KRESS

EXAMINATION METHODS AND SCIENTIFIC TERMS

X-RADIOGRAPHY

What is X-Radiography

Compared to ultraviolet and infrared light, x-rays have far more energy and thus much shorter wavelengths (less than 10 nm). This high energy enables x-rays to penetrate materials to varying degrees, generating an image that corresponds to an object’s density and composition. X-radiography can provide useful information relating to an artwork’s condition and construction and can reveal compositional changes, such as hidden images beneath visible paint layers.

History of X-Radiography

Discovered in the 1890s by a German physicist, x-radiography was not used on paintings until the 1930s (images of x-rays were first published in 1938). Today, x-ray images of paintings are still considered an important diagnostic tool for examination purposes.

Types of X-Ray Systems Used

There are three types of x-rays used in industry, soft rays (5-30 kV), diagnostic rays (30-150 kV), and therapeutic rays (150-500 kV). For the examination of easel paintings, scientists and conservators generally use the latter two, although other types of electromagnetic radiation has been used as well (e.g. autoradiography, electron emission, etc.). The X-ray tube is positioned in front of the artwork while the x-ray film is placed behind the painting. Today most museums have transitioned to using digital systems and are now able to manipulate the collected x-ray image using sophisticated software programs.

How Does X-Radiography Work?

X-rays are high in energy and when they pass through objects they cause momentary changes to occur at an atomic level. When x-rays pass through a painting, areas that appear white or opaque on the film either represent areas containing radio-opaque materials or areas where paint has been thickly applied. Materials that contain elements with higher atomic numbers (e.g. the lead in lead white/lead-tin yellow, the mercury in vermillion) will be noticeably visible on the x-ray film. Scholars are often able to obtain a distinctive x-ray image simply because lead white has been ubiquitously used throughout the history of Western easel painting. Today other types of non-destructive imaging like infrared reflectography are commonly performed in conjunction with x-radiography especially in cases when the x-ray image is difficult to interpret. In the case of cradled paintings, efforts are being made to develop automated digital techniques to virtually “erase” the distracting patterns created by the cradles in x-ray images.

References

Kirsh, Andrea, and Rustin S. Levenson. Seeing Through Paintings. New Haven: Yale University Press, 2000.

MacBeth, Rhona. “The Technical Examination and Documentation of Easel Paintings.” In The Conservation of Easel Paintings, edited by Rebecca Rushfield and Joyce Hill Stoner, 300-305. Routledge: London and New York, 2012.

Marijnissen, Roger H. The Masters’ and the Forger’s Secrets: X-ray Authentication of Paintings. Brussels: Mercatorfonds, 2011.

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