X-ray is an electromagnetic radiation. However, by possessing much shorter wavelengths than, say, visible light, X-rays have a far stronger penetrating power. As photons from X-rays pass through a painting, each material’s atoms absorb the photons differently. Pigments containing lead or mercury, for example, strongly absorb X-rays and, as such, prevent the X-rays from passing through the painting to expose the sensitive X-ray film. The heavier the material, the more resistance there is to the X-ray radiation. This means that X-rays can help elucidate not only the process of a painting’s construction but also its construction, making visible the nails and support mechanism.
Importantly, X-rays sometimes reveal pentimenti (changes in the drawn line of an artwork) which, when supported by accurate comparative data, can be useful in ascertaining the attribution of an artwork. X-rays can also be useful in establishing underdrawings. For example, the National Gallery’s Portrait of Frederik Rihel on Horseback (c.1663) which, in 2010, after X-radiography, was found to have a portrait previously hidden under the surface of paint which is oriented 90 degrees clockwise to the finished portrait.
From an entirely divergent context to Rembrandt, X-rays have been crucial, for example, in enabling researchers to reconsider the painting practices of modern and avant-garde artists like Kazimir Malevich. Although it may seem at first that Malevich’s Suprematist works, like Painterly Realism of a Football Player (1915), were painted over a painted base layer of colour, the X-rays conducted in 2014 at the Art Institute of Chicago illustrated what might be described as a halo formation around the compositional elements of the painting. X-ray, in this way, confirmed experts' knowledge of Malevich’s practice of painting the background of his paintings once the composition had since been established.
X-Rays are very useful in this respect because they show us a lot about the hand of an artist, although access to comparative data is limited as only a relatively small number of museum artworks have been X-ray scanned and had their results made public.
An MA-XRF scan simply involves an X-ray scanning the entire surface of a painting pixel by pixel, causing the emission of X-ray fluorescence radiation. From the energy of the emitted radiation, elements present can be identified, made visible through elemental distribution images which allow the visualisation of paint layers beneath the surface of the painting.
MA-XRF is a particularly useful tool for researchers because it is able to identify certain materials that are difficult to see otherwise, either with the naked eye or through other imaging techniques.
This process enables the determination of the elemental composition of any given sample, with the results being depicted on a spectrum as a number of peaks of various intensities each occurring at a characteristic wavelength, as illustrated below.
MA-XRF is particularly useful in identifying pigments and materials that are difficult to see otherwise, whether with the naked eye or through alternative imaging techniques. The pigments that are most likely to be blocked by X-rays are made of lead, meaning that many less heavy pigments, like umber, for example, will oftentimes be left unnoticed. This is a problem in the context of seventeenth-century painting, for example, because much of the underpainting of artists including Rembrandt and Rubens includes a heterogenous mixture of pigments, including earth pigments like umber. MA-XRF enables researchers to identify these pigments and, therefore, effectively visualise underdrawings and thus contribute to any assessment of an artwork’s authenticity.
When a painting is exposed to UV radiation, particular aged organic materials such as varnish and binders will fluoresce. UV fluorescence microscopy is very useful in identifying the distinctive layers of a painting and in revealing areas of retouching. Ultraviolet lights are used primarily as a tool to identify surface inconsistencies such as inpainting or fills.
A significant number of known forgeries have been intentionally damaged and retouched as a technique for forgers to artificially ‘age’ a painting.
Infrared radiation is invisible, lying at the lower energy side of the electromagnetic spectrum. Infrared radiation can cause the bonds in a molecule to vibrate, meaning that there will be a wide range of vibrational modes depending on the atoms that forming the sample. This is significant because, since every molecule has a unique structure, the vibrational modes will act almost like a fingerprint for the molecule. Infrared imaging, in this way, enables the recording of the variable absorption and reflectance of infrared light by an artwork. This can reveal different groups of pigments and inks as well as hidden details under the upper layers of a painting, including underdrawing, watermarks or hidden signatures.
At Hephaestus, we have seen a large number of scientific reports substantiating claims of authenticity from the misleading analysis of imaging results.