X-rays are a type of radiation which can be used for analysing aspects of a work of art not visible to the naked eye.
X-rays can pass through most solid objects, but they are obstructed by certain materials. The heavier the atoms of a substance, the more resistance it has to X-rays.
An X-radiograph records the areas of a work where the X-rays have been impeded (these areas appear white when printed as a photographic positive). Pigments containing heavy metals such as lead and mercury show up, as do the nails used in the construction of a painting’s support.
X-radiographs are useful for revealing changes that may have occurred at different stages in the development of a painting; losses to the paint layer show as dark areas. X-radiographs can be difficult to interpret because the image shows all of the layers of the work superimposed.