EXAMINATION METHODS AND SCIENTIFIC TERMS

X-RAY FLUORESCENCE

What is X-Ray Fluorescence?
History of X-Ray Fluorescence
Types of XRF Systems Commonly Used
How does X-Ray Fluorescence Work?
Related Links and Resources
References

What is X-Ray Fluorescence?

X-ray fluorescence (XRF) is a non-destructive technique that provides elemental information (generally for elements that have an atomic weight above aluminum) that is representative of an artwork’s surface. When applied to paintings, XRF can identify the presence of certain pigments (inorganic materials as opposed to organic dyestuffs) using an x-ray energy source, which can help answer questions relating to authenticity and provenance. As the technique does not require sampling, XRF is an excellent preliminary method that can be used to help determine whether additional sampling is necessary. Today XRF units can be found in museums and institutions worldwide, with nearly 1500 units being used for cultural heritage applications.

Care should be taken when interpreting results as peaks can arise from other sources (such as the instrument itself): therefore consultation with a conservation scientist is essential. Other complications such as the use of metal driers (e.g. driers containing lead or manganese), pigments in underlying paint layers, and mordants present in organic colorants (e.g. Ca, Sn) can lead to misinterpretation. While the technique is relatively easy to perform, data interpretation is often more complicated and requires familiarity with both the artwork and the system being used.

History of X-Ray Fluorescence

It is difficult to confirm exactly when XRF was first applied to the study of easel paintings but it was certainly being used to examine objects (e.g. archaeological materials) by the late 40s. A decade later scientists began using the technology to analyze surface coatings associated with painted surfaces and by the early 1970s XRF had gained popularity as a tool for cultural heritage research. As soon as handheld units were developed, XRF was extensively used in the geology field (to identify ores, soil components, etc.) as well as in the recycling industry for segregating alloys or various metallic objects. Since around 2000, however, these portable units have become a popular resource for in-situ analysis within archaeological and museum settings.

Types of XRF Systems Commonly Used

Scientists generally refer to the technique as micro or µXRF when it is used to analyze an extremely small area on the surface of an artwork. Nearly all XRF systems are described and marketed as being portable yet the handheld units are considered the easiest to transport and collect data in-situ (e.g. within a gallery setting). However, the less-transportable units generally have better detectors, can achieve a higher resolution, and have the ability to suppress the interference caused by atmospheric gases (by replacing nitrogen and oxygen with helium). On the other hand, these larger systems are bulkier and more difficult to maneuver from location to location.

The handheld units are valued for their portability as they are relatively easy to “point and shoot.” Drawbacks include lower resolution, less sensitivity to lower elements, and the fact that the instrument needs to be almost touching the surface of the artwork in order to eliminate potential interference with atmospheric gases. In both cases it is important to note the kind of detector and x-ray tube source used and whether analysis is being conducted under helium purge or vacuum (to eliminate the presence of oxygen/nitrogen to help in detecting lighter elements).

Today, enormous strides in XRF research have enabled scientists to couple these systems with scanning technology. This has allowed the analyst to gather elemental information across the entire surface of an artwork, instead of relying solely on a handful of small areas to identify and characterize certain pigments. This technology was first performed at synchrotron-radiation facilities in the early 2000’s; however, use of these facilities necessitates that the artwork be transported directly to the site, remaining in place for multiple days of data collection. Now, macro-XRF scanning systems have been constructed that can be brought directly to a museum or other institution, although several days are still required for collecting and interpreting the data.

How does X-Ray Fluorescence Work?

An x-ray beam first originates from an x-ray tube source inside the XRF instrument. The x-ray has sufficient energy to temporarily excite an electron from the inner shell of atoms present near the surface of the artwork. When electrons leave the inner shells they create vacancies, causing the atom to become temporarily unstable. Stability is regained when electrons from outer shells “drop down” into these newly created vacancies. As these outer electrons move from a higher energy state to a lower energy state, they release fluorescent x-rays, a specific quantity of energy that can be measured by the detector on the XRF. Generally, elements that are heavier than aluminum can be easily detected. Meanwhile, electrons from lighter elements produce fluorescent x-rays that are lower in energy and absorbed by the air prior to reaching the detector. This problem is often addressed by displacing the oxygen and/or nitrogen in the air with helium or by pulling a vacuum on the instrument.

Because electrons exist at discrete energy levels for each atom, the fluorescence emitted by each electron is unique and can therefore be used to identify different elements (e.g. lead, iron, mercury, etc.) within art materials. Each of the characteristic fluorescent x-rays then appear as individual peaks on the XRF spectrum. Atoms that are larger (have a higher atomic weight such as lead) generate multiple peaks as they contain more energy shells and thus more electrons.

Many complications can arise during analysis. If an atom is present in large quantities, it can produce sum peaks that may be mistaken for an atom that is not present. In addition, certain elements (like lead) can overlap peaks generated by other atoms (such as tin) making it difficult to confirm if two different pigments are actually present (e.g. lead white and lead-tin yellow). Enhancement peaks can also occur when fluorescent x-rays leaving one atom have enough energy to excite electrons in another element nearby. Underlying layers in a painting can also generate peaks as well as can metal-containing driers (e.g. lead, manganese) and mordants associated with organic colorants (e.g. Al, Ca, Sn). Finally the instrument itself and the x-ray tube source can generate peaks.

References

Lucchesi, Claude, “Analytical spectroscopy in the protective coatings industry” in Official Digest: Journal of Paint Technology and Engineering (Federation of Paint and Varnish Production Clubs) 30 (1958): 212-230.

Maggetti, Marino, and Giulio Galetti. “Chemical determination of the origin of the ‘black sigillata’ of Magdalensberg.” In Die Ausgrabungen auf dem Magdalensberg, Magdalensberg-Grabungsbericht, 15, edited by Hermann Vetters and Gernot Piccottini, 391-43. Verlag des Geschichtsvereins für Kärnten, Klagenfurt: Austria, 1949.

Shugar, Aaron N., and Jennifer L. Mass, eds. Applications, Possibilities, and Limitations of Handheld XRF in Art Conservation and Archaeology. Belgium: Leuven University Press, 2012.

Townsend, Joyce, and Jaap Boon. “Research and Instrumental Analysis in the Materials of Easel Paintings.” In The Conservation of Easel Paintings, edited by Rebecca Rushfield and Joyce Hill Stoner, 344-47. Routledge: London and New York, 2012.

Taft, W. Stanley Jr., and James Mayer. The Science of Paintings. New York: Springer, 2001.