Do you own a valuable painting? Perhaps it is what you do not see that makes it valuable.

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 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.

While Ultra-Violet examinations can be done in-house, heavy duty x-ray photography must be done at the laboratory level.

Art authenticators have been using x-rays to identify and authenticate paintings for more than 100 years. The first documented use of x-rays in art authentication was in 1896 in Frankfurt, Germany.

So how do x-rays work in the art authentication world? It's actually a simple process.

If you have ever had an x-ray performed at the doctor's office, you already have a basic understanding of how they work; x-rays are used to see different layers in your body that can't be seen by the naked eye.

There are two types of x-rays used in art authentication; stereoradiography which operates the same way as medical x-rays and autoradiography which uses beta particles. Each type of x-ray can show different things in a painting that would otherwise not be seen.

Like at your doctor's office, x-rays can see through different layers, but instead of flesh these x-rays see different layers of paint. X-rays can show where touch-ups have been made, or where places were painted over.

In order to create a new picture of the layers of the painting, the rays pass through the painting and create a negative of the darker areas on film. Think of it as reversed photography. After the rays are passed through the painting, old layers of paint can be seen and the investigation can begin. Is this consistent with the known preparation and painting method of the artist? Are the hidden compositions similar to the style that the artist used?

Time is also an element that radiographers use to make x-ray exposures. In the same way that you can under or over expose film in a camera, the same can be said for x-rays. Radiographers use a short x-ray exposure to show the deepest layers. The longer the exposure, the shallower the x-ray will be.

Through a combination of Morellian analysis, documentary research and x-ray examinations, authenticators can determine if a painting is the genuine article. For example; it is well-known that most artists would recycle their canvases. Painting over a rejected picture was a common practice. Red flags would go up for an art authenticator if there were no sketches, modifications (pentimenti) or anything at all below the surface of a painting. Some painters though, did not prepare a sketch. In general however, a perfect composition may indicate that the painting is a duplicate or a copy.

An educated authenticator with a very trained eye can distinguish styles and methods with the use of x-rays.

They are especially useful for examining paintings on panel (wood).