How can X-rays reveal an earlier painting?

A face can remain hidden inside a painting for centuries without the canvas becoming transparent. An artist may paint over an earlier figure, leaving its materials beneath the finished surface. Conservators can sometimes recover evidence of that earlier work by examining what passes through the layers.

An X-ray examination uses electromagnetic radiation with much shorter wavelengths than visible light. A beam passes through the painting toward a detector. Along the way, some radiation is absorbed or scattered out of the beam. This reduction is called attenuation. The amount depends on the materials, their thickness and the energy of the X-rays.

Paint is a mixture of materials, and different pigments can attenuate differently. Pigments containing elements such as lead or mercury often produce strong contrast against less attenuating surroundings. If an earlier painted face contains such material, its outline may affect the detector even when later paint completely hides the face from ordinary view. A radiograph is therefore evidence about the painting's materials, not a photograph taken by visible light through a transparent cover.

In a conventional display, regions with stronger attenuation appear lighter. Our invented example places a ring-shaped painted feature beneath a plain upper layer. Less radiation reaches the detector along paths through the strongly attenuating ring, and the displayed radiograph makes that ring light. If the hidden feature and surrounding paint attenuated almost equally, the outline could be difficult to distinguish.

The detector also records contributions from the support and every other layer along each path. A light patch might involve thick paint, a different material or even a metal fitting. Layers overlap in the image instead of arriving as neatly separated pages. Conservators compare the radiograph with surface examination and other analyses before deciding what a feature means.

This has revealed real changes in artworks. Getty researchers report that X-radiography in 1968 revealed another figure beneath Rembrandt's An Old Man in Military Costume. Later studies used additional techniques to map particular chemical elements and clarify that earlier composition. Those element maps are different measurements from a conventional radiograph, and a reconstructed color image still involves interpretation.

The finished surface is only one moment in a painting's life. Material traces can preserve an abandoned arrangement, a reused support or a later change. Science makes some of those traces accessible, allowing historians to ask how the work developed while keeping the observed evidence distinct from the story built around it.