Page 284 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 9: Excavation 263
be well preserved with details quite clearly visible on the surface. However, when they are picked up it is immediately obvious that their condition of preservation varies drastically. Some will be extremely light weighing as little as 10% of their original weight, whereas others will be almost their original weight. Sectioning the balls shows that the corrosion has proceeded from the outside inward, and there is a layer of graphitized iron surround- ing a core of solid iron. In the completely deteriorated case, there is no solid iron at all. The graphitized iron is very soft and needs to be handled with extreme care.
Wrought iron tends to produce a different situation. Removal of the surface concretion of reasonably well-preserved material reveals an iron object with a type of laminated or granular structure. This is a result of the differential corrosion taking place along the lines which join individual strips that have been hammered together in the process of forging the wrought iron. Thus, in the case of an anchor, the laminate patterns show how the anchor was constructed and how the iron billets were joined together in the forging process. In advanced cases of wrought iron deterio- ration, there may be no iron left in the concretion at all. In such cases, the concretions should not be discarded as they can be used to cast a replica of the object.
Thus, the problem of dealing with concretion is complex. The aim is to try and break open the concretion and recover the object, if there is one. For cast iron this is difficult because the object is usually very fragile com- pared with the hard concretion. Where possible, concretions should be recovered intact and broken open on land, preferably in the conservation laboratory. This is particularly true for complex concretions, where an x-ray can show much of the internal structure and composition before any mechanical work is carried out (Figures 9.16 and 17a).
Using a geological hammer and a short-handled sledgehammer is a very practical way of breaking up concretion under water (Figure 9.17b). The geological hammer can be carefully positioned on the required point of impact and its head struck with the sledgehammer. All this can be done without endangering one’s hands with surprising delicacy achieved with this method. The geological hammer should never be used alone to strike the concretion as it is impossible to control the point of impact.
In some situations quite robust solutions are required to extract large iron objects. During the recovery of the engine from the steamship Xantho, McCarthy (2000) used an oxygen powered thermal lance to great effect (Figure 9.18).
It is essential to attempt to remove concretions intact under water and to dismantle them in controlled conditions on land. So concretion excava-