By David Carson and Giacomo Chiari
Works of art and other cultural heritage objects can be composed of a nearly infinite variety of materials. Each material has a story to tell: how it was made, how it originally appeared, how it has changed over time, how it has been affected by other materials (materials either within the object or used in its preservation), and how it will respond to a new environment. To uncover these stories, scientists rely on observations made using scientific and analytical instrumentation.
It is rare for an instrument to be developed specifically for use in the study and conservation of cultural heritage, and in those few cases where it has been, it is even rarer for the instrument to be commercially viable. For example, in the early 1980s Kevex developed an air-path XRF spectrometer that allowed for the nondestructive collection of elemental information from objects—a tool that was of invaluable benefit to the field—but after the company had sold an instrument to every institution that wanted one, the market evaporated, and production ceased. Similar instruments have recently become commercially available, thanks to the dramatic growth in the number of museum-based science laboratories, and thanks to technological advances that make these instruments marketable to users beyond the cultural heritage community. Another example of a purpose-built instrument for conservation science is the microfadeometer, developed in the 1990s at Carnegie Mellon University by Paul Whitmore. The microfadeometer provides a method for determining the light fading characteristics of material on a very small spot, making it possible to do testing directly on an object. While the microfadeometer has not been marketed commercially, it can be constructed relatively easily from commercially available scientific components, and it has been widely adopted by the cultural heritage community.
TECHNOLOGY TRANSFER AND ADAPTATION
Because of the challenges associated with developing specialized instrumentation for use in the study and conservation of works of art, a more efficient way of advancing the available technology is the transfer of technology from other disciplines. Today's museum-based science laboratory relies on a variety of analytical technologies derived from multiple scientific disciplines, including chemistry, physics, engineering, medicine, biology, and materials science, and, more recently, from communication and imaging technologies. Many analytical instruments can be used in a conservation laboratory with little or no adaptation, including FTIR and Raman spectrometers, GC-MS, HPLC, SEM-EDS, ICP-MS, XRD, and XRF. However, because many of these instruments are invasive, their application is limited by the restricted availability of samples from cultural heritage objects. Therefore, in order to take full advantage of these existing analytical techniques, modification of such instruments may be necessary.
Another example of technology transfer and adaptation is polynomial texture mapping (PTM). PTM—developed by Tom Malzbender at Hewlett Packard (HP) for improving photorealism in three-dimensional rendering products—is a simple technique that provides the ability to look at the surfaces of an object under varying conditions. Requiring only a light source, a camera, and a reflecting sphere, PTM instrumentation records a series of images of an object while light rotates from different directions; the images are then combined and viewed using free software provided by HP.¹ PTM's ability to create a complete, integrated record of an object illuminated under direct and multiple raking light angles has potential for condition reporting and assessing physical changes in appearance over time. This technique has been used by the GCI to record the surface textures of paintings, mosaics, and wall paintings at the ancient site of Herculaneum. It has also been adapted for use with a standard microscope; by rotating the stage instead of the light source, the researcher can image microscopic textures.
WORK AT THE GCI
Transfer of technology and its adaptation for cultural heritage materials is an area the GCI has pursued over the past few years. An example of this is the Institute's development of a computer tomography (CT) scanner to examine bronze sculpture and other dense objects.
CT scanners are in widespread use in medicine, creating a three-dimensional X-ray image from a series of two-dimensional X-ray slices, each of which is generated by the rotation of the X-ray source and detector around the axis of the body. However, medical CT scanners are designed to examine the human body, which is largely composed of water and is of a particular shape. With the exception of their use in the examination of mummies, medical CT scanners have found little application in the study of art. Systems with larger energies and more open architecture are required to examine many of the other objects typically found in a museum collection. Several research groups in Germany and Italy have been at the forefront of applying these types of systems to works of art.
The GCI worked with physicists from the University of Bologna and Lawrence Livermore Laboratories in Berkeley, California, to develop our own CT scanner, applicable to the wide variety of materials and shapes found in works of art. Using the high-power X-ray source already in place at the GCI for conducting X-radiography of bronzes, the team constructed a simple arrangement using an X-ray-sensitive cesium-iodide scintillation screen, a large mirror, an astronomy-grade digital camera, and lots of lead shielding. In this adaptation, the object (rather than the X-ray source) is rotated—a design that vastly simplified the construction. The scanner is currently limited to imaging objects less than 44 cm in width. Nevertheless, it has been applied successfully to the study of a number of small bronze statues, revealing otherwise inaccessible features on the interior, casting flaws, and the structure of repairs.
Another example of the adaptation of an existing technique for use in the examination of cultural heritage materials is the development of a portable XRD/XRF instrument. While portable XRF spectrometers have been readily available from commercial sources for more than ten years, these instruments provide elemental information only; they do not provide the compound-specific information on crystalline material that is most appropriately probed through XRD analysis. Looking at technology developed by the Jet Propulsion Laboratory for the NASA Mars Exploration Rovers that allows the rovers to simultaneously conduct XRD and XRF analysis of rocks on Mars, GCI scientists saw an immediate application for conducting analysis in the field—for example, on wall paintings in remote tombs in China or Egypt. Unfortunately, as designed for the rover, the instrument requires a scoop of finely ground, powdered sample to be placed in a chamber and continuously agitated during analysis—a destructive technique inappropriate for the examination of wall paintings. The GCI modified the original design from a transmitted to a reflective mode, allowing it to operate completely noninvasively. This new instrument, named the Duetto XRD/XRF, has been used to identify pigments, grounds, and corrosion products in situ on manuscripts, paintings, and sculpture in the collection of the J. Paul Getty Museum. In addition, it has been used to examine paintings in the tomb of King Tutankhamen in Egypt, thus fulfilling its original intention—to allow for noninvasive analysis in the field.
These examples illustrate the importance of looking beyond the original intent of an instrument for opportunities where it may be applied—with some adaptation—to the examination of cultural heritage materials. The cost of developing new technologies will almost always be beyond the ability of most arts organizations to afford, but by looking to other fields and industries, we can enhance the tools used to examine works of art.
Nevertheless, it is important to avoid overestimating the power of emerging technologies; after all, in most cases, adaptations come at the cost of some functionality. While noninvasive and portable techniques are desirable and can be very useful in initial analyses, often the required information may be obtained only through the examination of removed samples, using tried-and-true technologies.
David Carson is the GCI's laboratory manager. Giacomo Chiari is the GCI's chief scientist.