PTT Grants Study Takes the Temperature of a Deteriorating Chinese Stele
The Nelson-Atkins Museum of Art in Kansas City, Missouri, is turning up the heat on research, monitoring artifacts that are threatened by weathering and environmental wear. The museum has just completed a PTTGrant project with the assistance of Wayne State University in Detroit, using thermal imaging to document voids and delamination in a Chinese stele dating to the Northern Wei Dynasty in the sixth century.
Now located at the Nelson-Atkins Museum, the stele is a dark gray limestone carved in low relief. Because of the layered nature of limestone, the object is rapidly delaminating (losing layers) of its surface, endangering the shallow carving. According to Kathleen Garland, object conservator for the museum, a reliable method of tracking the effects of consolidation was needed to prevent further damage.
“The block of limestone has a horizontal cut presumably made at the time that the sculpture was removed from its original location in China” Garland said. “Large amounts of delamination have occurred in the area of this cut.”
In spring 2000, John Twilley of Art Conservation Science, and Jerry Podany, head of Antiquities Conservation at the J.P. Getty Museum (both consultants on the possible treatment of the stone), suggested that infrared thermography might be a way to map the delaminating areas and monitor future damage.
The experiment involved a bank of 10 heat lamps set up to examine the stele, heating the surface of the limestone to body temperature for one minute prior to imaging. An infrared camera was used to image the stele to the depth of one centimeter.
The resulting images, called thermograms, clearly displayed defects as brighter than the surrounding area. The thermograms also revealed areas of delamination below the surface that were not detectable by gentle tapping, which is the traditional method for locating the voids. Rather than immediately applying a consolidant, the team performed a speculative treatment using a non-harmful acetone that would act similarly to a consolidant in real time.
“Since infrared thermography detects the voids below the surface by comparing thermal properties of air in the void, which are lower than the thermal properties of the limestone, we hypothesized that we would be able to detect when a void was filled,” Garland said. “A syringe of acetone was injected into a delaminating area just after the stone had been warmed with the infrared light for 10 minutes.”
The group selected acetone because it would not affect the surface of the artwork, and it is a good solvent for the consolidant, Paraloid B-72. As predicted, the team was able to watch the migration of the solvent in the delaminating area in real time.
Later, the team injected diluted solutions of B-72 and acetone into the void and successfully monitored the migration of the consolidant. Garland says the technique developed at Nelson-Atkins has promise as a safe methodology for conservators considering consolidation on fragile objects.
“To our knowledge, no other technique allows the conservator to watch the penetration of a consolidant below the surface in real time, and thus monitor the progress of the treatment,” she said.