Paintings often consist of several layers subsequently applied in time.
For revealing the images hidden beneath, discovering the chemistry of pigments, identifying the authors of unsigned works and probing the crack depths from damage or age, infrared radiation or X-ray are conventionally applied. The success of these techniques is usually affected by inherent limits to penetration depth and resolution; it would be very useful to have a non-invasive technique capable of high-resolution imaging of artwork.
To meet these requirements, terahertz region of electromagnetic spectrum can be utilized. Placed between the infrared and microwave regions, terahertz radiation can pass through different kinds of materials like plastic or canvas and reflects back slightly different depending on the chemical composition of each paint color. To analyze the structure of the paintings, the researchers look at the shape of the reflected wave at each layer of the paint. This kind of data reveals features of the painting hidden beneath layers of paint.
Recently, researchers at the University of Barcelona have utilized terahertz radiation to uncover the hidden carbon signature of a painting previously thought to be unsigned (Source: Physics Central). At the very bottom of the painting, the researchers found a signature. Blind to the visible eye, to X-ray, and to infrared analysis, the researchers identified what they believed to be Goya’s signature in the image created by the terahertz reflections. The researchers report that the signature was likely written with a pencil and that over the years, the top coat of varnish darkened and obscured the carbon signature.