X-rays can pass through many materials, and different materials block ('attenuate') the passage of X-rays to different degrees. As X-rays pass through an object, X-rays of a particular wavelength are attentuated more or less depending on the materials through which the X-rays pass and the thickness of the material. The attenuation of X-rays passing through the object makes a 'shadow pattern' which can be captured for study on photographic film, or by a reusable phosphor screen which can be read by a digital scanner, or directly by a digital detector. Each of these three techniques has advantages and disadvantages and all three are currently used at the Field Museum. The resulting X-radiographs allow conservators to look at features hidden below an object's surface. The different attenuation values can also be used to distinguish between materials which look the same under visible light but have different X-ray absorptions.
Stucco bust, Iraq, c.5th CE:
the X-ray image shows metal nails and bars used to attach fill materials in an early restoration attempt. If relative humidity is not carefully controlled, the metal could corrode and damage the original stucco. Less attenuating (grayer) material at left of image is a plaster of Paris restoration fill.
Sulka mask, Papua New Guinea, accessioned 1913:
this image was taken in several sections with a Siemens Axiom digital X-ray unit and then the images were enhanced and merged using Adobe Photoshop. The resulting image allows conservators to examine structural elements inside the mask's basketwork to determine the best way to support the mask in storage.
Double-barreled pistol, China, accessioned 1919:
X-ray taken to show construction. The high dynamic range of digital x-radiography allows both wood and metal components to be imaged in one shot.
Ceramic vessel, Peru, Chimú Culture (1100-1400 CE):
X-ray shows the complex interior structure of the pot: two chambers joined by hollow tubes with a hollow spout and the hollow construction of the two monkeys, each grasping the other's head, on top of the square chamber. Cracks in the handle covered in previous restorations are also visible.
Volcanic block, Pompeii, 79 CE:
film X-ray shows the artifacts inside this block of lava from a villa outside Pompeii: rings, coins, beads and a whetstone are visible.