There is a standard forensic procedure, used whenever it is desired to
recover engine numbers, firearm serial numbers, etc., which have been
removed from iron or steel objects by wear or abrasion.
The normal method is to polish the area of interest with diamond paste,
and
then lightly etch by swabbing with nital. The mark will then usually
appear, often with startling clarity, but will be fugitive and must be
photographed immediately. Anyone with experience of optical
metallography
will find this procedure easy to carry out. Residual traces of the
developed
mark are are easily polished out if required.
By and large, radiographic techniques are ineffective for this purpose,
as
they depend on differences in path mass (for a given atomic number
distribution), and this is normally only marginally affected by mechanical
deformation.
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