X-rays can reveal much about an artist's working process and can frequently show compositional changes. X-rays are located beyond UV in the electromagnetic spectrum. Like UV they are invisible but have even shorter wavelengths and greater energy than visible light. X-rays penetrate through paint layers and supports to varying degrees depending on the atomic weight of the material being x-rayed. Materials of low atomic weight allow x-rays to pass through easily and therefore appear dark on x-ray film, and those of high atomic weight block x-rays and appear white. For example, lead white paint is highly opaque to x-rays whereas carbon-based paints, including some blacks, are more transparent. The thickness of the material also determines the degree of opacity.

X-ray setup

In the study of The Old Guitarist, examinations with visible and ultraviolet radiation suggested the existence of two earlier compositions underneath the image of the old guitarist. X-radiographs revealed two faces peering out from behind the musician: 

(Contour 1) an older woman with her head bent forward, and (Contour 2) a young mother with a nursing child kneeling at her side. 

One or both figures appeared to be sitting with their left arms outstretched. In addition, the head of an animal was visible on the right. In this example the x-rays penetrated the upper paint layers which are thinly painted and less x-ray opaque, revealing x-ray absorbent layers beneath.

The Old Guitarist x-ray still image.

Since an x-ray penetrates down to the support layer of a painting, it can be used to examine the structure of the panel on which The Old Guitarist is painted. Detail 1 shows the wood grain of the panel. Detail 2 shows white lines that are actually cuts made into the wooden support before it was painted. When Picasso applied the ground laye,r the greater thickness of the ground material at the cuts shows up white on the x-ray film. It has been hypothesized that the wooden support for The Old Guitarist was originally a large breadboard, which would explain the multiple cuts and the joins at either end.

Detail 1: wood grain, center

Detail 2: cuts in panel, left center