Sometimes the artefact may be brought to the surface from the sea-bed and
the shape of the artefact may be readily apparent even though it is covered in a
thick layer of concretion. Even though the shape is obvious, the artefact may be
completely mineralised or only a small amount of uncorroded metal left. In the
worse case scenario, there may be a void beneath the concretion layers.
Alternatively, the shape of the artefact may be impossible to ascertain as the
concretion is just a solid lump (Figure 4). In all these cases it is advisable to
use electromagnetic radiation to examine what is beneath the concretion. X-rays
and -rays have been used for this purpose. These have a shorter wavelength
and higher energy than does visible light and are thus able to pass through
material which is totally opaque to visible light. The extent to which X-rays
and -rays can pass through a material depends upon the thickness of the arte-
fact and the atomic weights of the elements in the artefact. The light elements
such as C, N, O and H (found in the concretion) are virtually transparent, while
the commonly found metals such as Cu, Fe, Sn are medium absorbers but Pb
is a strong absorber. The artefact is irradiated from one side with penetrating
electromagnetic radiation and a photographic film is placed on the opposite
surface. A shadowgraph of the artefact is obtained. Scales, corrosion prod-
ucts, cracks, pores, etc., absorb little radiation and show up as darker regions
on the film. Solid metal, if present, shows up as very light areas. It is possible
to use a TV screen instead of the photographic film.
Metals 141
Figure 4Concretion recovered from seabed with no visible indication of metal object within