X-radiography is used throughout the museum world in the examination of a variety of art objects. Paintings were one of the first groups of art objects to be examined in this way and x-radiography has been used with paintings for well over 50 years.
The x-rays do not harm the art work. X-radiographs are traditionally produced by placing the painting between a photographic film and an x-ray emitter. The film is placed as close as is safe to the surface of the painting thus providing as near a life size image as possible. This method is slowly being replaced with Digital X-ray techniques that eliminate the need for film.
The materials used in a painting differ in their ability to absorb or transmit x-rays.
Paints, such as lead white, lead tin yellow, or vermillion that contain heavy metal elements, will absorb the x-rays and prevent them from blackening the film.
Materials that do not absorb x-rays, such as carbon black, will allow x-rays to pass through the object and blacken the film.
X-radiographs can be difficult to read, but fortunately, x-ray opaque pigments include most of the whites and yellows, meaning that light areas in a painting are mostly light on the radiograph.