A method of improving the mechanical properties of iron and steel, in which the metal is forged out, folded back on itself and the resulting ‘pile’ of metal is welded together. This process may be repeated several times so that the final metal consists of many fine layers.
Piling was used to homogenise steel produced by carburising strips of iron. Because the carbon has to diffuse into the metal from the surface it is difficult to produce homogeneous steel of any great thickness. If carried out successfully the piling process reduces the final thickness of each original strip to a fine lamination, while migration of carbon at welding heat further homogenises the carbon content. Thus, although exactly the same methods are used as in damascening (forging, folding and welding) the results and aims are different. The aim was to produce a more uniform consistent structure, whereas, damascening through the use of two different types of iron or steel deliberately produces a decorative pattern. The aim of piling (although not always achieved) was to produce a macro- and microscopically homogeneous steel, whereas, the aim of damascening was to produce a steel that behaved in a uniform manner in the bulk, but was patterned on the small scale. The production of ‘shear steel’, the highest-quality British product before the development of crucible steel, was essentially a large-scale version of piling, though given the thickness of the blister steel bar raw material, micro-homogeneity could not be achieved (Barraclough 1984, 45-6 and 65-7). Although, it is absolutely clear that some artefacts have been made by this technique, it is likely that some that been identified as having been made by this technique are, in fact, the result of either fortuitous banding, or the side effect of other parts of the manufacturing process.