Beneath the Surface of African Art: Thermoluminescence Sampling of Terracotta Sculptures from Africa

To test terracotta using TL dating, conservators take a powdered sample from an unobtrusive area of the object using a high-speed drill. Due to the fragile nature of these objects and the risk of causing damage through improper technique, only an experienced conservator or trained sampling agent should collect the sample. We reached out to objects conservator Lara Kaplan the sampling agent recommended by the TL testing firm Oxford Authentication. Lara agreed not only to sample our objects, but to take us through the whole sampling process.

The first step for sampling the objects was to examine them fully for past restorations and to locate the best areas to sample. Successful samples come from areas original to the piece rather than later restorations or additions. We confirmed locations on the object through further examination under UV light and magnification. Under UV, many restoration materials fluoresce (glow), which helped us to identify a few areas of old fills and repairs. Luckily, several relatively hidden areas on the bases or break edges of each terracotta sculpture were free of restorations—the perfect locations for sampling! After taking a photo of the selected locations for documentation purposes, Lara moved on to actually sampling the objects.

Lara brought a range of tools and materials with her for powdering the sample, including the specialized drill bits used with the Flex Shaft drill, a power rotary tool with a flexible shaft that extends the drill’s reach and allows operation from any angle. We set up our wet chemical room for sampling as it is our only windowless room and the whole process has to be undertaken in the dark using a red safety light (that’s right- the fragile ceramic has to be drilled in the DARK!). To avoid contamination, Lara first used a drill to abrade the surface of each sampling area, removing the top layer. After brushing off any loose dust and powder from the initial cleaning, Lara used a steady hand to drill straight into each terracotta. She first drilled a little way into each piece, discarding the powder to prevent contamination from the surface, then to several millimeters of depth, collecting the powdered sample on strategically placed weighing paper. Once 100mg of powdered sample accumulated for each object, the samples were carefully packed in light-tight envelopes and shipped to the testing company. Because of the large sample size and risk inherent in sampling objects, thermoluminescence dating is not an easy undertaking. It is, however, often the only way to accurately date many ceramic, terracotta, and cast metal objects.

Sampling went smoothly thanks to Lara’s experienced hand. Stay tuned for updates as we interpret the results of the testing. In the meantime, all four of these terracotta sculptures are back on view in the African galleries.