What do X-rays and infrared cameras reveal hiding under a painting's surface?

X-rays and infrared cameras reveal the hidden layers of a painting: sketches the artist drew first, second thoughts that were painted over, and sometimes an entirely different picture below. For an authenticator, these hidden layers are evidence about how a work was made.

Underdrawing is a preparatory drawing done on a painting ground before paint is applied. Infrared reflectography can show it because carbon black pigments absorb infrared light, while opaque pigments such as lead white are transparent to it. The method can reveal underlying layers without damaging the painting. X-rays work on the other side of the puzzle: white lead is detected by X-ray, so a conventional X-ray can detect earlier work present under the surface of a painting.

Artists change their minds, and the changes are called pentimenti, from the Italian for 'repentance'. A pentimento is the presence or emergence of earlier images, forms or strokes that have been changed and painted over. Pentimenti are valuable for detection, because they help decide whether a painting is the prime version by the original artist or a copy. In general, the more pentimenti, the more likely a painting is to be the prime version, while copies normally have few if any.

There is a plain forgery check too. If a painting supposedly from the 17th century lies on top of one showing people in 19th-century dress, the scientist will assume the top painting is not authentic.