X-ray Diffraction Analysis
When X-rays are fired at a crystalline sample placed in the
X-ray camera, a proportion are diffracted by the regular crystal
structure. These diffracted X-rays produce a pattern of lighter and
darker lines on a film. The pattern on the film depends on what is
in the sample and by reference to standard data, this pattern can
be used as a kind of 'fingerprint' to identify a wide variety of
materials. Such materials include corrosion products on metals,
pigments used on wall paintings, and inlaid decoration in
jewellery. Unlike techniques such as X-ray fluorescence (XRF),
atomic absorption spectrometry (AAS), inductively-coupled
plasma/mass spectrometry (ICP/MS) and other methods of chemical
analysis that provide information on elemental composition, XRD
allows a particular mineral to be identified. For example,
limestone and shell were both used as white inlays on Anglo Saxon
brooches. The problem is that both limestone (the mineral calcite)
and shell (the mineral aragonite) consist chemically of calcium
carbonate, so that the chemical analysis alone cannot distinguish
between them. Using XRD it was a simple matter to identify the
white inlay on the brooch as aragonite, showing that shell had been
used.
X-ray diffraction analysis is particularly useful in the study
of museum objects because it is almost non-destructive. It requires
a very small sample, much less than the size of a pin-head, to use
the X-ray camera.
Further Reading
R. Jenkins and R.L. Snyder, Introduction to X-ray powder
diffractometry (Wiley, New york, 1996)