Looking Beneath the Paint Surface
Rather than removing samples of the paint layer, non-invasive methods examine an intact painting by using the nonvisible regions of the electromagnetic spectrum. Ultraviolet, infrared, and X-rays can reveal many secrets that are invisible to the human eye.
Ultraviolet rays are best at showing surface features. The varnish that is the top layer of most paintings will fluoresce when exposed to ultraviolet. This fluorescence shows whether the original varnish has been disturbed.
Infrared rays can penetrate the layers of paint, so infrared imaging can be used to detect charcoal sketches and other images that are often hidden beneath the painted surface.
X-rays are used to look through a painting. X-rays are absorbed by dense materials and pass through others. Pigments that include metal atoms, such as white lead, show up clearly, as do metal objects used in the painting's construction. One forger was caught when X-rays revealed a machine-made nail in the wooden panel under a painting that was supposedly painted in the 1500s! Machine-made nails were not manufactured until the 1800s.