X-ray diffraction analysis (XRD) is a technique used to study crystalline materials (those which have a fixed, highly ordered internal structure). Many pigments and the materials used in grounds are crystalline and so they can be identified using XRD.
In XRD, a narrow beam of X-rays is focused on a sample and is scattered in a highly specific way. The scattered X-rays can be recorded electronically or on a strip of photographic film. Each unique crystal structure generates a ‘fingerprint’ pattern of the scattered X-ray beam. The ‘fingerprint’ allows for identification of the material.