Radiography of a Baining Mask
posted August 31st, 2012
After rehousing some 15 Sulka masks (read about that here: http://fieldmuseum.org/users/ashley-jehle/blog/rehousing-sulka-masks), we set our sights on the museum's collection of Baining masks. But before we could break out the power tools and begin rehousing, we decided to X-ray one of the Baining masks in order to better understand its structure.
The Baining peoples of New Britain, Papua New Guinea make masks to be danced to commemorate events such as births, deaths, and harvest season. This mask entered the collection at the Field Museum in 1913 and has been stored in a similar way as the Sulka Masks since the 1980’s.
Baining masks can vary in form and shape, but they are mostly made of plant materials and are often decorated with red and black pigment. This mask is constructed of a bamboo pole and a frame made of wood and rattan, which are covered with leaves and barkcloth. The wooden and rattan frame creates the structure of the mask and is lashed together with what appears to be rattan epidermis. The leaves act as filler over the frame. The outer covering is barkcloth, or tapa, which is made from the beaten inner bark of certain trees, like breadfruit and ficus. The barkcloth is stitched together around the frame and is painted with black and red designs.
In this detail, you can see the veins in the leaves, and the two small dots at the top and bottom are from the fiducials.
The Field Museum has a Siemens Multix digital X-ray system. For taking X-rays from the side, the digital plate can be mounted to a “bucky” and the tube support synchronized so that when the plate is moved vertically the X-ray tube follows it. The Multix is a medical X-ray unit, and the digital plate measures about 14 inches by 17 inches. Since the mask is over 8 feet long (and over 3 feet tall), it could not be X-rayed overall. Therefore, the object was X-rayed in smaller, overlapping sections, and these X-radiographs were merged digitally in Photoshop.
One of the complications that arises from X-raying sections and stitching them together is geometric distortion of the image. The shape and size of the mask gets distorted at the edges of the X-radiograph as a result of the way the X-ray beam is emitted from the tube. Think of the way hand puppet shadows look depending on the distance between the flashlight and your hand.
What made this digital image patchworking more precise and easier to put together were fiducials! (What a fun word, right?) Regenstein Conservator J.P. Brown first came across fiducial “CT-Spots” during a recent CT scanning project in collaboration with Christ Hospital in Oak Lawn. Fiducials are basically stickers that contain a high attenuating material which will appear white in the X-radiograph. These stickers were placed on the cheesecloth wrappings, and these areas were overlapped so that the fiducials appear near the edges of each X-radiograph section.
The fiducials appear as small white dots in the X-radiographs. Note that the lighter colored trio of squares in the images is an effect from the digital plate. It’s something that does not appear when X-raying humans, but appears when X-raying lower attenuating things like plant material.
As you can see, the X-radiograph shows us the internal framework of the mask and provides information on how it was constructed. Understanding the mask’s structure will help us develop storage mounts which adequately support and protect the masks for long term storage.