"The hammering of the iron, at first by hand and later by water powered hammers, played a necessary part in the production of quality iron. The slag which remained in the wrought iron from the earliest smelting processes and later from any of the several pig iron refining processes, was evenly distributed as uniformly oriented, elongated threads or "stringers" throughout the hot iron by the blows of the hammer. This fibrous structure was the source of good toughness and corrosion resistance of wrought iron. The best wrought iron would contain as many as 250,000 uniformly orientated hairlike stringers of slag per square inch of cross section. Only after the earliest studies of iron with the microscope towards the end of the nineteeth century were the reasons behind the beneficial effect of hammering well understood- but awareness had come long before understanding. In the early descriptions of iron and steel making, old horseshoes and horseshoe nails are frequently cited as a necessary ingredient of the best quality products. This practice is an extension of the well foundered but somewhat obsessive commitment to hammering. The constant pounding of these shoes and nails on cobblestone pavements served as hammering which, it was thought, continued to improve the iron of which they were made even as they were being worn beyond usefulness."
Pg 22-23 Fighting Iron A metal handbook for the Arms Collectors 1999 by Art Gogan.
Hope this clears things up a little.
Jeff
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