Synchrotron culture

Most of us are familiar with the first X-ray image in medicine: Wilhelm Röntgen’s wife’s hand, complete with wedding ring, in 1895. However, few people know that when Röntgen first communicated the result, he also reported the first X-ray radiography of a painting – a piece of wood painted with lead white pigments. Three months later, a friend of Röntgen described the results: “In a wooden box the tablets containing metallic pigments, such as cinnabar, chromium yellow and Berlin blue can be separated from those tablets that do not contain metallic pigments, for example carmine and gamboges, with the cover closed.” Subjecting paintings and other works of art to X-ray radiography caught on fast in the cultural-heritage community, and today synchrotrons are at the forefront of this powerful technique.

Toxic revelation

One criticism commonly associated with such studies, which recently include chemical analysis of Van Gogh paint samples at the ESRF, is that their impact is due to the beauty of objects rather than the scientific value of the work. Research on fascinating artefacts certainly appeals to a wide audience, but working on a famous work of art is not sufficient in itself to get beam time on the ESRF. The ESRF’s Environmental and Cultural Heritage Science (EC) review panel is vigilant to the scientific value of the proposals. One can’t underestimate the importance of such experiments for historical knowledge or for preserving artefacts for future generations. Besides its scientific relevance, the media interest increases the visibility of synchrotron facilities in general.

In 2009, researchers used the ESRF to study cadmium sulphide yellow pigment from James Ensor’s 1921 Still Life With Cabbage, which fades over time as small white globules appear on the paint surface (Credit: Kröller müller museum).

I encountered synchrotrons at the ESRF’s ID22 beamline in 2001, while I was training as a chemist at the Centre of Research and Restoration of French Museums beneath the Louvre Museum in Paris. Our team studied mummy skins and also modern skins that had been treated with chemicals mimicking ancient cosmetics, which contained lead, mercury and arsenic – all of which are now known to be highly dangerous. Focusing on lead, we assessed the toxicity and studied the chemical reactions in these ancient practices by combining infrared micro-spectroscopy with X-ray micro-fluorescence to follow drug penetration within the skin. The former was done at the LURE facility, while the latter was at ID21, but since 2004 it has been possible to combine both techniques at the ESRF.

At that time there had already been some studies at ID21 into the degradation mechanisms in wooden warships or in metallic artefacts. But since then, particularly during the last five years, cultural-heritage studies have taken off at the ESRF and elsewhere. My research has since shifted from cosmetics to paintings, but both fields are based on similar mixtures of pigments and binders, for which the mechanical properties are important.

The composition of whitish/transparent globules that form on the surface of Ensor’s painting (top) was revealed at the ESRF. µ-XRF was used to map the cadmium (red), sulphur (green) and lead (blue) in the white rectangle.  Credit: Anal. Chem. 81, 2600–2610 (2009).

Broad impact

The complex mixture of inorganic, organic and even hybrid materials in artistic materials drives innovative techniques that benefit the entire synchrotron community. An example is the fitting programme PyMca, which is not only used daily on many ESRF beamlines for X-ray fluorescence but also in the wider scientific community. The software was initially developed to get rid of complications caused by the interference of emission lines from light and heavy elements in X-ray fluorescence, which can determine the elemental composition of the complex mixtures and proved crucial in the analysis of the sfumato technique that da Vinci used for his Mona Lisa. More recently, ID21 staff have developed a wavelength-dispersive spectrometer that is ideal for such line separation. It already has several applications for analysis of ancient materials, and is another example of a technique driven by cultural heritage research that will benefit other fields.

A decade ago, the cultural heritage community viewed synchrotrons as inaccessible, even fearsome, entities. Step by step, researchers have realised that the techniques on offer, be they infrared imaging or X-ray diffraction, are natural extensions of familiar laboratory techniques. The next challenge is to make techniques that are more specific to synchrotrons (refer to graph), such as X-ray absorption spectroscopy, more accessible to the cultural-heritage community.

State of the art
A decade ago, studying art or ancient objects with synchrotrons was perceived as rather exotic, but in the past five years the ESRF has been at the forefront of a surge in such research. An important step was the establishment of the Environmental and Cultural Heritage Science (EC) review panel committee in 2005. More recently, France’s SOLEIL source has established a cultural-heritage institute (IPANEMA) with a dedicated beamline (PUMA), which is currently under construction, demonstrating a reciprocal interest between synchrotron and cultural-heritage communities. Dedicated international conferences, such as Synchrotron Radiation for Art and Archaeology, which started at Grenoble in 2005, are also driving momentum. Synchrotrons including the ESRF, SOLEIL, ELETTRA, PETRAIII, Spring8, DIAMOND and the Australian Synchrotron advertise their cultural-heritage science on their web pages, seeing it as an ideal way to showcase the capabilities of new instruments.

XAS on the rise

Most cultural-heritage studies employ X-ray fluorescence or diffraction, natural extensions of laboratory techniques to deliver information about local elemental composition and crystallographic phases. But X-ray absorption spectroscopy (XAS) is an underused tool that reveals the chemical state of a sample, allowing researchers to address its mode of manufacture or ways in which to preserve an artefact. XAS publications are on the rise, addressing a widening range of materials (right) but the technique is yet to be widely adopted by the cultural-heritage community.

Matthew Chalmers

This article appeared in ESRFnews, June 2011. 

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