X-rays are not very good for thick objects and the equipment is not
portable. Radioisotopes such as Co^60 , Ir^192 and Cs^137 in the form of a wire and
sealed in an aluminium capsule are used as a source of -rays. The whole unit
is stored in a lead or tungsten alloy, which is opaque to -rays. Radioisotopes
are portable and it is possible to distinguish between two metals beneath a
concretion. The subsequent radiograph of the undistinguished lump in Figure 4
(which turned out to be a rudder pintle), is shown in Figure 5. Once the shape
of the underlying metal is revealed, the conservator is able to delicately
remove the concretion without destroying the shape of the object as shown in
Figure 6.
If the extent of corrosion of the artefact while buried is uncertain, a radio-
graph may be taken to ascertain the amount of metal left uncorroded. This was
often helpful in deciding the most suitable method for subsequent conservation.
4.1 Acid Pickling
In the metal finishing, the use of acids for the removal of corrosion products
prior to coating is widely practiced. The acids used depend on the metal or alloy
being treated as it is essential that once the corrosion products have been
removed, there is no attack on the exposed metal surface. For use in conserva-
tion the aim is to selectively dissolve the rust and leave the parent metal unaf-
fected. Thus 5 M HCl plus hexamine as corrosion inhibitor has been used for
142 Chapter 6
Figure 5Radiograph of concretion showing shape of the object which was later identified
as a rudder pintle