is exposed to an extremely narrow beam of high energy electrons. Some of these electrons are reflected from the surface of the sample, allowing this to be imaged on a monitor screen, usually in the form of a ‘back-scattered electron image’ or BEI. Other electrons lose energy by interacting with the atoms present in the sample, prompting the emission of x-rays; every chemical element reacts differently in terms of the quantity of x-rays produced. These are visualized in a graph, known as a spectrum, whose peaks indicate which elements are present in the sample.
This SEM image shows a pure lead-white ground, probably applied to the canvas by Van Gogh himself. It differs from the substances generally found on commercially prepared canvases, which tend to feature a thicker ground layer in which the lead white has been mixed with cheaper fillers. EDX has been used to confirm the composition. The EDX graph (picture below right) from the circled region (picture upper right) upper reveals the characteristic lead (Pb) spectrum.
Transmission Electron Microscope (TEM)
With an electron microscope, featuring electromagnetic lenses and special detectors, it is possible to examine very small particles. In a Transmission Electron Microscope, electrons are speeded up in a vacuum until their wavelength is extremely short. Beams of these fast-moving electrons are focused on an extremely thin paint sample (see nano-machining with the FIB) which absorbs or scatters them, thereby forming an image of the particles in the sample. A TEM can achieve a magnification factor of approximately one million, making it possible to view objects as small as an atom.
This sample is approximately 12 μm (microns) thick, 12 thousandths of a millimetre! This revealed that the large particles have a complex porous structure while the paint chiefly consists of evenly dispersed extremely fine white pigment particles in binding medium.
A light source can be placed behind the painting so that it shines through the canvas to highlight any breaks, cracks, loss of paint particles and thin areas.
Related: normal light, raking light, ultraviolet light.
Under ultraviolet light, the chemical elements of the pigments, binders and varnishes fluoresce in different colours. Each colour is indicative of a specific element, which enables the various pigments and types of varnish to be identified. UV fluorescence also allows later retouches to be distinguished, as they fluoresce quite differently than the original paint layer.
The right side picture shows Head of a woman in ultraviolet light. The resinous varnish is shown up as a greenish-blue fluorescence. The darker areas are where varnish has been removed for further examination and show the fluorescence of the oil paint underneath the varnish.
Like infrared reflectography, X-radiography is used to study the material underneath the upper paint layers. It is the same technique that is used in medicine to study the internal organs of the human body. X-radiation is electromagnetic radiation that has a shorter wavelength than visible light. X-rays penetrate fabrics quite easily and are obstructed (absorbed) by denser materials. Pigments with heavy metals therefore absorb more X-radiation than other pigments. These differences are shown up on X-ray images: thicker applied brushstrokes or brushstrokes with paint containing heavy metals show up lighter or even white. They can reveal, for instance, any changes made during the painting process such as overpainting.
See also: Re-using materials - Baskets of potatoes.