X-ray use has become a common practice among art authenticators. Not only does it unlock secrets underneath paintings, but it helps to establish authenticity. Types of paper, materials, preparatory sketches, changes to the composition, and other clues can be discovered through the use of an x-ray to prove the nature and origin of a painting.
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-rays can also be used to detect traces of minerals and other elements within the paint. These traces can be clues to when the painting was executed and where. For example, this x-ray of Vermeer’s “The Girl With a Pearl Earring” reveals that there were traces of lead in the paint that he used.
During Vermeer’s day, lead was a primary component in white paint. These brighter areas on the x-ray show where Vermeer used white, therefore creating the luminous glow that this picture has become famous for. Even though this is unmistakably a Vermeer, this specific applied technique confirms the painting was produced at the time when lead was in use.