NDT is primarily used in the industrial marketplace (automotive, aerospace, the power industry, transportation, military equipment, and many others), but from time to time it is also used in more unusual circumstances. Over the years, many historical items have been tested using NDT methods. For example, the Liberty Bell; the Confederate submarine, the H.L. Hunley; and artifacts from René-Robert Cavelier, Sieur de la Salle’s ship, La Belle have been inspected for preservation and conservation. Even famous paintings have been X-rayed to determine condition and to reveal if other paintings are beneath the top painting.
Two of the most unusual things to be radiographed are the Fighting African Elephants on display in the Field Museum of Natural History in Chicago, Illinois. On display since 1909, the elephants are located in Stanley Field Hall of the museum and immediately greet visitors as giant, imposing figures (Figure 1). The elephant display was created for the museum by Chief Taxidermist Carl Akeley. On a zoology expedition to Africa in 1905, Akeley and his wife Delia shot the elephants in Kenya. The skins were preserved and brought back to the US—along with the tusks and hair—for taxidermy.
Over time, the elephants were exposed to natural light, as well as humidity and dry air, which have caused cracks in the skin. These minor fissures have been mended over time, but with more than 100 years of exposure, the museum felt it was time for a major renovation. To perform proper repairs and restoration, the scientists had to know what was immediately below the skin and the condition of the base that held the elephants together.
Lawrence Heaney, curator of mammals at the Field Museum, said, “They have done well for over 100 years, [but] they are evocative and historically important, [and] they would need significant repairs if they are to last another 100 years.”
The elephants were preserved so long ago, few records exist. Museum officials knew that the base of each pachyderm was fashioned using wood and steel, and the skins were attached to chicken wire, but little else.
The decision was made to X-ray the animals so that a more accurate record of their structures could be discerned and to determine if there was any damage or weakening of the internal structures. At the time, the museum was open every day, so the work was performed at night after closing.
Because X-ray was to be used, the area had to be secured to ensure that no museum employees would be exposed to radiation. The X-rays were taken using computed digital radiography with 14 in. × 17 in. photostimulable phosphor imaging plates. The plates were held in place by a tripod as the imaging moved from section to section over the torso and head of each elephant (Figure 2). The X-ray tube was mounted on a portable lift to allow it to be easily moved around and raised above the elephants.
The exposures were made at 170 kV and 4 mA at approximately 60 in. from source to detector, for approximately 3 minutes.
After exposure, the imaging plates (Figure 3) were processed through a plate reader and the images were transferred to a monitor. As the work progressed, the images were viewed on a computer monitor that was part of the computed digital X-ray system (Figure 4).
One specific challenge the radiographers faced was that they were not able to use location markers or lead letters on the elephants because of the fragile nature of the hides. In order to correctly map the elephants with the images, each exposure had a different lead letter placed on the cassette and the location of each image was drawn onto an actual photograph of the elephant. More than 30 X-ray images were taken of the elephant shown with the tusk up in the air.
The images were extremely valuable to the museum and provided exactly the information needed to proceed with the restoration. They confirmed there were no bones inside the structures. In fact, the internal structures were found to be similar to the chicken wire steel used to fasten the skin (Figure 5). The images also showed exactly how the heads were attached to the bodies.
Radiation safety posed unique issues due to the exposure geometry and large open area of the museum. As previously mentioned, all exposures were taken after hours to ensure no employees or guests entered the radiation area. Barriers were placed at all entrances and four survey meters were used. One industrial radiographer was stationed on the second floor to measure radiation levels. Industrial radiographers used the three remaining meters on the first floor to monitor dosage rates at the various entrances and near the museum representatives who observed the work. All radiographers wore film badges and dosimeters as well. Dosages were well below the 2 milliroentgen limits permitted, with no readings at most locations.
Situations such as this provide a different side of how NDT can be used to solve everyday problems. It also provides scientists and archaeologists with new means for examining historical items without disassembling or damaging them.
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Revised from “Radiography Has a Night at the Museum,” published in the October 2019 issue of ASNT’s The NDT Technician written by Stuart Kleven, Alloyweld Inspection Co., skleven@alloyweldinspection.com, and Amanda Young, McNDT Pipeline Ltd., amanda@mcndt.com.
Totally cool!
Nuevamente la radiografía nos demuestra la versatilidad de su uso y en este caso específico fue invaluable para los restauradores el conocer la estructura soporte de estas piezas.