Revealing Hidden Layers
Revealing Hidden Layers
Terahertz imaging, a technology developed for industrial and scientific purposes, is proving to be increasingly useful in studying art works and artifacts. And David Citrin ’85, a professor of Electrical and Computer Engineering at the Georgia Institute of Technology, is on the front lines of that effort.
Earlier this year, Citrin and a cross-disciplinary research team—made up of imaging scientists, a chemist and an art historian—used terahertz imaging to uncover an inscription of the Lord’s Prayer hidden beneath corrosion on a lead funerary cross from the 16th century. His team has also studied the 17th-century painting Madonna in Preghiera, by the workshop of Giovanni Battista Salvi da Sassoferrato, and a Byzantine coin. The process allows researchers to see through layers of paint, corrosion and other materials without damaging the artifact.
Williams Today reached out to Citrin via email to learn more about the technique and his inspiration for working in a field that combines art and science. (Responses have been edited for clarity and space.)
Can you explain what terahertz imaging is?
Terahertz waves are electromagnetic radiation, similar to microwaves, light and X-rays. Whereas microwaves have a wavelength of millimeters, terahertz waves are shorter—about the same as the diameter of a human hair. The shorter wavelength allows us to pick up more detail than we could with microwaves. Terahertz waves can see through many materials that are electrically insulating (e.g., plastics, ceramics, glass, paper, textiles, wood and paint) but not those that are electrical conductors, such as metals.
With terahertz waves, we can image in three dimensions, not just the surface but also the interior of a variety of objects, retaining details that are hard to get at without breaking the object open. Since these techniques do not damage the item under investigation, terahertz imaging is used for what is called in an industrial setting “nondestructive evaluation.” It has been applied to paintings on canvas and wood, wall paintings, mummies, manuscript pages, ceramic, glass and wood objects, among others.
In the context of our work on the lead cross, the terahertz waves pass through the layer of corrosion and are then reflected off the metallic lead below. This is what enabled us to obtain information about the inscription underneath the corrosion.
What do the results of the imaging mean?
Though there are different types of terahertz imaging that can be carried out, our main aim is that we can obtain—if we are lucky—information about the interior structure of objects nondestructively. For example, we have mapped out the thickness of multiple paint layers in oil paintings on canvas. Such work potentially helps us to understand the processes and materials used in producing a painting. This knowledge might have art-historical importance, and it might also be relevant to conservation efforts to arrest degradation of endangered objects.
The technique can identify pigments used in a painting, which assists in authentication of a painting when we know what pigments were available or used by a certain artist or in a given period or location. Other research groups have identified underdrawings on paintings. Because charcoal, pencil and other carbon-based materials strongly reflect the incident terahertz radiation, we gain insight into how the artist’s conception of the work evolved during its execution.
Terahertz imaging has also been used to detect re-paintings or changes made by the artist over the course of production of the work or by restorers. We recently studied a painting the owner thought was by a famous painter and suspected it contained an overpainted signature. We couldn’t find a signature, but subsequent chemical analysis carried out by others detected the prevalent use of a pigment that was not developed until after the purported artist’s death. And one researcher we know who was using terahertz techniques detected metal foil that was used in a wall painting that was subsequently covered in a restoration.
In ongoing work, we are measuring the thickness of a surface layer formed on certain types of Roman coarse ware pottery due to the chemistry that occurs during firing in the kiln. Though I speculate here, we might be able to characterize the specific kiln in which an object was fired and thus identify where it was produced. This may provide information of relevance to understanding long-distance trade in the Roman empire.
Does terahertz imaging have applications beyond examining historical artifacts?
Actually, we got into the archeometry business through our work on industrial materials. Most of our work is on materials such as paint on steel for structural applications, oxide layers on metals related to steel production, glass and plastics used in microelectronics, and fiber composites used in aerospace, automotive and energy applications.
In aircraft, for example, terahertz imaging may identify non-visible damage in the fiber-composite components—used to replace metals and reduce fuel consumption—before catastrophic failure occurs.
Did anything from your Williams experience lead you to this type of research?
Most definitely! During my years at Williams, I took an art history survey course with Professor Emeritus Zirka Filipczak and have been interested in art ever since. During graduate school studying physics at the University of Illinois, I sat in several courses on art and architectural history. It was fascinating!
Working at the interface between a STEM field and the humanities has been fascinating and rewarding. Few of my colleagues in the sciences and engineering get private tours of museums.
You can read more about David Citrin’s ’85 research in coverage on the Georgia Tech website.
Photograph by John Toon.
Regina Velázquez is an assistant editor and senior writer in the Office of Communications.