X-ray Diffraction Analysis

X-ray powder diffraction is a technique which uses a finely collimated beam of X-rays to characterise the crystallographic structure of natural and manufactured materials. The technique has been put to a number of uses in materials research, but in the cultural heritage field its most valuable application is in identifying materials such as corrosion on metals, inlaid decoration on jewellery and pigments on wall paintings. An advantage of the method is that the sample size required can be extremely small – a few mm2 of grains of powder - and with some types of equipment, analysis can be performed directly onto the surface of small, flattish items.

All minerals and compounds have unique crystallographic structures so XRD can be used to precisely identify a mineral or compound by comparing its diffraction data against a database of known minerals and compounds. X-ray fluorescence analysis and other methods of chemical analysis will provide information on elemental composition but XRD identifies the exact mineral, or mixture of minerals. For example, white inlays on Anglo Saxon garnet brooches from Kent were sometimes made of shell and sometimes of chalk paste, which must originally have looked different from each other. These materials are chemically the same (calcium carbonate) but XRD is very easily able to distinguish between chalk (the mineral calcite) and shell (usually aragonite).

Examples of the application of powder XRD in arts and cultural heritage: -
Identification of minerals, pigments, rocks, gemstones, corrosion products on metals,
inclusions and phases in ceramics, opacifiers in glass and enamel, niello,
salt growths on glass, stone and ceramics
and old conservation materials such as fillers and colourants

Crystallographic structure can be calculated using Bragg’s Law

                                                   nλ = 2d sin θ

where n is 1, λ (lamda) is the (known) wavelength of the incident X-ray beam, d is the interatomic spacing (in ångstroms), θ (theta) is the angle between the incident X-ray beam and the lattice plane.

The identification of the material is made by matching its d spaces with the ICDD (International Centre for Diffraction Data) database.

 

Further Reading:

R. Jenkins and R.L. Snyder, Introduction to X-ray powder diffractometry (Wiley, New york, 1996)