RTI at Kurru!

JANELLE BATKIN-HALL, Graduate Intern in Conservation

I’ve just returned from a fantastic six-week fieldwork experience at the El Kurru archaeological site in North Sudan. There, Kelsey conservator Suzanne Davis and I documented ancient figural and geometric graffiti in a funerary temple at the site. Each day, Suzanne and I would make our way through a maze of mudbrick alleys to the edge of the village, where the funerary temple and several royal burial tombs and pyramids are located in the desert.

We photographed  the graffiti using a process called reflectance transformation imaging (RTI) (fig. 1). RTI is an excellent technique for documentation because each pixel records surface texture in addition to color. Since sandstone is subject to ongoing disintegration and loss, the resulting RTI images provide an excellent record of the graffiti’s current condition, as well as a highly detailed image of the column’s surface texture.

Fig-1
Figure 1. Conservators Suzanne Davis (left) and Janelle Batkin-Hall performing RTI imaging at El Kurru. (photo by Walter de Winter)

The Kurru graffiti were documented using highlight image capture where the camera remains fixed and a portable flash is moved at intervals which create a dome of light over the surface.  In a single photo sequence of one object (or in this case, graffito), approximately 48 digital images are taken. Two reflective black spheres are also fixed within the image frame, and the reflection of the flash on these spheres allows the processing software to calculate the light direction for each image. The resulting images are combined with software, resulting in a single file. In this file, the viewer can move the light source across the surface in order to examine the surface details and topography from any angle (fig. 2).  As a result of using this technique, 64 “new” ancient graffiti were positively identified and additional surface details became visible. In a couple of instances, a graffito was initially misidentified. For example, in 2015 a particular graffito was identified as an arrow. After performing RTI, it was clearly a human figure.

Fig-2_RTIViewer_Screen_Capture
Figure 2. Screen capture of bull graffito using CHI’s RTIViewer software.

For me, this was a great experience because I was able to use a technique I recently learned in graduate school. Being able to apply it onsite and share the results with our colleagues was very rewarding.