In ancient China most pigments were made from finely ground minerals. Like painting over a scratch on your car, damaged paint on artifacts may be touched up. To our eyes, there may be no difference between two paints of the same color. X-ray diffraction analysis can detect differences in composition, if they exist. If a mineral pigment was not used until the 17th century then its presence on a 2000-year-old statue is significant.
X-ray diffraction (XRD) analysis is useful for identifying samples of unknown minerals. It has long been used on geological materials, but has numerous applications in the study of archaeological materials and antiquities. There are two XRD techniques, the single-crystal method and the powder method. As the names imply, the first uses a single crystal and the second uses a powdered sample. The single-crystal method is mostly used for structural analysis of the crystal, while the primary application of the powder method is mineral identification. Following is a basic explanation of how X-rays are diffracted:
Diffraction of an x-ray beam striking a crystal occurs because the l [wavelength] of the x-ray beam is similar to the spacing of atoms in minerals.... When an x-ray beam encounters the regular, 3-D arrangement of atoms in a crystal most of the x-rays will destructively interfere with each other and cancel each other out, but in some specific directions they constructively interfere and reinforce one another. It is these reinforced (diffracted) x-rays that produce the characteristic x-ray diffraction patterns that [are] used for mineral ID. (Gorring 1998)
At TK, XRD analysis is used for mineral identification, rather than the determination of crystalline structure. Corrosion products on metals, and pigments made from ground minerals are both suitable materials for analysis. The powdered sample may be formed into a pellet or smeared on a glass slide. Monochromatic X-rays are focused on the sample, which is rotated to ensure that all possible diffractions are obtained. The older, photographic method of analysis involved the use of a sheet of film to record the pattern of the diffracted rays. The more exact modern method utilizes an electronic instrument, a diffractometer, to record the intensity of the X-rays. Comparison of the resulting pattern with a database of patterns for known substances makes identification of the substance possible.
A sample of the unknown pigment or mineral is removed from the artifact, sealed in a plastic vial and labeled. It is not generally necessary to send documentation with the sample, as the purpose of this test is to identify an unknown, not to authenticate an artifact. Documentation of the sampling is retained at TK, including photographs of the sampled object, the specific site from which each sample was obtained, and the nature of the sample (pigment, corrosion, etc.). TK uses H&M Analytical Services (Allentown, New Jersey) for XRD analysis.
Many of the pigments on Chinese antiquities are mineral based. While the variety of pigments used is quite extensive, certain pigments are not known to have been used during specific time periods. Some synthetic pigments are readily identifiable as inconsistent with antiquity. Others, such as white lead (basic lead carbonate), have been manufactured for centuries. The identification of a pigment inconsistent with the age of the artifact is considered cause for significant additional study of the sampled object.
Numerous clues may be present concerning the rough age of pigments on an artifact. Some pigments are unstable in certain environments, resulting in a color shift over time. In the case of pigments, such as ultramarine blue, that can currently be obtained in either the natural ground-mineral form or the synthetic form there are microscopic differences, such as grain shape, which can identify the method of manufacture. Natural mineral pigments tend to contain impurities, detectable by XRD, which are usually lacking in synthesized versions.
X-ray diffraction (XRD) can be used to identify unknown crystalline substances, like corrosion products on metal. Analysis of pigments may detect materials inconsistent with the purported age of the sample. Impurity patterns may support classification of a sample as natural or man-made. XRD is not used to authenticate antiquities, though impurity patterns in natural mineral pigments may be linked to specific geographic regions and previously analyzed materials known to have been used in antiquity.
1 Additional information concerning H & M Analytical is available at the web site www.h-and-m-analytical.com
Gorring, Matthew. 1998 Oct 22. Geos 443 mineralogy [monograph on the Internet]. Upper Montclair (NJ): Montclair State Univ, College of Science and Mathematics, Dept of earth and environmental studies; [updated 1999 Oct 18; cited 2003 Oct 13]. X-ray diffraction: Diffraction and the Bragg equation. Available from: www.csam.montclair.edu
Gettens, Rutherford J; Stout, George L. 1966. Painting Materials; A Short Encyclopedia. New York: Dover Publications. 333 p. ISBN: 0-486-21597-0.
Heiney, Paul A. c1996. High Resolution X-ray diffraction [monograph on the Internet]. Philadelphia (PA): Univ of Pennslvania, School of Arts and Sciences, Dept of Physics and Astronomy; [updated 1996 Dec 30; cited 2003 Oct 13]. Part 2, What is X-ray diffraction (XRD). Available from: http://dept.physics.upenn.edu/~heiney/talks/hires/whatis.html This site can be accessed directly, or through a link at the H & M Analytical web site, services page, available from: www.h-and-m-analytical.com Follow the links for XRD.
Tissue, Brian M. 1995. The Chemistry Hypermedia Project [database on the Internet]. Blacksburg (VA): Virginia Polytechnic Institute and State Univ; [updated 2003 Jun; cited 2003 Nov 14]. Light: Electromagnetic Spectrum. Available from: www.chem.vt.edu